Nouveau - Aug 2020 - [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

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 at lst.de>
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.

On Wed, Aug 19, 2020 at 03:57:53PM +0200, Tomasz Figa
wrote:
> > > 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.
Ok, I'll switch to just ignoring it for the next version.
> 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.
It isn't just disputable, but by kernel policies simply is not relevant.