Messages in this thread Patch in this message | | | Date | Sun, 21 Nov 2021 17:46:21 +0000 | From | Paul Cercueil <> | Subject | Re: [PATCH 15/15] Documentation: iio: Document high-speed DMABUF based API |
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Hi Jonathan,
Le dim., nov. 21 2021 at 15:10:26 +0000, Jonathan Cameron <jic23@kernel.org> a écrit : > On Mon, 15 Nov 2021 14:22:43 +0000 > Paul Cercueil <paul@crapouillou.net> wrote: > >> Document the new DMABUF based API. >> >> Signed-off-by: Paul Cercueil <paul@crapouillou.net> > > 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?
I have this line in the file: For more information about manipulating DMABUF objects, see: :ref:`dma-buf`.
For the :ref: to work I need a label at the reference point, if I understood correctly.
>> 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 :)
Their I2C ADC will support it, as long as the driver supports the dmaengine buffer interface. I can make that explicit, yes.
>> + >> +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?
Alright.
Cheers, -Paul
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