[PATCH v3] Documentation: gpu: Mention the requirements for new properties

[Laurent Pinchart]

On Tue, Jun 15, 2021 at 01:16:56PM +0300, Pekka Paalanen wrote:
> On Tue, 15 Jun 2021 12:45:57 +0300 Laurent Pinchart wrote:
> > On Tue, Jun 15, 2021 at 07:15:18AM +0000, Simon Ser wrote:
> > > On Tuesday, June 15th, 2021 at 09:03, Pekka Paalanen wrote:
> > >   
> > > > indeed it will, but what else could one do to test userspace KMS
> > > > clients in generic CI where all you can have is virtual hardware? Maybe
> > > > in the long run VKMS needs to loop back to a userspace daemon that
> > > > implements all the complex processing and returns the writeback result
> > > > via VKMS again? That daemon would then need a single upstream, like the
> > > > kernel, where it is maintained and correctness verified.  
> > > 
> > > The complex processing must be implemented even without write-back, because
> > > user-space can ask for CRCs of the CRTC.
> > >   
> > > > Or an LD_PRELOAD that hijacks all KMS ioctls and implements virtual
> > > > stuff in userspace? Didn't someone already have something like that?
> > > > It would need to be lifted to be a required part of kernel UAPI
> > > > submissions, I suppose like IGT is nowadays.  
> > > 
> > > FWIW, I have a mock libdrm [1] for libliftoff. This is nowhere near a full
> > > software implementation with write-back connectors, but allows to expose
> > > virtual planes and check atomic commits in CI.
> > > 
> > > [1]: https://github.com/emersion/libliftoff/blob/master/test/libdrm_mock.c
> > >   
> > > > For compositor developers like me knowing the exact formulas would be a huge
> > > > benefit as it would allow me to use KMS to off-load precision-sensitive
> > > > operations (e.g.  professional color management). Otherwise, compositors
> > > > probably need a switch: "high quality color management? Then do not use KMS
> > > > features."  
> > > 
> > > I think for alpha blending there are already rounding issues depending on the
> > > hardware. I wouldn't keep my hopes up for any guarantee that all hw uses the
> > > exact same formulae for color management stuff.  
> > 
> > Good, because otherwise you would be very quickly disappointed :-)
> > 
> > For scaling we would also need to replicate the exact same filter taps,
> > which are often not documented.
> 
> That is where the documented tolerances come into play.

This is something I've experimented with a while ago, when developing
automated tests for the rcar-du driver. When playing with different
input images we had to constantly increases tolerances, up to a point
where the tests started to miss real problems :-(

> Userspace projects need screenshot-based testing, and we need to know
> how much tolerance we should allow or expect.
> 
> Good reminder about CRCs. CRCs have zero tolerance, so they are not
> useful for testing properties that have any leeway, are they?

-- 
Regards,

Laurent Pinchart