On Tue, Mar 29, 2022 at 10:47:23AM +0100, Paul Cercueil wrote: > Hi Daniel, > > Le mar., mars 29 2022 at 10:54:43 +0200, Daniel Vetter <daniel@xxxxxxxx> a > écrit : > > 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. > > I don't see how io_uring would help us. It's an async I/O framework, does it > allow us to access a kernel buffer without copying the data? Does it allow > us to zero-copy the data to a network interface? With networking, do you mean rdma, or some other kind of networking? Anything else than rdma doesn't support dma-buf, and I don't think it will likely ever do so. Similar it's really tricky to glue dma-buf support into the block layer. Wrt io_uring, yes it's async, but that's not the point. The point is that with io_uring you pre-register ranges for reads and writes to target, which in combination with O_DIRECT, makes it effectively (and efficient!) zero-copy. Plus it has full integration with both networking and normal file io, which dma-buf just doesn't have. Like you _cannot_ do zero copy from a dma-buf into a normal file. You absolutely can do the same with io_uring. > > 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). > > We want both. Using dma-bufs for the driver/userspace interface is a > convenience as we then have a unique API instead of two distinct ones. > > Why should CPU access from userspace be the exception? It works fine for IIO > dma-bufs. You keep warning about this being a terrible design, but I simply > don't see it. It depends really on what you're trying to do, and there's extremely high chances it will simply not work. Unless you want to do zero copy with a gpu, or something which is in that ecosystem of accelerators and devices, then dma-buf is probably not what you're looking for. -Daniel > > Cheers, > -Paul > > > > + > > > +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 > > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
