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

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Trying to send this once more as text only.

Am 04.03.24 um 14:40 schrieb Christian König:
Am 04.03.24 um 14:28 schrieb Nuno Sá:
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@xxxxxxxxxxxxxxx>

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

As part of the interface, 3 new IOCTLs have been added:

   Attach the DMABUF object identified by the given file descriptor to the

   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.

   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?


Yes, that's broken as well. The dma_resv_usage_rw() function is supposed to be used while retrieving fences.

In other words your "if (dma_to_ram) ..." above is incorrect as well. That one should look more like this:
  * Writes needs to wait for other writes and reads, but readers only have to wait for writers.

retl = dma_resv_wait_timeout(dmabuf->resv, dma_resv_usage_rw(dma_to_ram), timeout);


- Nuno Sá

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