Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

[Tomasz Figa <tfiga@xxxxxxxxxxxx>]

On Wed, Aug 19, 2020 at 3:55 PM Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Wed, Aug 19, 2020 at 01:16:51PM +0200, Tomasz Figa wrote:
> > Hi Christoph,
> >
> > On Wed, Aug 19, 2020 at 8:56 AM Christoph Hellwig <hch@xxxxxx> wrote:
> > >
> > > 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