linux-kernel - [PATCH] videobuf2: always re-init queue memory consistency attribute

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Message-Id: <20200609060417.8066-1-sergey.senozhatsky@gmail.com>
Date:   Tue,  9 Jun 2020 15:04:17 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [PATCH] videobuf2: always re-init queue memory consistency attribute

We need a combination of two factors in order to permit modification
of queue's memory consistency attribute in set_queue_consistency():
  (1) queue should allow user-space cache hints
  (2) queue should be used for MMAP-ed I/O

Therefore the code in videobuf2 core looks as follows:

	q->memory = req->memory;
	set_queue_consistency(q, consistent);

This works when we do something like this (suppose that queue allows
cache hints)

	reqbufs.memory = V4L2_MEMORY_DMABUF;
	reqbufs.flags = 0;
	doioctl(node, VIDIOC_REQBUFS, &reqbufs);

	reqbufs.memory = V4L2_MEMORY_MMAP;
	reqbufs.flags = V4L2_FLAG_MEMORY_NON_CONSISTENT;
	doioctl(node, VIDIOC_REQBUFS, &reqbufs);

However, this doesn't work the other way around

	reqbufs.memory = V4L2_MEMORY_MMAP;
	reqbufs.flags = V4L2_FLAG_MEMORY_NON_CONSISTENT;
	doioctl(node, VIDIOC_REQBUFS, &reqbufs);

	reqbufs.memory = V4L2_MEMORY_DMABUF;
	reqbufs.flags = 0;
	doioctl(node, VIDIOC_REQBUFS, &reqbufs);

because __vb2_queue_free() doesn't clear queue's ->dma_attrs
once its don't freeing queue buffers, and by the time we call
set_queue_consistency() the queue is DMABUF so (2) is false
and we never clear the stale consistency attribute.

Re-implement set_queue_consistency() - it must always clear
queue's non-consistency attr and set it only if (1) and (2).

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 drivers/media/common/videobuf2/videobuf2-core.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 7e081716b8da..37d0186ba330 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -704,12 +704,11 @@ EXPORT_SYMBOL(vb2_verify_memory_type);
 
 static void set_queue_consistency(struct vb2_queue *q, bool consistent_mem)
 {
+	q->dma_attrs &= ~DMA_ATTR_NON_CONSISTENT;
+
 	if (!vb2_queue_allows_cache_hints(q))
 		return;
-
-	if (consistent_mem)
-		q->dma_attrs &= ~DMA_ATTR_NON_CONSISTENT;
-	else
+	if (!consistent_mem)
 		q->dma_attrs |= DMA_ATTR_NON_CONSISTENT;
 }
 
-- 
2.27.0