On Thu, 17 Jun 2021 00:05:24 +0300 Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > 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 :-( What should we infer from that? That the hardware is broken and exposing those KMS properties is a false promise? If a driver on certain hardware cannot correctly implement a KMS property over the full domain of the input space, should that driver then simply not expose the KMS property at all? But I would assume that the vendor still wants to expose the features in upstream kernels, yet they cannot use the standard KMS properties for that. Should the driver then expose vendor-specific properties with the disclaimer that the result is not always what one would expect, so that userspace written and tested explicitly for that hardware can still work? That is, a sufficient justification for a vendor-specific KMS property would be that a standard property already exists, but the hardware is too buggy to make it work. IOW, give up trying to make sense. I would like to move towards a direction where *hardware* design and testing is eventually guided by Linux KMS property definitions and their tests. If we could have a rule that if a driver cannot correctly implement a property then it must not expose the property, maybe in the long term that might start having an effect? My underlying assumption is that generic userspace will not use vendor-specific properties. Or, since we have atomic commits with TEST_ONLY, should it be driver's responsibility to carefully inspect the full state and reject the commit if the hardware is incapable of implementing it correctly? Vendor-specific userspace would know to avoid failing configurations to begin with. I suppose that might put an endless whack-a-mole game on drivers though. Thanks, pq
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