On Wed, Aug 19, 2020 at 3:55 PM Christoph Hellwig [off-list ref] wrote:

On Wed, Aug 19, 2020 at 01:16:51PM +0200, Tomasz Figa wrote:

quoted

Hi Christoph,

On Wed, Aug 19, 2020 at 8:56 AM Christoph Hellwig [off-list ref] wrote:

quoted

The V4L2-FLAG-MEMORY-NON-CONSISTENT flag is entirely unused,

Could you explain what makes you think it's unused? It's a feature of
the UAPI generally supported by the videobuf2 framework and relied on
by Chromium OS to get any kind of reasonable performance when
accessing V4L2 buffers in the userspace.

Because it doesn't do anything except on PARISC and non-coherent MIPS,
so by definition it isn't used by any of these media drivers.

It's still an UAPI feature, so we can't simply remove the flag, it
must stay there as a no-op, until the problem is resolved.

Also, it of course might be disputable as an out-of-tree usage, but
selecting CONFIG_DMA_NONCOHERENT_CACHE_SYNC makes the flag actually do
something on other platforms, including ARM64.

Best regards,
Tomasz