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@xxxxxxxxxxxxxxx> > --- > 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 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
