Re: [PATCH v7 3/6] iio: core: Add new DMABUF interface infrastructure

["Nuno Sá" <noname.nuno@gmail.com>]

On Mon, 2024-03-04 at 13:44 +0100, Christian König wrote:
> Am 23.02.24 um 13:14 schrieb Nuno Sa:
> > From: Paul Cercueil <paul@crapouillou.net>
> > 
> > Add the necessary infrastructure to the IIO core to support a new
> > optional DMABUF based interface.
> > 
> > With this new interface, DMABUF objects (externally created) can be
> > attached to a IIO buffer, and subsequently used for data transfer.
> > 
> > A userspace application can then use this interface to share DMABUF
> > objects between several interfaces, allowing it to transfer data in a
> > zero-copy fashion, for instance between IIO and the USB stack.
> > 
> > The userspace application can also memory-map the DMABUF objects, and
> > access the sample data directly. The advantage of doing this vs. the
> > read() 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.
> > 
> > As part of the interface, 3 new IOCTLs have been added:
> > 
> > IIO_BUFFER_DMABUF_ATTACH_IOCTL(int fd):
> >   Attach the DMABUF object identified by the given file descriptor to the
> >   buffer.
> > 
> > IIO_BUFFER_DMABUF_DETACH_IOCTL(int fd):
> >   Detach the DMABUF object identified by the given file descriptor from
> >   the buffer. Note that closing the IIO buffer's file descriptor will
> >   automatically detach all previously attached DMABUF objects.
> > 
> > IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *):
> >   Request a data transfer to/from the given DMABUF object. Its file
> >   descriptor, as well as the transfer size and flags are provided in the
> >   "iio_dmabuf" structure.
> > 
> > These three IOCTLs have to be performed on the IIO buffer's file
> > descriptor, obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
> 
> A few nit picks and one bug below, apart from that looks good to me.

Hi Christian,

Thanks for looking at it. I'll just add some comment on the bug below and for
the other stuff I hope Paul is already able to step in and reply to it. My
assumption (which seems to be wrong) is that the dmabuf bits should be already
good to go as they should be pretty similar to the USB part of it.

...

> 
> > +	if (dma_to_ram) {
> > +		/*
> > +		 * If we're writing to the DMABUF, make sure we don't have
> > +		 * readers
> > +		 */
> > +		retl = dma_resv_wait_timeout(dmabuf->resv,
> > +					     DMA_RESV_USAGE_READ, true,
> > +					     timeout);
> > +		if (retl == 0)
> > +			retl = -EBUSY;
> > +		if (retl < 0) {
> > +			ret = (int)retl;
> > +			goto err_resv_unlock;
> > +		}
> > +	}
> > +
> > +	if (buffer->access->lock_queue)
> > +		buffer->access->lock_queue(buffer);
> > +
> > +	ret = dma_resv_reserve_fences(dmabuf->resv, 1);
> > +	if (ret)
> > +		goto err_queue_unlock;
> > +
> > +	dma_resv_add_fence(dmabuf->resv, &fence->base,
> > +			   dma_resv_usage_rw(dma_to_ram));
> 
> That is incorrect use of the function dma_resv_usage_rw(). That function 
> is for retrieving fences and not adding them.
> 
> See the function implementation and comments, when you use it like this 
> you get exactly what you don't want.
> 

Does that mean that the USB stuff [1] is also wrong?

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git/tree/drivers/usb/gadget/function/f_fs.c?h=usb-next#n1669

- Nuno Sá