Re: [PATCH 1/2] drm: add crtc background color property

[Harry Wentland]

On 2021-07-14 3:35 a.m., Pekka Paalanen wrote:
> On Tue, 13 Jul 2021 09:54:35 -0400
> Harry Wentland <harry.wentland@xxxxxxx> wrote:
> 
>> On 2021-07-13 3:52 a.m., Pekka Paalanen wrote:
>>> On Mon, 12 Jul 2021 12:15:59 -0400
>>> Harry Wentland <harry.wentland@xxxxxxx> wrote:
>>>   
>>>> On 2021-07-12 4:03 a.m., Pekka Paalanen wrote:  
>>>>> On Fri, 9 Jul 2021 18:23:26 +0200
>>>>> Raphael Gallais-Pou <raphael.gallais-pou@xxxxxxxxxxx> wrote:
>>>>>     
>>>>>> On 7/9/21 10:04 AM, Pekka Paalanen wrote:    
>>>>>>> On Wed, 7 Jul 2021 08:48:47 +0000
>>>>>>> Raphael GALLAIS-POU - foss <raphael.gallais-pou@xxxxxxxxxxx> wrote:
>>>>>>>      
>>>>>>>> Some display controllers can be programmed to present non-black colors
>>>>>>>> for pixels not covered by any plane (or pixels covered by the
>>>>>>>> transparent regions of higher planes).  Compositors that want a UI with
>>>>>>>> a solid color background can potentially save memory bandwidth by
>>>>>>>> setting the CRTC background property and using smaller planes to display
>>>>>>>> the rest of the content.
>>>>>>>>
>>>>>>>> To avoid confusion between different ways of encoding RGB data, we
>>>>>>>> define a standard 64-bit format that should be used for this property's
>>>>>>>> value.  Helper functions and macros are provided to generate and dissect
>>>>>>>> values in this standard format with varying component precision values.
>>>>>>>>
>>>>>>>> Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@xxxxxxxxxxx>
>>>>>>>> Signed-off-by: Matt Roper <matthew.d.roper@xxxxxxxxx>
>>>>>>>> ---
>>>>>>>>   drivers/gpu/drm/drm_atomic_state_helper.c |  1 +
>>>>>>>>   drivers/gpu/drm/drm_atomic_uapi.c         |  4 +++
>>>>>>>>   drivers/gpu/drm/drm_blend.c               | 34 +++++++++++++++++++++--
>>>>>>>>   drivers/gpu/drm/drm_mode_config.c         |  6 ++++
>>>>>>>>   include/drm/drm_blend.h                   |  1 +
>>>>>>>>   include/drm/drm_crtc.h                    | 12 ++++++++
>>>>>>>>   include/drm/drm_mode_config.h             |  5 ++++
>>>>>>>>   include/uapi/drm/drm_mode.h               | 28 +++++++++++++++++++
>>>>>>>>   8 files changed, 89 insertions(+), 2 deletions(-)  
>>>
>>> ...
>>>   
>>>>>>> The question about full vs. limited range seems unnecessary to me, as
>>>>>>> the background color will be used as-is in the blending stage, so
>>>>>>> userspace can just program the correct value that fits the pipeline it
>>>>>>> is setting up.
>>>>>>>
>>>>>>> One more question is, as HDR exists, could we need background colors
>>>>>>> with component values greater than 1.0?      
>>>>>>
>>>>>> AR4H color format should cover that case, isn't it ?    
>>>>>
>>>>> Yes, but with the inconvenience I mentioned.
>>>>>
>>>>> This is a genuine question though, would anyone actually need
>>>>> background color values > 1.0. I don't know of any case yet where it
>>>>> would be required. It would imply that plane blending happens in a
>>>>> color space where >1.0 values are meaningful. I'm not even sure if any
>>>>> hardware supporting that exists.
>>>>>
>>>>> Maybe it would be best to assume that only [0.0, 1.0] pixel value range
>>>>> is useful, and mention in the commit message that if someone really
>>>>> needs values outside of that, they should create another background
>>>>> color property. Then, you can pick a simple unsigned integer pixel
>>>>> format, too. (I didn't see any 16 bit-per-channel formats like that in
>>>>> drm_fourcc.h though.)
>>>>>     
>>>>
>>>> I don't think we should artificially limit this to [0.0, 1.0]. As you
>>>> mentioned above when talking about full vs limited, the userspace
>>>> understands what's the correct value that fits the pipeline. If that
>>>> pipeline is FP16 with > 1.0 values then it would make sense that the
>>>> background color can be > 1.0.  
>>>
>>> Ok. The standard FP32 format then for ease of use and guaranteed enough
>>> range and precision for far into the future?
>>>   
>>
>> I don't have a strong preference for FP16 vs FP32. My understanding is
>> that FP16 is enough to represent linearly encoded data in a way that
>> looks smooth to humans.
>>
>> scRGB uses FP16 with linear encoding in a range of [-0.5, 7.4999].
>>
>>> Or do you want to keep it in 64 bits total, so the UABI can pack
>>> everything into a u64 instead of needing to create a blob?
>>>
>>> I don't mind as long as it's clearly documented what it is and how it
>>> works, and it carries enough precision.
>>>
>>> But FP16 with its 10 bits of precision might be too little for integer
>>> 12-16 bpc pipelines and sinks?
> 
> The 10 bits worries me still.
> 
> If you have a pipeline that works in [0.0, 1.0] range only, then FP16
> limits precision to 10 bits (in the upper half of the range?).
> 
>>>
>>> If the values can go beyond [0.0, 1.0] range, then does the blending
>>> hardware and the degamma/ctm/gamma coming afterwards cope with them, or
>>> do they get clamped anyway?
>>>   
>>
>> That probably depends on the HW and how it's configured. AMD HW can handle
>> values above and below [0.0, 1.0].
> 
> Right, so how would userspace know what will happen?
> 
> Or do we need to specify that while values outside that unit range are
> expressable, it is hardware-specific on how they will behave, so
> generic userspace should not attempt to use values outside of the unit
> range?
> 
> I guess this caveat should be documented for everything, not just for
> background color? LUT inputs and outputs, CTM input and output ranges,
> FB formats...
> 

I'm not sure we should artificially limit the interface at this point, or
document hypotheticals. At this point I don't even know whether going beyond
[0.0, 1.0] would be a challenge for any HW that supports floating point
formats.

Harry

> 
> Thanks,
> pq
>