[RFC PATCH 1/3] drm: Introduce color fill properties for drm plane

[Jessica Zhang]

On 11/11/2022 1:45 AM, Daniel Vetter wrote:
> On Wed, Nov 09, 2022 at 05:44:37PM -0800, Jessica Zhang wrote:
>>
>>
>> On 11/9/2022 5:59 AM, Daniel Vetter wrote:
>>> On Wed, Nov 09, 2022 at 04:53:45PM +0300, Dmitry Baryshkov wrote:
>>>> On 09/11/2022 16:52, Daniel Vetter wrote:
>>>>> On Tue, Nov 08, 2022 at 06:25:29PM +0000, Simon Ser wrote:
>>>>>> On Saturday, October 29th, 2022 at 13:23, Dmitry Baryshkov <dmitry.baryshkov at linaro.org> wrote:
>>>>>>
>>>>>>> On 29/10/2022 01:59, Jessica Zhang wrote:
>>>>>>>
>>>>>>>> Add support for COLOR_FILL and COLOR_FILL_FORMAT properties for
>>>>>>>> drm_plane. In addition, add support for setting and getting the values
>>>>>>>> of these properties.
>>>>>>>>
>>>>>>>> COLOR_FILL represents the color fill of a plane while COLOR_FILL_FORMAT
>>>>>>>> represents the format of the color fill. Userspace can set enable solid
>>>>>>>> fill on a plane by assigning COLOR_FILL to a uint64_t value, assigning
>>>>>>>> the COLOR_FILL_FORMAT property to a uint32_t value, and setting the
>>>>>>>> framebuffer to NULL.
>>>>>>>
>>>>>>> I suppose that COLOR_FILL should override framebuffer rather than
>>>>>>> requiring that FB is set to NULL. In other words, if color_filL_format
>>>>>>> is non-zero, it would make sense to ignore the FB. Then one can use the
>>>>>>> color_fill_format property to quickly switch between filled plane and
>>>>>>> FB-backed one.
>>>>>>
>>>>>> That would be inconsistent with the rest of the KMS uAPI. For instance,
>>>>>> the kernel will error out if CRTC has active=0 but a connector is still
>>>>>> linked to the CRTC. IOW, the current uAPI errors out if the KMS state
>>>>>> is inconsistent.
>>>>>
>>>>> So if the use-case here really is to solid-fill a plane (and not just
>>>>> provide a background color for the crtc overall), then I guess we could
>>>>> also extend addfb to make that happen. We've talked in the past about
>>>>> propertery-fying framebuffer objects, and that would sort out this uapi
>>>>> wart. And I agree the color fill vs PLANE_ID issue is a bit ugly at least.
>>>>>
>>>>> But if the use-cases are all background color then just doing the crtc
>>>>> background color would be tons simpler (and likely also easier to support
>>>>> for more hardware).
>>>>
>>>> No. The hardware supports multiple color-filled planes, which do not have to
>>>> cover the whole CRTC.
>>>
>>> The use case here means the userspace use-case. What the hw can do on any
>>> given chip kinda doesnt matter, which is why I'm asking. KMD uapi is not
>>> meant to reflect 100% exactly what a specific chip can do, but instead:
>>> - provide features userspace actually needs. If you want per-plane fill,
>>>     you need userspace that makes use of per-plane fill, and if all you have
>>>     is crtc background, then that's it.
>>
>> Hey Daniel,
>>
>> The userspace use case we're trying to support is the Android HWC SOLID_FILL
>> hint here [1], which is specifying per-plane fill.
> 
> Does surfaceflinger actually use this for more than background fills? Yes
> I'm annoying, but if we can simplify the kernel driver implementation
> burden by asking compositors to do the math and simplify things, then I
> think we should.

AFAIK surfaceflinger allows apps to use this for cases beyond just 
background fill -- an app, for example, can pass the hint for a plane 
that only partially covers a screen and the driver would be expected to 
fill just that ROI.

> 
> We also need an open source implementation for this that works and is
> tested end-to-end. There's the drm_hwc project, but last time I've checked
> there's really not much happpening there unfortunately.

FWIW, Simon mentioned in a separate reply that Wayland supports a 1x1 FB 
support protocol [1] for a similar purpose as this RFC series. I can 
also create an IGT test meanwhile showing an example of E2E usage.

Thanks,

Jessica

[1] 
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/104

> -Daniel
> 
>>
>> Thanks,
>>
>> Jessica Zhang
>>
>> [1] https://android.googlesource.com/platform/hardware/interfaces/+/refs/heads/master/graphics/composer/aidl/android/hardware/graphics/composer3/Composition.aidl#52
>>
>>> - we should create uapi with an eye towards what's actually possible on a
>>>     reasonable set of drivers and hw. Sometimes that means a slightly more
>>>     restricted set so that it's possible to implement in more places,
>>>     especially if that restricted feature set still gets the job done for
>>>     userspace.
>>>
>>> Cheers, Daniel
>>> -- 
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch