On Mon, 15 Nov 2021 14:22:43 +0000 Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote: > Document the new DMABUF based API. > > Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx> Hi Paul, A few trivial things inline but looks good to me if we do end up using DMABUF anyway. Jonathan > --- > 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: > + Why this change? > Buffer Sharing and Synchronization > ================================== > > diff --git a/Documentation/iio/dmabuf_api.rst b/Documentation/iio/dmabuf_api.rst > new file mode 100644 > index 000000000000..b4e120a4ef0c > --- /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. I would note somewhere that this interface is optional for a given IIO driver. I don't want people to start assuming their i2c ADC will support this and wondering why it doesn't work :) > + > +The advantage of this DMABUF based interface vs. the fileio > +interface, is that it avoids an extra copy of the data between the > +kernel and userspace. This is particularly userful 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. > + > +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 (either opened from the corresponding /dev/iio:deviceX, or > +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..9ce799fbf262 100644 > --- a/Documentation/iio/index.rst > +++ b/Documentation/iio/index.rst > @@ -10,3 +10,5 @@ Industrial I/O > iio_configfs > > ep93xx_adc > + > + dmabuf_api Given this is core stuff rather than driver specific, perhaps move it up a few lines?
