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

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Simon Ser <contact@emersion.fr>
Cc: "Pekka Paalanen" <ppaalanen@gmail.com>,
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Subject: Re: [PATCH v3] Documentation: gpu: Mention the requirements for new properties
Date: Tue, 15 Jun 2021 12:45:57 +0300	[thread overview]
Message-ID: <YMh21WBrADbZDcbp@pendragon.ideasonboard.com> (raw)
In-Reply-To: <ouNaZaqkV1d_wPRESVBQHxvMhmJ53xIrgtPfDs8mB88AN3FEWt7cq031k8ZqCva1Ob0TCNTnsWqNDS0l5NXfejXIL7YUky3XGdjmh1_hefk=@emersion.fr>

On Tue, Jun 15, 2021 at 07:15:18AM +0000, Simon Ser wrote:
> On Tuesday, June 15th, 2021 at 09:03, Pekka Paalanen <ppaalanen@gmail.com> 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.

-- 
Regards,

Laurent Pinchart