Branch out dma-buf related documentation into its own rst file to allow adding it to the sphinx documentation generated.

While at it, move dma-buf-sharing.txt into rst as the dma-buf guide too; adjust MAINTAINERS accordingly.

v2: - Removed authorship as suggested by Jani, - Address review comments from Jonathan Corbet and Markus Heiser.

Signed-off-by: Sumit Semwal sumit.semwal@linaro.org --- Documentation/DocBook/device-drivers.tmpl | 41 --- Documentation/dma-buf-sharing.txt | 482 ---------------------------- Documentation/dma-buf/guide.rst | 507 ++++++++++++++++++++++++++++++ Documentation/dma-buf/intro.rst | 82 +++++ MAINTAINERS | 2 +- 5 files changed, 590 insertions(+), 524 deletions(-) delete mode 100644 Documentation/dma-buf-sharing.txt create mode 100644 Documentation/dma-buf/guide.rst create mode 100644 Documentation/dma-buf/intro.rst

diff --git a/Documentation/DocBook/device-drivers.tmpl b/Documentation/DocBook/device-drivers.tmpl index 9c10030eb2be..a93fbffa9742 100644 --- a/Documentation/DocBook/device-drivers.tmpl +++ b/Documentation/DocBook/device-drivers.tmpl @@ -128,47 +128,6 @@ X!Edrivers/base/interface.c !Edrivers/base/platform.c !Edrivers/base/bus.c </sect1> - <sect1> - <title>Buffer Sharing and Synchronization</title> - <para> - The dma-buf subsystem provides the framework for sharing buffers - for hardware (DMA) access across multiple device drivers and - subsystems, and for synchronizing asynchronous hardware access. - </para> - <para> - This is used, for example, by drm "prime" multi-GPU support, but - is of course not limited to GPU use cases. - </para> - <para> - The three main components of this are: (1) dma-buf, representing - a sg_table and exposed to userspace as a file descriptor to allow - passing between devices, (2) fence, which provides a mechanism - to signal when one device as finished access, and (3) reservation, - which manages the shared or exclusive fence(s) associated with - the buffer. - </para> - <sect2><title>dma-buf</title> -!Edrivers/dma-buf/dma-buf.c -!Iinclude/linux/dma-buf.h - </sect2> - <sect2><title>reservation</title> -!Pdrivers/dma-buf/reservation.c Reservation Object Overview -!Edrivers/dma-buf/reservation.c -!Iinclude/linux/reservation.h - </sect2> - <sect2><title>fence</title> -!Edrivers/dma-buf/fence.c -!Iinclude/linux/fence.h -!Edrivers/dma-buf/seqno-fence.c -!Iinclude/linux/seqno-fence.h -!Edrivers/dma-buf/fence-array.c -!Iinclude/linux/fence-array.h -!Edrivers/dma-buf/reservation.c -!Iinclude/linux/reservation.h -!Edrivers/dma-buf/sync_file.c -!Iinclude/linux/sync_file.h - </sect2> - </sect1> <sect1><title>Device Drivers DMA Management</title> !Edrivers/base/dma-coherent.c !Edrivers/base/dma-mapping.c diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt deleted file mode 100644 index ca44c5820585..000000000000 --- a/Documentation/dma-buf-sharing.txt +++ /dev/null @@ -1,482 +0,0 @@ - DMA Buffer Sharing API Guide - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - - Sumit Semwal - <sumit dot semwal at linaro dot org> - <sumit dot semwal at ti dot com> - -This document serves as a guide to device-driver writers on what is the dma-buf -buffer sharing API, how to use it for exporting and using shared buffers. - -Any device driver which wishes to be a part of DMA buffer sharing, can do so as -either the 'exporter' of buffers, or the 'user' of buffers. - -Say a driver A wants to use buffers created by driver B, then we call B as the -exporter, and A as buffer-user. - -The exporter -- implements and manages operations[1] for the buffer -- allows other users to share the buffer by using dma_buf sharing APIs, -- manages the details of buffer allocation, -- decides about the actual backing storage where this allocation happens, -- takes care of any migration of scatterlist - for all (shared) users of this - buffer, - -The buffer-user -- is one of (many) sharing users of the buffer. -- doesn't need to worry about how the buffer is allocated, or where. -- needs a mechanism to get access to the scatterlist that makes up this buffer - in memory, mapped into its own address space, so it can access the same area - of memory. - -dma-buf operations for device dma only --------------------------------------- - -The dma_buf buffer sharing API usage contains the following steps: - -1. Exporter announces that it wishes to export a buffer -2. Userspace gets the file descriptor associated with the exported buffer, and - passes it around to potential buffer-users based on use case -3. Each buffer-user 'connects' itself to the buffer -4. When needed, buffer-user requests access to the buffer from exporter -5. When finished with its use, the buffer-user notifies end-of-DMA to exporter -6. when buffer-user is done using this buffer completely, it 'disconnects' - itself from the buffer. - - -1. Exporter's announcement of buffer export - - The buffer exporter announces its wish to export a buffer. In this, it - connects its own private buffer data, provides implementation for operations - that can be performed on the exported dma_buf, and flags for the file - associated with this buffer. All these fields are filled in struct - dma_buf_export_info, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro. - - Interface: - DEFINE_DMA_BUF_EXPORT_INFO(exp_info) - struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info) - - If this succeeds, dma_buf_export allocates a dma_buf structure, and - returns a pointer to the same. It also associates an anonymous file with this - buffer, so it can be exported. On failure to allocate the dma_buf object, - it returns NULL. - - 'exp_name' in struct dma_buf_export_info is the name of exporter - to - facilitate information while debugging. It is set to KBUILD_MODNAME by - default, so exporters don't have to provide a specific name, if they don't - wish to. - - DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info, - zeroes it out and pre-populates exp_name in it. - - -2. Userspace gets a handle to pass around to potential buffer-users - - Userspace entity requests for a file-descriptor (fd) which is a handle to the - anonymous file associated with the buffer. It can then share the fd with other - drivers and/or processes. - - Interface: - int dma_buf_fd(struct dma_buf *dmabuf, int flags) - - This API installs an fd for the anonymous file associated with this buffer; - returns either 'fd', or error. - -3. Each buffer-user 'connects' itself to the buffer - - Each buffer-user now gets a reference to the buffer, using the fd passed to - it. - - Interface: - struct dma_buf *dma_buf_get(int fd) - - This API will return a reference to the dma_buf, and increment refcount for - it. - - After this, the buffer-user needs to attach its device with the buffer, which - helps the exporter to know of device buffer constraints. - - Interface: - struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, - struct device *dev) - - This API returns reference to an attachment structure, which is then used - for scatterlist operations. It will optionally call the 'attach' dma_buf - operation, if provided by the exporter. - - The dma-buf sharing framework does the bookkeeping bits related to managing - the list of all attachments to a buffer. - -Until this stage, the buffer-exporter has the option to choose not to actually -allocate the backing storage for this buffer, but wait for the first buffer-user -to request use of buffer for allocation. - - -4. When needed, buffer-user requests access to the buffer - - Whenever a buffer-user wants to use the buffer for any DMA, it asks for - access to the buffer using dma_buf_map_attachment API. At least one attach to - the buffer must have happened before map_dma_buf can be called. - - Interface: - struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, - enum dma_data_direction); - - This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the - "dma_buf->ops->" indirection from the users of this interface. - - In struct dma_buf_ops, map_dma_buf is defined as - struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, - enum dma_data_direction); - - It is one of the buffer operations that must be implemented by the exporter. - It should return the sg_table containing scatterlist for this buffer, mapped - into caller's address space. - - If this is being called for the first time, the exporter can now choose to - scan through the list of attachments for this buffer, collate the requirements - of the attached devices, and choose an appropriate backing storage for the - buffer. - - Based on enum dma_data_direction, it might be possible to have multiple users - accessing at the same time (for reading, maybe), or any other kind of sharing - that the exporter might wish to make available to buffer-users. - - map_dma_buf() operation can return -EINTR if it is interrupted by a signal. - - -5. When finished, the buffer-user notifies end-of-DMA to exporter - - Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to - the exporter using the dma_buf_unmap_attachment API. - - Interface: - void dma_buf_unmap_attachment(struct dma_buf_attachment *, - struct sg_table *); - - This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the - "dma_buf->ops->" indirection from the users of this interface. - - In struct dma_buf_ops, unmap_dma_buf is defined as - void (*unmap_dma_buf)(struct dma_buf_attachment *, - struct sg_table *, - enum dma_data_direction); - - unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like - map_dma_buf, this API also must be implemented by the exporter. - - -6. when buffer-user is done using this buffer, it 'disconnects' itself from the - buffer. - - After the buffer-user has no more interest in using this buffer, it should - disconnect itself from the buffer: - - - it first detaches itself from the buffer. - - Interface: - void dma_buf_detach(struct dma_buf *dmabuf, - struct dma_buf_attachment *dmabuf_attach); - - This API removes the attachment from the list in dmabuf, and optionally calls - dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits. - - - Then, the buffer-user returns the buffer reference to exporter. - - Interface: - void dma_buf_put(struct dma_buf *dmabuf); - - This API then reduces the refcount for this buffer. - - If, as a result of this call, the refcount becomes 0, the 'release' file - operation related to this fd is called. It calls the dmabuf->ops->release() - operation in turn, and frees the memory allocated for dmabuf when exported. - -NOTES: -- Importance of attach-detach and {map,unmap}_dma_buf operation pairs - The attach-detach calls allow the exporter to figure out backing-storage - constraints for the currently-interested devices. This allows preferential - allocation, and/or migration of pages across different types of storage - available, if possible. - - Bracketing of DMA access with {map,unmap}_dma_buf operations is essential - to allow just-in-time backing of storage, and migration mid-way through a - use-case. - -- Migration of backing storage if needed - If after - - at least one map_dma_buf has happened, - - and the backing storage has been allocated for this buffer, - another new buffer-user intends to attach itself to this buffer, it might - be allowed, if possible for the exporter. - - In case it is allowed by the exporter: - if the new buffer-user has stricter 'backing-storage constraints', and the - exporter can handle these constraints, the exporter can just stall on the - map_dma_buf until all outstanding access is completed (as signalled by - unmap_dma_buf). - Once all users have finished accessing and have unmapped this buffer, the - exporter could potentially move the buffer to the stricter backing-storage, - and then allow further {map,unmap}_dma_buf operations from any buffer-user - from the migrated backing-storage. - - If the exporter cannot fulfill the backing-storage constraints of the new - buffer-user device as requested, dma_buf_attach() would return an error to - denote non-compatibility of the new buffer-sharing request with the current - buffer. - - If the exporter chooses not to allow an attach() operation once a - map_dma_buf() API has been called, it simply returns an error. - -Kernel cpu access to a dma-buf buffer object --------------------------------------------- - -The motivation to allow cpu access from the kernel to a dma-buf object from the -importers side are: -- fallback operations, e.g. if the devices is connected to a usb bus and the - kernel needs to shuffle the data around first before sending it away. -- full transparency for existing users on the importer side, i.e. userspace - should not notice the difference between a normal object from that subsystem - and an imported one backed by a dma-buf. This is really important for drm - opengl drivers that expect to still use all the existing upload/download - paths. - -Access to a dma_buf from the kernel context involves three steps: - -1. Prepare access, which invalidate any necessary caches and make the object - available for cpu access. -2. Access the object page-by-page with the dma_buf map apis -3. Finish access, which will flush any necessary cpu caches and free reserved - resources. - -1. Prepare access - - Before an importer can access a dma_buf object with the cpu from the kernel - context, it needs to notify the exporter of the access that is about to - happen. - - Interface: - int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, - enum dma_data_direction direction) - - This allows the exporter to ensure that the memory is actually available for - cpu access - the exporter might need to allocate or swap-in and pin the - backing storage. The exporter also needs to ensure that cpu access is - coherent for the access direction. The direction can be used by the exporter - to optimize the cache flushing, i.e. access with a different direction (read - instead of write) might return stale or even bogus data (e.g. when the - exporter needs to copy the data to temporary storage). - - This step might fail, e.g. in oom conditions. - -2. Accessing the buffer - - To support dma_buf objects residing in highmem cpu access is page-based using - an api similar to kmap. Accessing a dma_buf is done in aligned chunks of - PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns - a pointer in kernel virtual address space. Afterwards the chunk needs to be - unmapped again. There is no limit on how often a given chunk can be mapped - and unmapped, i.e. the importer does not need to call begin_cpu_access again - before mapping the same chunk again. - - Interfaces: - void *dma_buf_kmap(struct dma_buf *, unsigned long); - void dma_buf_kunmap(struct dma_buf *, unsigned long, void *); - - There are also atomic variants of these interfaces. Like for kmap they - facilitate non-blocking fast-paths. Neither the importer nor the exporter (in - the callback) is allowed to block when using these. - - Interfaces: - void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long); - void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *); - - For importers all the restrictions of using kmap apply, like the limited - supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2 - atomic dma_buf kmaps at the same time (in any given process context). - - dma_buf kmap calls outside of the range specified in begin_cpu_access are - undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on - the partial chunks at the beginning and end but may return stale or bogus - data outside of the range (in these partial chunks). - - Note that these calls need to always succeed. The exporter needs to complete - any preparations that might fail in begin_cpu_access. - - For some cases the overhead of kmap can be too high, a vmap interface - is introduced. This interface should be used very carefully, as vmalloc - space is a limited resources on many architectures. - - Interfaces: - void *dma_buf_vmap(struct dma_buf *dmabuf) - void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr) - - The vmap call can fail if there is no vmap support in the exporter, or if it - runs out of vmalloc space. Fallback to kmap should be implemented. Note that - the dma-buf layer keeps a reference count for all vmap access and calls down - into the exporter's vmap function only when no vmapping exists, and only - unmaps it once. Protection against concurrent vmap/vunmap calls is provided - by taking the dma_buf->lock mutex. - -3. Finish access - - When the importer is done accessing the CPU, it needs to announce this to - the exporter (to facilitate cache flushing and unpinning of any pinned - resources). The result of any dma_buf kmap calls after end_cpu_access is - undefined. - - Interface: - void dma_buf_end_cpu_access(struct dma_buf *dma_buf, - enum dma_data_direction dir); - - -Direct Userspace Access/mmap Support ------------------------------------- - -Being able to mmap an export dma-buf buffer object has 2 main use-cases: -- CPU fallback processing in a pipeline and -- supporting existing mmap interfaces in importers. - -1. CPU fallback processing in a pipeline - - In many processing pipelines it is sometimes required that the cpu can access - the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid - the need to handle this specially in userspace frameworks for buffer sharing - it's ideal if the dma_buf fd itself can be used to access the backing storage - from userspace using mmap. - - Furthermore Android's ION framework already supports this (and is otherwise - rather similar to dma-buf from a userspace consumer side with using fds as - handles, too). So it's beneficial to support this in a similar fashion on - dma-buf to have a good transition path for existing Android userspace. - - No special interfaces, userspace simply calls mmap on the dma-buf fd, making - sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always* - used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with - -EAGAIN or -EINTR, in which case it must be restarted. - - Some systems might need some sort of cache coherency management e.g. when - CPU and GPU domains are being accessed through dma-buf at the same time. To - circumvent this problem there are begin/end coherency markers, that forward - directly to existing dma-buf device drivers vfunc hooks. Userspace can make - use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence - would be used like following: - - mmap dma-buf fd - - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write - to mmap area 3. SYNC_END ioctl. This can be repeated as often as you - want (with the new data being consumed by the GPU or say scanout device) - - munmap once you don't need the buffer any more - - For correctness and optimal performance, it is always required to use - SYNC_START and SYNC_END before and after, respectively, when accessing the - mapped address. Userspace cannot rely on coherent access, even when there - are systems where it just works without calling these ioctls. - -2. Supporting existing mmap interfaces in importers - - Similar to the motivation for kernel cpu access it is again important that - the userspace code of a given importing subsystem can use the same interfaces - with a imported dma-buf buffer object as with a native buffer object. This is - especially important for drm where the userspace part of contemporary OpenGL, - X, and other drivers is huge, and reworking them to use a different way to - mmap a buffer rather invasive. - - The assumption in the current dma-buf interfaces is that redirecting the - initial mmap is all that's needed. A survey of some of the existing - subsystems shows that no driver seems to do any nefarious thing like syncing - up with outstanding asynchronous processing on the device or allocating - special resources at fault time. So hopefully this is good enough, since - adding interfaces to intercept pagefaults and allow pte shootdowns would - increase the complexity quite a bit. - - Interface: - int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *, - unsigned long); - - If the importing subsystem simply provides a special-purpose mmap call to set - up a mapping in userspace, calling do_mmap with dma_buf->file will equally - achieve that for a dma-buf object. - -3. Implementation notes for exporters - - Because dma-buf buffers have invariant size over their lifetime, the dma-buf - core checks whether a vma is too large and rejects such mappings. The - exporter hence does not need to duplicate this check. - - Because existing importing subsystems might presume coherent mappings for - userspace, the exporter needs to set up a coherent mapping. If that's not - possible, it needs to fake coherency by manually shooting down ptes when - leaving the cpu domain and flushing caches at fault time. Note that all the - dma_buf files share the same anon inode, hence the exporter needs to replace - the dma_buf file stored in vma->vm_file with it's own if pte shootdown is - required. This is because the kernel uses the underlying inode's address_space - for vma tracking (and hence pte tracking at shootdown time with - unmap_mapping_range). - - If the above shootdown dance turns out to be too expensive in certain - scenarios, we can extend dma-buf with a more explicit cache tracking scheme - for userspace mappings. But the current assumption is that using mmap is - always a slower path, so some inefficiencies should be acceptable. - - Exporters that shoot down mappings (for any reasons) shall not do any - synchronization at fault time with outstanding device operations. - Synchronization is an orthogonal issue to sharing the backing storage of a - buffer and hence should not be handled by dma-buf itself. This is explicitly - mentioned here because many people seem to want something like this, but if - different exporters handle this differently, buffer sharing can fail in - interesting ways depending upong the exporter (if userspace starts depending - upon this implicit synchronization). - -Other Interfaces Exposed to Userspace on the dma-buf FD ------------------------------------------------------- - -- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only - with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow - the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other - llseek operation will report -EINVAL. - - If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all - cases. Userspace can use this to detect support for discovering the dma-buf - size using llseek. - -Miscellaneous notes -------------------- - -- Any exporters or users of the dma-buf buffer sharing framework must have - a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. - -- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set - on the file descriptor. This is not just a resource leak, but a - potential security hole. It could give the newly exec'd application - access to buffers, via the leaked fd, to which it should otherwise - not be permitted access. - - The problem with doing this via a separate fcntl() call, versus doing it - atomically when the fd is created, is that this is inherently racy in a - multi-threaded app[3]. The issue is made worse when it is library code - opening/creating the file descriptor, as the application may not even be - aware of the fd's. - - To avoid this problem, userspace must have a way to request O_CLOEXEC - flag be set when the dma-buf fd is created. So any API provided by - the exporting driver to create a dmabuf fd must provide a way to let - userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd(). - -- If an exporter needs to manually flush caches and hence needs to fake - coherency for mmap support, it needs to be able to zap all the ptes pointing - at the backing storage. Now linux mm needs a struct address_space associated - with the struct file stored in vma->vm_file to do that with the function - unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd - with the anon_file struct file, i.e. all dma_bufs share the same file. - - Hence exporters need to setup their own file (and address_space) association - by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap - callback. In the specific case of a gem driver the exporter could use the - shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then - zap ptes by unmapping the corresponding range of the struct address_space - associated with their own file. - -References: -[1] struct dma_buf_ops in include/linux/dma-buf.h -[2] All interfaces mentioned above defined in include/linux/dma-buf.h -[3] https://lwn.net/Articles/236486/ diff --git a/Documentation/dma-buf/guide.rst b/Documentation/dma-buf/guide.rst new file mode 100644 index 000000000000..7407d255e9da --- /dev/null +++ b/Documentation/dma-buf/guide.rst @@ -0,0 +1,507 @@ + +.. _dma-buf-guide: + +============================ +DMA Buffer Sharing API Guide +============================ + +This document serves as a guide to device-driver writers on what is the dma-buf +buffer sharing API, how to use it for exporting and using shared buffers. + +Any device driver which wishes to be a part of DMA buffer sharing, can do so as +either the 'exporter' of buffers, or the 'user' of buffers. + +Say a driver A wants to use buffers created by driver B, then we call B as the +exporter, and A as buffer-user. + +The exporter + +* implements and manages operations for the buffer via + :c:type:`struct dma_buf_ops <dma_buf_ops>` +* allows other users to share the buffer by using dma_buf sharing APIs, +* manages the details of buffer allocation, +* decides about the actual backing storage where this allocation happens, +* takes care of any migration of scatterlist - for all (shared) users of this + buffer, + +The buffer-user + +* is one of (many) sharing users of the buffer. +* doesn't need to worry about how the buffer is allocated, or where. +* needs a mechanism to get access to the scatterlist that makes up this buffer + in memory, mapped into its own address space, so it can access the same area + of memory. + +dma-buf operations for device dma only +====================================== + +The dma_buf buffer sharing API usage contains the following steps: + +1. Exporter announces that it wishes to export a buffer +2. Userspace gets the file descriptor associated with the exported buffer, and + passes it around to potential buffer-users based on use case +3. Each buffer-user 'connects' itself to the buffer +4. When needed, buffer-user requests access to the buffer from exporter +5. When finished with its use, the buffer-user notifies end-of-DMA to exporter +6. when buffer-user is done using this buffer completely, it 'disconnects' + itself from the buffer. + + +Exporter's announcement of buffer export +---------------------------------------- + +The buffer exporter announces its wish to export a buffer. In this, it +connects its own private buffer data, provides implementation for operations +that can be performed on the exported :c:type:`dma_buf`, and flags for the file +associated with this buffer. All these fields are filled in struct +:c:type:`dma_buf_export_info`, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro. + +Interface: + :c:type:`DEFINE_DMA_BUF_EXPORT_INFO(exp_info) <DEFINE_DMA_BUF_EXPORT_INFO>` + + :c:func:`struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)<dma_buf_export>` + +If this succeeds, dma_buf_export allocates a dma_buf structure, and +returns a pointer to the same. It also associates an anonymous file with this +buffer, so it can be exported. On failure to allocate the dma_buf object, +it returns NULL. + +``exp_name`` in struct dma_buf_export_info is the name of exporter - to +facilitate information while debugging. It is set to ``KBUILD_MODNAME`` by +default, so exporters don't have to provide a specific name, if they don't +wish to. + +DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info, +zeroes it out and pre-populates exp_name in it. + +Userspace gets a handle to pass around to potential buffer-users +---------------------------------------------------------------- + +Userspace entity requests for a file-descriptor (fd) which is a handle to the +anonymous file associated with the buffer. It can then share the fd with other +drivers and/or processes. + +Interface: + :c:func:`int dma_buf_fd(struct dma_buf *dmabuf, int flags) <dma_buf_fd>` + +This API installs an fd for the anonymous file associated with this buffer; +returns either ``fd``, or error. + +Each buffer-user 'connects' itself to the buffer +------------------------------------------------ + +Each buffer-user now gets a reference to the buffer, using the fd passed to it. + +Interface: + :c:func:`struct dma_buf *dma_buf_get(int fd) <dma_buf_get>` + +This API will return a reference to the dma_buf, and increment refcount forit. + +After this, the buffer-user needs to attach its device with the buffer, which +helps the exporter to know of device buffer constraints. + +Interface: + :c:func:`struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev) <dma_buf_attach>` + +This API returns reference to an attachment structure, which is then used +for scatterlist operations. It will optionally call the 'attach' dma_buf +operation, if provided by the exporter. + +The dma-buf sharing framework does the bookkeeping bits related to managing +the list of all attachments to a buffer. + +.. note:: + + Until this stage, the buffer-exporter has the option to choose not to actually + allocate the backing storage for this buffer, but wait for the first + buffer-user to request use of buffer for allocation. + +When needed, buffer-user requests access to the buffer +------------------------------------------------------ + +Whenever a buffer-user wants to use the buffer for any DMA, it asks for +access to the buffer using dma_buf_map_attachment API. At least one attach to +the buffer must have happened before map_dma_buf can be called. + +Interface: + :c:func:`struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, enum dma_data_direction) <dma_buf_map_attachment>` + +This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the +``dma_buf->ops->`` indirection from the users of this interface. + +In struct :c:type:`dma_buf_ops`, ``map_dma_buf`` is defined as + ``struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, enum dma_data_direction);`` + +It is one of the buffer operations that must be implemented by the exporter. +It should return the sg_table containing scatterlist for this buffer, mapped +into caller's address space. + +If this is being called for the first time, the exporter can now choose to +scan through the list of attachments for this buffer, collate the requirements +of the attached devices, and choose an appropriate backing storage for the +buffer. + +Based on enum :c:type:`dma_data_direction`, it might be possible to have multiple users +accessing at the same time (for reading, maybe), or any other kind of sharing +that the exporter might wish to make available to buffer-users. + +``map_dma_buf()`` operation can return -EINTR if it is interrupted by a signal. + + +When finished, the buffer-user notifies end-of-DMA to exporter +-------------------------------------------------------------- + +Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to +the exporter using the dma_buf_unmap_attachment API. + +Interface: + :c:func:`void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *) <dma_buf_unmap_attachment>` + +This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the +"dma_buf->ops->" indirection from the users of this interface. + +In struct dma_buf_ops, unmap_dma_buf is defined as + ``void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *, enum dma_data_direction)`` + +``unmap_dma_buf()`` signifies the end-of-DMA for the attachment provided. Like +``map_dma_buf``, this API also must be implemented by the exporter. + + +when buffer-user is done using this buffer, it 'disconnects' itself from the buffer. +------------------------------------------------------------------------------------ + +After the buffer-user has no more interest in using this buffer, it should +disconnect itself from the buffer: + + * it first detaches itself from the buffer. + + Interface: + :c:func:`void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *dmabuf_attach) <dma_buf_detach>` + + This API removes the attachment from the list in dmabuf, and optionally calls + ``dma_buf->ops->detach()``, if provided by exporter, for any housekeeping bits. + + * Then, the buffer-user returns the buffer reference to exporter. + + Interface: + :c:func:`void dma_buf_put(struct dma_buf *dmabuf) <dma_buf_put>` + + This API then reduces the refcount for this buffer. + + If, as a result of this call, the refcount becomes 0, the 'release' file + operation related to this fd is called. It calls the + ``dmabuf->ops->release()`` operation in turn, and frees the memory allocated + for dmabuf when exported. + +NOTES +----- + +Importance of attach-detach and {map,unmap}_dma_buf operation pairs +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + + The attach-detach calls allow the exporter to figure out backing-storage + constraints for the currently-interested devices. This allows preferential + allocation, and/or migration of pages across different types of storage + available, if possible. + + Bracketing of DMA access with {map,unmap}_dma_buf operations is essential + to allow just-in-time backing of storage, and migration mid-way through a + use-case. + +Migration of backing storage if needed +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + + If after + + * at least one map_dma_buf has happened, + * and the backing storage has been allocated for this buffer, + + another new buffer-user intends to attach itself to this buffer, it might + be allowed, if possible for the exporter. + + In case it is allowed by the exporter: + + * if the new buffer-user has stricter 'backing-storage constraints', and the + exporter can handle these constraints, the exporter can just stall on the + map_dma_buf until all outstanding access is completed (as signalled by + unmap_dma_buf). + + * Once all users have finished accessing and have unmapped this buffer, the + exporter could potentially move the buffer to the stricter backing-storage, + and then allow further {map,unmap}_dma_buf operations from any buffer-user + from the migrated backing-storage. + + * If the exporter cannot fulfill the backing-storage constraints of the new + buffer-user device as requested, dma_buf_attach() would return an error to + denote non-compatibility of the new buffer-sharing request with the current + buffer. + + + If the exporter chooses not to allow an attach() operation once a + map_dma_buf() API has been called, it simply returns an error. + +Kernel cpu access to a dma-buf buffer object +============================================ + +The motivation to allow cpu access from the kernel to a dma-buf object from the +importers side are: + +* fallback operations, e.g. if the devices is connected to a usb bus and the + kernel needs to shuffle the data around first before sending it away. + +* full transparency for existing users on the importer side, i.e. userspace + should not notice the difference between a normal object from that subsystem + and an imported one backed by a dma-buf. This is really important for drm + opengl drivers that expect to still use all the existing upload/download + paths. + +Access to a dma_buf from the kernel context involves three steps: + +1. Prepare access, which invalidate any necessary caches and make the object + available for cpu access. + +2. Access the object page-by-page with the dma_buf map apis + +3. Finish access, which will flush any necessary cpu caches and free reserved + resources. + +Prepare access +-------------- + +Before an importer can access a dma_buf object with the cpu from the kernel +context, it needs to notify the exporter of the access that is about to +happen. + +Interface: + :c:func:`int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction) <dma_buf_begin_cpu_access>` + +This allows the exporter to ensure that the memory is actually available for +cpu access - the exporter might need to allocate or swap-in and pin the +backing storage. The exporter also needs to ensure that cpu access is +coherent for the access direction. The direction can be used by the exporter +to optimize the cache flushing, i.e. access with a different direction (read +instead of write) might return stale or even bogus data (e.g. when the +exporter needs to copy the data to temporary storage). + +This step might fail, e.g. in oom conditions. + +Access the buffer +----------------- + +To support dma_buf objects residing in highmem cpu access is page-based using +an api similar to kmap. Accessing a dma_buf is done in aligned chunks of +``PAGE_SIZE`` size. Before accessing a chunk it needs to be mapped, which returns +a pointer in kernel virtual address space. Afterwards the chunk needs to be +unmapped again. There is no limit on how often a given chunk can be mapped +and unmapped, i.e. the importer does not need to call ``begin_cpu_access()`` again +before mapping the same chunk again. + +Interfaces: + :c:func:`void *dma_buf_kmap(struct dma_buf *, unsigned long) <dma_buf_kmap>` + + :c:func:`void dma_buf_kunmap(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap>` + +There are also atomic variants of these interfaces. Like for kmap they +facilitate non-blocking fast-paths. Neither the importer nor the exporter (in +the callback) is allowed to block when using these. + +Interfaces: + :c:func:`void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long) <dma_buf_kmap_atomic>` + + :c:func:`void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap_atomic>` + +For importers all the restrictions of using kmap apply, like the limited +supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2 +atomic dma_buf kmaps at the same time (in any given process context). + +``dma_buf kmap`` calls outside of the range specified in ``begin_cpu_access`` are +undefined. If the range is not ``PAGE_SIZE`` aligned, kmap needs to succeed on +the partial chunks at the beginning and end but may return stale or bogus +data outside of the range (in these partial chunks). + +Note that these calls need to always succeed. The exporter needs to complete +any preparations that might fail in ``begin_cpu_access()``. + +For some cases the overhead of kmap can be too high, a vmap interface +is introduced. This interface should be used very carefully, as vmalloc +space is a limited resources on many architectures. + +Interfaces: + :c:func:`void *dma_buf_vmap(struct dma_buf *dmabuf) <dma_buf_vmap>` + + :c:func:`void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr) <dma_buf_vunmap>` + +The vmap call can fail if there is no vmap support in the exporter, or if it +runs out of vmalloc space. Fallback to kmap should be implemented. Note that +the dma-buf layer keeps a reference count for all vmap access and calls down +into the exporter's vmap function only when no vmapping exists, and only +unmaps it once. Protection against concurrent vmap/vunmap calls is provided +by taking the ``dma_buf->lock`` mutex. + +Finish access +------------- + +When the importer is done accessing the CPU, it needs to announce this to +the exporter (to facilitate cache flushing and unpinning of any pinned +resources). The result of any dma_buf kmap calls after end_cpu_access is +undefined. + +Interface: + :c:func:`void dma_buf_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direction dir) <dma_buf_end_cpu_access>` + + +Direct Userspace Access/mmap Support +==================================== + +Being able to mmap an export dma-buf buffer object has 2 main use-cases: + +* CPU fallback processing in a pipeline and +* supporting existing mmap interfaces in importers. + +CPU fallback processing in a pipeline +------------------------------------- + +In many processing pipelines it is sometimes required that the cpu can access +the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid +the need to handle this specially in userspace frameworks for buffer sharing +it's ideal if the dma_buf ``fd`` itself can be used to access the backing storage +from userspace using mmap. + +Furthermore Android's ION framework already supports this (and is otherwise +rather similar to dma-buf from a userspace consumer side with using fds as +handles, too). So it's beneficial to support this in a similar fashion on +dma-buf to have a good transition path for existing Android userspace. + +No special interfaces, userspace simply calls mmap on the dma-buf fd, making +sure that the cache synchronization ioctl (``DMA_BUF_IOCTL_SYNC``) is *always* +used when the access happens. Note that ``DMA_BUF_IOCTL_SYNC`` can fail with +-EAGAIN or -EINTR, in which case it must be restarted. + +Some systems might need some sort of cache coherency management e.g. when +CPU and GPU domains are being accessed through dma-buf at the same time. To +circumvent this problem there are begin/end coherency markers, that forward +directly to existing dma-buf device drivers vfunc hooks. Userspace can make +use of those markers through the ``DMA_BUF_IOCTL_SYNC`` ioctl. The sequence +would be used like following: + + * mmap dma-buf fd + * for each drawing/upload cycle in CPU + + 1. SYNC_START ioctl, + 2. read/write to mmap area + 3. SYNC_END ioctl. + + This can be repeated as often as you want (with the new data being + consumed by the GPU or say scanout device) + + * munmap once you don't need the buffer any more + +For correctness and optimal performance, it is always required to use +``SYNC_START`` and ``SYNC_END`` before and after, respectively, when accessing the +mapped address. Userspace cannot rely on coherent access, even when there +are systems where it just works without calling these ioctls. + +Supporting existing mmap interfaces in importers +------------------------------------------------ + +Similar to the motivation for kernel cpu access it is again important that +the userspace code of a given importing subsystem can use the same interfaces +with a imported dma-buf buffer object as with a native buffer object. This is +especially important for drm where the userspace part of contemporary OpenGL, +X, and other drivers is huge, and reworking them to use a different way to +mmap a buffer rather invasive. + +The assumption in the current dma-buf interfaces is that redirecting the +initial mmap is all that's needed. A survey of some of the existing +subsystems shows that no driver seems to do any nefarious thing like syncing +up with outstanding asynchronous processing on the device or allocating +special resources at fault time. So hopefully this is good enough, since +adding interfaces to intercept pagefaults and allow pte shootdowns would +increase the complexity quite a bit. + +Interface: + :c:func:`int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *, unsigned long) <dma_buf_mmap>` + +If the importing subsystem simply provides a special-purpose mmap call to set +up a mapping in userspace, calling ``do_mmap`` with ``dma_buf->file`` will equally +achieve that for a dma-buf object. + +Implementation notes for exporters +---------------------------------- + +Because dma-buf buffers have invariant size over their lifetime, the dma-buf +core checks whether a vma is too large and rejects such mappings. The +exporter hence does not need to duplicate this check. + +Because existing importing subsystems might presume coherent mappings for +userspace, the exporter needs to set up a coherent mapping. If that's not +possible, it needs to fake coherency by manually shooting down ptes when +leaving the cpu domain and flushing caches at fault time. Note that all the +``dma_buf`` files share the same anon inode, hence the exporter needs to replace +the ``dma_buf`` file stored in ``vma->vm_file`` with it's own if pte shootdown is +required. This is because the kernel uses the underlying inode's address_space +for vma tracking (and hence pte tracking at shootdown time with +``unmap_mapping_range``). + +If the above shootdown dance turns out to be too expensive in certain +scenarios, we can extend dma-buf with a more explicit cache tracking scheme +for userspace mappings. But the current assumption is that using mmap is +always a slower path, so some inefficiencies should be acceptable. + +Exporters that shoot down mappings (for any reasons) shall not do any +synchronization at fault time with outstanding device operations. +Synchronization is an orthogonal issue to sharing the backing storage of a +buffer and hence should not be handled by dma-buf itself. This is explicitly +mentioned here because many people seem to want something like this, but if +different exporters handle this differently, buffer sharing can fail in +interesting ways depending upong the exporter (if userspace starts depending +upon this implicit synchronization). + +Other Interfaces Exposed to Userspace on the dma-buf FD +------------------------------------------------------- + +* Since kernel 3.12 the dma-buf FD supports the llseek system call, but only + with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow + the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other + llseek operation will report -EINVAL. + + If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all + cases. Userspace can use this to detect support for discovering the dma-buf + size using llseek. + +Miscellaneous notes +------------------- + +* Any exporters or users of the dma-buf buffer sharing framework must have + a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. + +* In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set + on the file descriptor. This is not just a resource leak, but a + potential security hole. It could give the newly exec'd application + access to buffers, via the leaked fd, to which it should otherwise + not be permitted access. + +The problem with doing this via a separate fcntl() call, versus doing it +atomically when the fd is created, is that this is inherently racy in a +multi-threaded app (See `here https://lwn.net/Articles/236486/`_). The issue +is made worse when it is library code opening/creating the file descriptor, +as the application may not even be aware of the fd's. + +To avoid this problem, userspace must have a way to request ``O_CLOEXEC`` +flag be set when the dma-buf fd is created. So any API provided by +the exporting driver to create a dmabuf fd must provide a way to let +userspace control setting of ``O_CLOEXEC`` flag passed in to ``dma_buf_fd()``. + +* If an exporter needs to manually flush caches and hence needs to fake + coherency for mmap support, it needs to be able to zap all the ptes pointing + at the backing storage. Now linux mm needs a struct address_space associated + with the struct file stored in ``vma->vm_file`` to do that with the function + unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd + with the ``anon_file`` struct file, i.e. all dma_bufs share the same file. + +Hence exporters need to setup their own file (and address_space) association +by setting ``vma->vm_file`` and adjusting ``vma->vm_pgoff`` in the ``dma_buf`` mmap +callback. In the specific case of a gem driver the exporter could use the +shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then +zap ptes by unmapping the corresponding range of the struct address_space +associated with their own file. diff --git a/Documentation/dma-buf/intro.rst b/Documentation/dma-buf/intro.rst new file mode 100644 index 000000000000..4ecdbea50e75 --- /dev/null +++ b/Documentation/dma-buf/intro.rst @@ -0,0 +1,82 @@ +================================== +Buffer Sharing and Synchronization +================================== + + +Introduction +------------ + +The dma-buf subsystem provides the framework for sharing buffers for +hardware (DMA) access across multiple device drivers and subsystems, and +for synchronizing asynchronous hardware access. + +This is used, for example, by drm "prime" multi-GPU support, but is of +course not limited to GPU use cases. + +The three main components of this are: + +* dma-buf_: represents an sg_table, and is exposed to userspace as a file + descriptor to allow passing between devices, + +* fence_: which provides a mechanism to signal when one device has finished + access, and + +* reservation_: manages the shared or exclusive fence(s) associated with the + buffer. + +Please refer to :ref:`dma-buf-guide` for more details. + +.. _dma-buf: + +dma-buf +~~~~~~~ + +.. kernel-doc:: drivers/dma-buf/dma-buf.c + :export: + +.. kernel-doc:: include/linux/dma-buf.h + :internal: + +.. _fence: + +fence +~~~~~ + +.. kernel-doc:: drivers/dma-buf/fence.c + :export: + +.. kernel-doc:: include/linux/fence.h + :internal: + +.. kernel-doc:: drivers/dma-buf/fence-array.c + :export: + +.. kernel-doc:: include/linux/fence-array.h + :internal: + +.. kernel-doc:: drivers/dma-buf/seqno-fence.c + :export: + +.. kernel-doc:: include/linux/seqno-fence.h + :internal: + +.. kernel-doc:: drivers/dma-buf/sync_file.c + :export: + +.. kernel-doc:: include/linux/sync_file.h + :internal: + +.. _reservation: + +reservation +~~~~~~~~~~~ + +.. kernel-doc:: drivers/dma-buf/reservation.c + :doc: Reservation Object Overview + +.. kernel-doc:: drivers/dma-buf/reservation.c + :export: + +.. kernel-doc:: include/linux/reservation.h + :internal: + diff --git a/MAINTAINERS b/MAINTAINERS index 0bbe4b105c34..dd422c6e54fe 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -3838,7 +3838,7 @@ F: drivers/dma-buf/ F: include/linux/dma-buf* F: include/linux/reservation.h F: include/linux/*fence.h -F: Documentation/dma-buf-sharing.txt +F: Documentation/dma-buf/ T: git git://git.linaro.org/people/sumitsemwal/linux-dma-buf.git

SYNC FILE FRAMEWORK