| Message ID | 20211115142243.60605-4-paul@crapouillou.net (mailing list archive) |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | iio: buffer-dma: write() and new DMABUF based API | expand |
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? > 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?
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
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
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