On Tue, 18 May 2021 10:32:48 -0400 Harry Wentland harry.wentland@amd.com wrote:
On 2021-05-17 4:34 a.m., Pekka Paalanen wrote:
On Fri, 14 May 2021 17:04:51 -0400 Harry Wentland harry.wentland@amd.com wrote:
On 2021-04-30 8:53 p.m., Sebastian Wick wrote:
On 2021-04-26 20:56, Harry Wentland wrote:
...
Another reason I'm proposing to define the color space (and gamma) of a plane is to make this explicit. Up until the color space and gamma of a plane or framebuffer are not well defined, which leads to drivers assuming the color space and gamma of a buffer (for blending and other purposes) and might lead to sub-optimal outcomes.
Blending only is "correct" with linear light so that property of the color space is important. However, why does the kernel have to be involved here? As long as user space knows that for correct blending the data must represent linear light and knows when in the pipeline blending happens it can make sure that the data at that point in the pipeline contains linear light.
The only reason the kernel needs to be involved is to take full advantage of the available HW without requiring KMS clients to be aware of the difference in display HW.
Can you explain in more tangible examples, why you think so, please?
Is it because hardware does not fit the KMS UAPI model of the abstract pixel pipeline?
I'd wager no HW is designed to meet KMS UAPI, rather KMS UAPI is designed to abstract HW.
Of course, but you are in big trouble in any case if there is a fundamental mismatch. You may have to declare that all existing KMS properties for this stuff will be mutually exclusive with your new properties, so that you can introduce a new generic abstract pipeline in KMS.
By mutually exclusive I mean that a driver must advertise only one or the other set of properties and never both. If you want to support userspace that doesn't understand the alternative set, maybe you also need a drm client cap to switch to the alternative set per-drm-client.
Or is it because you have fixed-function hardware elements that you can only make use of when userspace uses an enum-based UAPI?
One example is our degamma on our latest generation HW, where we have fixed-function "degamma" (rather de-EOTF):
https://gitlab.freedesktop.org/agd5f/linux/-/blob/amd-staging-drm-next/drive...
Ok.
I would totally agree that the driver does not want to be analysing LUT entries to decipher if it could use a fixed-function element or not. It would introduce uncertainty in the UAPI. So fixed-function elements would need their own properties, but I don't know if that is feasible as generic UAPI or if it should be driver-specific (and so left unused by generic userspace).
For the CRTC LUTs we actually do a linearity check to program the HW into bypass when the LUT is linear since the KMS LUT definition doesn't map well onto the LUT definition used by our HW and leads to rounding errors and failing IGT kms_color tests (if I remember this correctly).
https://gitlab.freedesktop.org/agd5f/linux/-/blob/amd-staging-drm-next/drive...
Hence the suggestion to define pre-defined TFs right at a KMS level for usecases where we can assume the display will tonemap the content.
Right. Explaining this would have been a good introduction in your cover letter.
Maybe you want to define new KMS properties that shall be mutually exclusive with the existing KMS GAMMA/CTM/DEGAMMA properties and clearly document them as such.
Thanks, pq