Hi Jonathan,
Le dim., févr. 13 2022 at 18:46:16 +0000, Jonathan Cameron
<jic23@xxxxxxxxxx> a écrit :
On Mon, 7 Feb 2022 12:59:21 +0000
Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote:
Hi Jonathan,
This is the V2 of my patchset that introduces a new userspace
interface
based on DMABUF objects to complement the fileio API, and adds
write()
support to the existing fileio API.
Hi Paul,
It's been a little while. Perhaps you could summarize the various view
points around the appropriateness of using DMABUF for this?
I appreciate it is a tricky topic to distil into a brief summary but
I know I would find it useful even if no one else does!
So we want to have a high-speed interface where buffers of samples are
passed around between IIO devices and other devices (e.g. USB or
network), or made available to userspace without copying the data.
DMABUF is, at least in theory, exactly what we need. Quoting the
documentation
(https://www.kernel.org/doc/html/v5.15/driver-api/dma-buf.html):
"The dma-buf subsystem provides the framework for sharing buffers for
hardware (DMA) access across multiple device drivers and subsystems,
and for synchronizing asynchronous hardware access. This is used, for
example, by drm “prime” multi-GPU support, but is of course not
limited to GPU use cases."
The problem is that right now DMABUF is only really used by DRM, and to
quote Daniel, "dma-buf looks like something super generic and useful,
until you realize that there's a metric ton of gpu/accelerator bagage
piled in".
Still, it seems to be the only viable option. We could add a custom
buffer-passing interface, but that would mean implementing the same
buffer-passing interface on the network and USB stacks, and before we
know it we re-invented DMABUFs.
Cheers,
-Paul
Changes since v1:
- the patches that were merged in v1 have been (obviously) dropped
from
this patchset;
- the patch that was setting the write-combine cache setting has
been
dropped as well, as it was simply not useful.
- [01/12]:
* Only remove the outgoing queue, and keep the incoming queue,
as we
want the buffer to start streaming data as soon as it is
enabled.
* Remove IIO_BLOCK_STATE_DEQUEUED, since it is now functionally
the
same as IIO_BLOCK_STATE_DONE.
- [02/12]:
* Fix block->state not being reset in
iio_dma_buffer_request_update() for output buffers.
* Only update block->bytes_used once and add a comment about
why we
update it.
* Add a comment about why we're setting a different state for
output
buffers in iio_dma_buffer_request_update()
* Remove useless cast to bool (!!) in iio_dma_buffer_io()
- [05/12]:
Only allow the new IOCTLs on the buffer FD created with
IIO_BUFFER_GET_FD_IOCTL().
- [12/12]:
* 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.
The patches not listed here have not been modified since v1.
Cheers,
-Paul
Alexandru Ardelean (1):
iio: buffer-dma: split iio_dma_buffer_fileio_free() function
Paul Cercueil (11):
iio: buffer-dma: Get rid of outgoing queue
iio: buffer-dma: Enable buffer write support
iio: buffer-dmaengine: Support specifying buffer direction
iio: buffer-dmaengine: Enable write support
iio: core: Add new DMABUF interface infrastructure
iio: buffer-dma: Use DMABUFs instead of custom solution
iio: buffer-dma: Implement new DMABUF based userspace API
iio: buffer-dmaengine: Support new DMABUF based userspace API
iio: core: Add support for cyclic buffers
iio: buffer-dmaengine: Add support for cyclic buffers
Documentation: iio: Document high-speed DMABUF based API
Documentation/driver-api/dma-buf.rst | 2 +
Documentation/iio/dmabuf_api.rst | 94 +++
Documentation/iio/index.rst | 2 +
drivers/iio/adc/adi-axi-adc.c | 3 +-
drivers/iio/buffer/industrialio-buffer-dma.c | 610
++++++++++++++----
.../buffer/industrialio-buffer-dmaengine.c | 42 +-
drivers/iio/industrialio-buffer.c | 60 ++
include/linux/iio/buffer-dma.h | 38 +-
include/linux/iio/buffer-dmaengine.h | 5 +-
include/linux/iio/buffer_impl.h | 8 +
include/uapi/linux/iio/buffer.h | 30 +
11 files changed, 749 insertions(+), 145 deletions(-)
create mode 100644 Documentation/iio/dmabuf_api.rst
