Re: [PATCH 16/16] drm/amd/display: Don't restrict bpc to 8 bpc

["Michel Dänzer" <michel.daenzer@mailbox.org>]

On 12/14/22 16:46, Alex Deucher wrote:
> On Wed, Dec 14, 2022 at 4:01 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
>> On Tue, 13 Dec 2022 18:20:59 +0100
>> Michel Dänzer <michel.daenzer@mailbox.org> wrote:
>>> On 12/12/22 19:21, Harry Wentland wrote:
>>>> This will let us pass kms_hdr.bpc_switch.
>>>>
>>>> I don't see any good reasons why we still need to
>>>> limit bpc to 8 bpc and doing so is problematic when
>>>> we enable HDR.
>>>>
>>>> If I remember correctly there might have been some
>>>> displays out there where the advertised link bandwidth
>>>> was not large enough to drive the default timing at
>>>> max bpc. This would leave to an atomic commit/check
>>>> failure which should really be handled in compositors
>>>> with some sort of fallback mechanism.
>>>>
>>>> If this somehow turns out to still be an issue I
>>>> suggest we add a module parameter to allow users to
>>>> limit the max_bpc to a desired value.
>>>
>>> While leaving the fallback for user space to handle makes some sense
>>> in theory, in practice most KMS display servers likely won't handle
>>> it.
>>>
>>> Another issue is that if mode validation is based on the maximum bpc
>>> value, it may reject modes which would work with lower bpc.
>>>
>>>
>>> What Ville (CC'd) suggested before instead (and what i915 seems to be
>>> doing already) is that the driver should do mode validation based on
>>> the *minimum* bpc, and automatically make the effective bpc lower
>>> than the maximum as needed to make the rest of the atomic state work.
>>
>> A driver is always allowed to choose a bpc lower than max_bpc, so it
>> very well should do so when necessary due to *known* hardware etc.
>> limitations.
>>
> 
> In the amdgpu case, it's more of a preference thing.  The driver would
> enable higher bpcs at the expense of refresh rate and it seemed most
> users want higher refresh rates than higher bpc. 

I wrote the above because I thought that this patch might result in some modes getting pruned because they can't work with the max bpc. However, I see now that cbd14ae7ea93 ("drm/amd/display: Fix incorrectly pruned modes with deep color") should prevent that AFAICT.

The question then is: What happens if user space tries to use a mode which doesn't work with the max bpc? Does the driver automatically lower the effective bpc as needed, or does the atomic commit (check) fail? The latter would seem bad.


> I guess the driver can select a lower bpc at its discretion to produce
> what it thinks is the best default, but what about users that don't want
> the default?

They can choose the lower refresh rate?


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