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

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.

On Thu, Aug 20, 2020 at 06:43:47AM +0200, Christoph Hellwig
wrote:
> 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.
So I took a deeper look.  I don't really think it qualifies as a UAPI
in our traditional sense.  For one it only appeared in 5.9-rc1, so we
can trivially expedite the patch into 5.9-rc and not actually make it
show up in any released kernel version.  And even as of the current
Linus' tree the only user is a test driver.  So I really think the best
way to go ahead is to just revert it ASAP as the design wasn't thought
out at all.

On Thu, Aug 20, 2020 at 7:20 AM Christoph Hellwig <hch at lst.de>
wrote:
>
> On Thu, Aug 20, 2020 at 06:43:47AM +0200, Christoph Hellwig wrote:
> > 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.
>
> So I took a deeper look.  I don't really think it qualifies as a UAPI
> in our traditional sense.  For one it only appeared in 5.9-rc1, so we
> can trivially expedite the patch into 5.9-rc and not actually make it
> show up in any released kernel version.  And even as of the current
> Linus' tree the only user is a test driver.  So I really think the best
> way to go ahead is to just revert it ASAP as the design wasn't thought
> out at all.
The UAPI and V4L2/videobuf2 changes are in good shape and the only
wrong part is the use of DMA API, which was based on an earlier email
guidance anyway, and a change to the synchronization part . I find
conclusions like the above insulting for people who put many hours
into designing and implementing the related functionality, given the
complexity of the videobuf2 framework and how ill-defined the DMA API
was, and would feel better if such could be avoided in future
communication.

That said, we can revert it on the basis of the implementation issues,
but I feel like we wouldn't get anything by doing so, because as I
said, the design is sane and most of the implementation is fine as
well. Instead. I'd suggest simply removing the use of the attribute
being removed, so that the feature stays no-op until the DMA API
provides a way to implement it or we just migrate videobuf2 to stop
using the DMA API as much as possible, like many drivers in the DRM
subsystem did.

Best regards,
Tomasz