Re: [PATCH v5 0/3] Add "link bpc" DRM property

[Michel Dänzer]

On 3/31/26 14:38, Pekka Paalanen wrote:
> On Tue, 31 Mar 2026 10:01:59 +0200
> Michel Dänzer <michel.daenzer@xxxxxxxxxxx> wrote:
>> On 3/26/26 14:53, Pekka Paalanen wrote:
>>> On Tue, 24 Mar 2026 17:44:21 +0100
>>> Michel Dänzer <michel.daenzer@xxxxxxxxxxx> wrote:
>>>   
>>>> * There's no clear use case.
>>>>
>>>> This is generally a requirement for new KMS UAPI.
>>>>
>>>> The practical usefulness of the corresponding weston MR is dubious
>>>> per the concern above.  
>>>
>>> I think the example of RGB 10 bpc to be degraded to YCbCr 10 bpc
>>> rather than RGB 8 bpc is an excellent use case.  
>>
>> This series and the corresponding Weston MR aren't enough to address
>> that use case though, are they? All they achieve is logging a
>> potentially misleading warning.
>>
>> It might make sense to combine this series and the Weston MR with
>> whatever else is needed for that use case.
> 
> What do you believe is missing?

For the stated use case, e.g. a mechanism to control RGB vs YCbCr?


> Informing the user that the display quality may not be as expected is
> the point.

The warning implies that the "link bpc" value is expected to match the "max bpc" value, which generally isn't the case.


>>> I had another use case in
>>> https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/1850#note_3115686  
>>
>> That would need to take dithering into account as well?
> 
> Yes, dithering could be an adverse effect or not sufficient. Hence the
> 'link bpc' property should not consider any kind of dithering, to be on
> the safe side. I fully expect dithering to become explicitly
> controllable, as policy belongs in userspace.

I agree that would be ideal, alas it's not current reality.


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast