Document the new DMABUF based API.
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..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. + +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