On Tue, Mar 29, 2022 at 10:11:14AM +0100, Paul Cercueil wrote:
Hi Daniel,
Le mar., mars 29 2022 at 10:33:32 +0200, Daniel Vetter daniel@ffwll.ch a écrit :
On Tue, Feb 15, 2022 at 05:43:35PM +0000, Paul Cercueil wrote:
Hi Jonathan,
Le dim., févr. 13 2022 at 18:46:16 +0000, Jonathan Cameron jic23@kernel.org a écrit :
On Mon, 7 Feb 2022 12:59:21 +0000 Paul Cercueil paul@crapouillou.net 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.
dma-buf also doesn't support sharing with network and usb stacks, so I'm a bit confused why exactly this is useful?
There is an attempt to get dma-buf support in the network stack, called "zctap". Last patchset was sent last november. USB stack does not support dma-buf, but we can add it later I guess.
So yeah unless there's some sharing going on with gpu stuff (for data processing maybe) I'm not sure this makes a lot of sense really. Or at least some zero-copy sharing between drivers, but even that would minimally require a dma-buf import ioctl of some sorts. Which I either missed or doesn't exist.
We do want zero-copy between drivers, the network stack, and the USB stack. It's not just about having a userspace interface.
I think in that case we need these other pieces too. And we need acks from relevant subsystems that these other pieces are a) ready for upstream merging and also that the dma-buf side of things actually makes sense.
If there's none of that then just hand-roll your buffer handling code (xarray is cheap to use in terms of code for this), you can always add dma-buf import/export later on when the need arises.
Scrolling through patches you only have dma-buf export, but no importing, so the use-case that works is with one of the existing subsystems that supporting dma-buf importing.
I think minimally we need the use-case (in form of code) that needs the buffer sharing here.
I'll try with zctap and report back.
Do you have a link for this? I just checked dri-devel on lore, and it's not there. Nor anywhere else.
We really need all the pieces, and if block layer reaction is anything to judge by, dma-buf wont happen for networking either. There's some really nasty and fairly fundamental issues with locking and memory reclaim that make this utter pain or outright impossible. -Daniel
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 FDreturned by
IIO_BUFFER_GET_FD_IOCTL. * Move the page up a bit in the index since it is corestuff
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
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch