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Date:   Fri, 27 Nov 2020 23:30:33 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To:     Hans Verkuil <hverkuil@xs4all.nl>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Tomasz Figa <tfiga@chromium.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Re: [PATCH] media: vb2: always set buffer cache sync hints
Message-ID: <X8ENifLanjYuhF/r@jagdpanzerIV.localdomain>
References: <20201127094136.1051071-1-sergey.senozhatsky@gmail.com>
 <0dbfa509-8c82-7470-c18b-24ab5c92dc4b@xs4all.nl>
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On (20/11/27 13:59), Hans Verkuil wrote:
> 
> I think this needs a comment, basically explaining what you said in
> the commit log. It's not obvious from the code that this is a
> workaround.

Something like this?

---

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 5499013cf82e..21b2b0ae3629 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -414,6 +414,16 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
                vb->index = q->num_buffers + buffer;
                vb->type = q->type;
                vb->memory = memory;
+               /*
+                * A workaround fix. We need to set these flags here so that
+                * videobuf2 core will always call ->prepare()/->finish()
+                * cache sync/flush on vb2 buffers. Otherwise, for backends
+                * that don't rely on V4L2 (perhaps dvb) these flags will
+                * always be false and, hence, videobuf2 core will skip cache
+                * sync/flush operations.
+                */
+               vb->need_cache_sync_on_prepare = 1;
+               vb->need_cache_sync_on_finish = 1;
                for (plane = 0; plane < num_planes; ++plane) {
                        vb->planes[plane].length = plane_sizes[plane];
                        vb->planes[plane].min_length = plane_sizes[plane];