On Mon, Feb 07, 2022 at 01:01:40PM +0000, Paul Cercueil wrote:

Document the new DMABUF based API.

v2: - Explicitly state that the new interface is optional and is not implemented by all drivers. - The IOCTLs can now only be called on the buffer FD returned by IIO_BUFFER_GET_FD_IOCTL. - Move the page up a bit in the index since it is core stuff and not driver-specific.

Signed-off-by: Paul Cercueil paul@crapouillou.net

Documentation/driver-api/dma-buf.rst | 2 + Documentation/iio/dmabuf_api.rst | 94 ++++++++++++++++++++++++++++ Documentation/iio/index.rst | 2 + 3 files changed, 98 insertions(+) create mode 100644 Documentation/iio/dmabuf_api.rst

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index 2cd7db82d9fe..d3c9b58d2706 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -1,3 +1,5 @@ +.. _dma-buf:

Buffer Sharing and Synchronization

diff --git a/Documentation/iio/dmabuf_api.rst b/Documentation/iio/dmabuf_api.rst new file mode 100644 index 000000000000..43bb2c1b9fdc --- /dev/null +++ b/Documentation/iio/dmabuf_api.rst @@ -0,0 +1,94 @@ +=================================== +High-speed DMABUF interface for IIO +===================================

+1. Overview +===========

+The Industrial I/O subsystem supports access to buffers through a file-based +interface, with read() and write() access calls through the IIO device's dev +node.

+It additionally supports a DMABUF based interface, where the userspace +application can allocate and append DMABUF objects to the buffer's queue. +This interface is however optional and is not available in all drivers.

+The advantage of this DMABUF based interface vs. the read() +interface, is that it avoids an extra copy of the data between the +kernel and userspace. This is particularly useful for high-speed +devices which produce several megabytes or even gigabytes of data per +second.

+The data in this DMABUF interface is managed at the granularity of +DMABUF objects. Reducing the granularity from byte level to block level +is done to reduce the userspace-kernelspace synchronization overhead +since performing syscalls for each byte at a few Mbps is just not +feasible.

+This of course leads to a slightly increased latency. For this reason an +application can choose the size of the DMABUFs as well as how many it +allocates. E.g. two DMABUFs would be a traditional double buffering +scheme. But using a higher number might be necessary to avoid +underflow/overflow situations in the presence of scheduling latencies.

So this reads a lot like reinventing io-uring with pre-registered O_DIRECT memory ranges. Except it's using dma-buf and hand-rolling a lot of pieces instead of io-uring and O_DIRECT.

At least if the entire justification for dma-buf support is zero-copy support between the driver and userspace it's _really_ not the right tool for the job. dma-buf is for zero-copy between devices, with cpu access from userpace (or kernel fwiw) being very much the exception (and often flat-out not supported at all). -Daniel

+2. User API +===========

+``IIO_BUFFER_DMABUF_ALLOC_IOCTL(struct iio_dmabuf_alloc_req *)`` +----------------------------------------------------------------

+Each call will allocate a new DMABUF object. The return value (if not +a negative errno value as error) will be the file descriptor of the new +DMABUF.

+``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *)`` +--------------------------------------------------------

+Place the DMABUF object into the queue pending for hardware process.

+These two IOCTLs have to be performed on the IIO buffer's file +descriptor, obtained using the `IIO_BUFFER_GET_FD_IOCTL` ioctl.

+3. Usage +========

+To access the data stored in a block by userspace the block must be +mapped to the process's memory. This is done by calling mmap() on the +DMABUF's file descriptor.

+Before accessing the data through the map, you must use the +DMA_BUF_IOCTL_SYNC(struct dma_buf_sync *) ioctl, with the +DMA_BUF_SYNC_START flag, to make sure that the data is available. +This call may block until the hardware is done with this block. Once +you are done reading or writing the data, you must use this ioctl again +with the DMA_BUF_SYNC_END flag, before enqueueing the DMABUF to the +kernel's queue.

+If you need to know when the hardware is done with a DMABUF, you can +poll its file descriptor for the EPOLLOUT event.

+Finally, to destroy a DMABUF object, simply call close() on its file +descriptor.

+For more information about manipulating DMABUF objects, see: :ref:`dma-buf`.

+A typical workflow for the new interface is:

• for block in blocks:

•  DMABUF_ALLOC block
  

•  mmap block
  

• enable buffer
• while !done

•  for block in blocks:
  

•    DMABUF_ENQUEUE block
  

•    DMABUF_SYNC_START block
  

•    process data
  

•    DMABUF_SYNC_END block
  

• disable buffer
• for block in blocks:

•  close block
  

diff --git a/Documentation/iio/index.rst b/Documentation/iio/index.rst index 58b7a4ebac51..669deb67ddee 100644 --- a/Documentation/iio/index.rst +++ b/Documentation/iio/index.rst @@ -9,4 +9,6 @@ Industrial I/O iio_configfs

• dmabuf_api
• ep93xx_adc

-- 2.34.1