[Linaro-mm-sig] [RFC] dma-shared-buf: Add buffer sharing framework

Hans Verkuil hverkuil at xs4all.nl
Fri Aug 26 09:06:38 UTC 2011


On Wednesday, August 10, 2011 14:11:04 Sumit Semwal wrote:
> This RFC is aimed at introducing the buffer sharing framework for review.
> 
> Since almost all the discussion about buffer sharing objects happened on
> linaro-mm-sig list, I am sending it first for review within the list, and
> will share it with other lists after first-set of review comments from here.
> 
> --
> 
> This is the first step in defining a buffer sharing framework.
> A new buffer object is added, with hooks to allow for easy sharing of
> this buffer object across devices.
> 
> The idea was first mooted at the Linaro memory management mini-summit in
> Budapest in May 2011, as part of multiple things needed for a 'unified memory
> management framework'. It took a more concrete shape at Linaro memory-management
> mini-summit in Cambridge, Aug 2011.
> 
> The framework allows:
> - a new buffer-object to be created, which associates a file pointer with each
>    user-buffer and associated allocator-defined operations on that buffer.
>    This operation is called the 'export' operation.
> - this exported buffer-object to be shared with the other entity by asking for
>    its 'file-descriptor (fd)', and sharing the fd across.
> - a received fd to get the buffer object back, where it can be accessed using
>    the associated allocator-defined operations.
> - the exporter and importer to share the scatterlist using get_ and put_
>    operations.
> 
> Some file operations are provided as wrappers over the allocator-defined
> operations, which allows usage of fops(eg mmap) on the associated 'fd'.
> 
> This is based on design suggestions from many people at both the mini-summits,
> most notably from Arnd Bergmann <arnd at arndb.de>, and Rob Clark <rob at ti.com>.
> 
> The implementation is inspired from proof-of-concept patch-set from
> Tomasz Stanislawski <t.stanislaws at samsung.com>, who demonstrated buffer sharing
> between two v4l2 devices.
> 
> Signed-off-by: Sumit Semwal <sumit.semwal at ti.com>
> ---
>  drivers/base/Kconfig    |   10 +++
>  drivers/base/Makefile   |    1 +
>  drivers/base/dma-buf.c  |  196 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h |  105 +++++++++++++++++++++++++
>  4 files changed, 312 insertions(+), 0 deletions(-)
>  create mode 100644 drivers/base/dma-buf.c
>  create mode 100644 include/linux/dma-buf.h
> 
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index d57e8d0..5398ce8 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -168,4 +168,14 @@ config SYS_HYPERVISOR
>  	bool
>  	default n
>  
> +config DMA_SHARED_BUFFER
> +	bool "Buffer framework to be shared between drivers"
> +	default n
> +	depends on ANON_INODES
> +	help
> +	  This option enables the framework for buffer-sharing between
> +	  multiple drivers. A buffer is associated with a file using driver
> +	  APIs extension; the file's descriptor can then be passed on to other
> +	  driver.
> +
>  endmenu
> diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> index 4c5701c..bd95732 100644
> --- a/drivers/base/Makefile
> +++ b/drivers/base/Makefile
> @@ -8,6 +8,7 @@ obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
>  obj-y			+= power/
>  obj-$(CONFIG_HAS_DMA)	+= dma-mapping.o
>  obj-$(CONFIG_HAVE_GENERIC_DMA_COHERENT) += dma-coherent.o
> +obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf.o
>  obj-$(CONFIG_ISA)	+= isa.o
>  obj-$(CONFIG_FW_LOADER)	+= firmware_class.o
>  obj-$(CONFIG_NUMA)	+= node.o
> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> new file mode 100644
> index 0000000..e35b385
> --- /dev/null
> +++ b/drivers/base/dma-buf.c
> @@ -0,0 +1,196 @@
> +/*
> + * Framework for buffer objects that can be shared across devices/subsystems.
> + *
> + * Copyright(C) 2011 Texas Instruments Inc. All rights reserved.
> + * Author: Sumit Semwal <sumit.semwal at ti.com>
> + *
> + * Many thanks to linaro-mm-sig list, and specially
> + * Arnd Bergmann <arnd at arndb.de> and Rob Clark <rob at ti.com>
> + * for their support in creation and refining of this idea.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along with
> + * this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/fs.h>
> +#include <linux/slab.h>
> +#include <linux/dma-buf.h>
> +#include <linux/anon_inodes.h>
> +
> +static int is_dma_buf_file(struct file *);
> +
> +/* file operation wrappers for dma buf ops */
> +static ssize_t dma_buf_read(struct file *file, char __user *buf, size_t size,
> +				loff_t *offset)
> +{
> +	struct dma_buf *dmabuf;
> +
> +	if (!is_dma_buf_file(file))
> +		return -EINVAL;
> +
> +	dmabuf = file->private_data;
> +	return dmabuf->ops->read(dmabuf, buf, size);

The 'read' ops should be optional. Not all drivers will support this.

> +}
> +
> +static ssize_t dma_buf_write(struct file *file, char __user *buf, size_t size,
> +				loff_t *offset)
> +{
> +	struct dma_buf *dmabuf;
> +
> +	if (!is_dma_buf_file(file))
> +		return -EINVAL;
> +
> +	dmabuf = file->private_data;
> +	return dmabuf->ops->write(dmabuf, buf, size);

Ditto for write...

> +}
> +
> +static int dma_buf_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct dma_buf *dmabuf;
> +
> +	if (!is_dma_buf_file(file))
> +		return -EINVAL;
> +
> +	dmabuf = file->private_data;
> +	return dmabuf->ops->mmap(dmabuf, vma);

...and mmap.

I would expect to see an ioctl op as well, if only to pass the size of the
buffer back to userspace.

And if we support read/write, then we need a seek as well.

Frankly, I'm not sure about the read and write. I don't really see the use
case. The whole point of buffer sharing is to avoid copying buffers, and that's
exactly what read and write do.

> +}
> +
> +static int dma_buf_release(struct inode *inode, struct file *file)
> +{
> +	struct dma_buf *dmabuf;
> +
> +	if (!is_dma_buf_file(file))
> +		return -EINVAL;
> +
> +	dmabuf = file->private_data;
> +
> +	dmabuf->ops->release(dmabuf);
> +	kfree(dmabuf);
> +	return 0;
> +}
> +
> +static const struct file_operations dma_buf_fops = {
> +	.mmap		= dma_buf_mmap,
> +	.read		= dma_buf_read,
> +	.write		= dma_buf_write,
> +	.release	= dma_buf_release,
> +};
> +
> +/*
> + * is_dma_buf_file - Check if struct file* is associated with dma_buf
> + */
> +static inline int is_dma_buf_file(struct file *file)
> +{
> +	return file->f_op == &dma_buf_fops;
> +}
> +
> +/**
> + * dma_buf_export - Creates a new dma_buf, and associates an anon file
> + * with this buffer,so it can be exported.
> + * Also 'attach' the allocator specific data and ops to the buffer.
> + *
> + * @priv:	Attach private data of allocator to this buffer
> + * @ops:	Attach allocator-defined dma buf ops to the new buffer.
> + *
> + * Returns, on success, a newly created dma_buf object, which wraps the
> + * supplied private data and operations for dma_buf_ops. On failure to
> + * allocate the dma_buf object, it can return NULL.
> + *
> + */
> +struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops)
> +{
> +	struct dma_buf *dmabuf;
> +	struct file *file;
> +
> +	BUG_ON(!priv || !ops);
> +
> +	dmabuf = kzalloc(sizeof(struct dma_buf), GFP_KERNEL);
> +	if (dmabuf == NULL)
> +		return dmabuf;
> +
> +	dmabuf->priv = priv;
> +	dmabuf->ops = ops;
> +
> +	file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, 0);
> +
> +	dmabuf->file = file;
> +	file->private_data = dmabuf;
> +
> +	return dmabuf;
> +}
> +EXPORT_SYMBOL(dma_buf_export);
> +
> +/**
> + * dma_buf_fd - returns a file descriptor for the given dma_buf
> + * @dmabuf:	[in]	pointer to dma_buf for which fd is required.
> + *
> + * On success, returns an associated 'fd'. Else, returns error.
> + */
> +int dma_buf_fd(struct dma_buf *dmabuf)
> +{
> +	int error, fd;
> +
> +	if (!dmabuf->file)
> +		return -EINVAL;
> +
> +	error = get_unused_fd_flags(0);
> +	if (error < 0)
> +		return error;
> +	fd = error;
> +
> +	fd_install(fd, dmabuf->file);
> +
> +	return fd;
> +}
> +EXPORT_SYMBOL(dma_buf_fd);
> +
> +/**
> + * dma_buf_get - returns the dma_buf structure related to an fd
> + * @fd:	[in]	fd associated with the dma_buf to be returned
> + *
> + * On success, returns the dma_buf structure associated with an fd; uses
> + * file's refcounting done by fget to increase refcount. returns ERR_PTR
> + * otherwise.
> + */
> +struct dma_buf *dma_buf_get(int fd)
> +{
> +	struct file *file;
> +
> +	file = fget(fd);
> +
> +	if (!file)
> +		return ERR_PTR(-EBADF);
> +
> +	if (!is_dma_buf_file(file)) {
> +		fput(file);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	return file->private_data;
> +}
> +EXPORT_SYMBOL(dma_buf_get);
> +
> +/**
> + * dma_buf_put - decreases refcount of the buffer
> + * @dmabuf:	[in]	buffer to reduce refcount of
> + *
> + * Uses file's refcounting done implicitly by fput()
> + */
> +void dma_buf_put(struct dma_buf *dmabuf)
> +{
> +	BUG_ON(!dmabuf->file);
> +
> +	fput(dmabuf->file);
> +
> +	return;
> +}
> +EXPORT_SYMBOL(dma_buf_put);
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> new file mode 100644
> index 0000000..c22896a
> --- /dev/null
> +++ b/include/linux/dma-buf.h
> @@ -0,0 +1,105 @@
> +/*
> + * Header file for dma buffer sharing framework.
> + *
> + * Copyright(C) 2011 Texas Instruments Inc. All rights reserved.
> + * Author: Sumit Semwal <sumit.semwal at ti.com>
> + *
> + * Many thanks to linaro-mm-sig list, and specially
> + * Arnd Bergmann <arnd at arndb.de> and Rob Clark <rob at ti.com>
> + * for their support in creation and refining of this idea.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along with
> + * this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#ifndef __DMA_BUF_H__
> +#define __DMA_BUF_H__
> +
> +#include <linux/file.h>
> +#include <linux/err.h>
> +#include <linux/device.h>
> +#include <linux/scatterlist.h>
> +
> +struct dma_buf;
> +
> +/**
> + * struct dma_buf_ops - operations possible on struct dmabuf
> + * @get_scatterlist: returns list of scatter pages allocated, increases
> + *		     usecount of the buffer
> + * @put_scatterlist: decreases usecount of buffer, might deallocate scatter
> + *		     pages
> + * @mmap: map this buffer
> + * @read: read from this buffer
> + * @write: write to this buffer
> + * @release: release this buffer; to be called after the last dma_buf_put
> + * @sync_sg_for_cpu: sync the sg list for cpu
> + * @sync_sg_for_device: synch the sg list for device
> + */
> +struct dma_buf_ops {
> +	/* allow buffer to not be pinned when DMA is not happening */
> +	struct scatterlist * (*get_scatterlist)(struct dma_buf *);
> +	void (*put_scatterlist)(struct dma_buf *, struct scatterlist *);
> +
> +	/* allow allocator to mmap/read/write to take care of cache attrib */
> +	int (*mmap)(struct dma_buf *, struct vm_area_struct *);
> +	ssize_t (*read)(struct dma_buf *, void *, size_t);
> +	ssize_t (*write)(struct dma_buf *, void *, size_t);
> +	/* after final dma_buf_put() */
> +	void (*release)(struct dma_buf *);
> +
> +	/* allow allocator to take care of cache ops */
> +	void (*sync_sg_for_cpu) (struct dma_buf *, struct device *);
> +	void (*sync_sg_for_device)(struct dma_buf *, struct device *);
> +};
> +
> +/**
> + * struct dma_buf - shared buffer object
> + * @file: file pointer used for sharing buffers across, and for refcounting.
> + * @ops: dma_buf_ops associated with this buffer object
> + * @priv: user specific private data
> + */
> +struct dma_buf {
> +	struct file *file;
> +	struct dma_buf_ops *ops;
> +	void *priv;
> +};
> +
> +#ifdef CONFIG_DMA_SHARED_BUFFER
> +
> +struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops);
> +int dma_buf_fd(struct dma_buf *dmabuf);
> +struct dma_buf *dma_buf_get(int fd);
> +void dma_buf_put(struct dma_buf *dmabuf);
> +
> +#else
> +static inline struct dma_buf *dma_buf_export(void *priv,
> +						struct dma_buf_ops *ops)
> +{
> +	return ERR_PTR(-ENODEV);
> +}
> +
> +static inline int dma_buf_fd(struct dma_buf *dmabuf)
> +{
> +	return -ENODEV;
> +}
> +
> +static inline struct dma_buf *dma_buf_get(int fd)
> +{
> +	return ERR_PTR(-ENODEV);
> +}
> +
> +static inline void dma_buf_put(struct dma_buf *dmabuf)
> +{
> +	return;
> +}
> +#endif /* CONFIG_DMA_SHARED_BUFFER */
> +
> +#endif /* __DMA_BUF_H__ */
> 

Regards,

	Hans



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