[RFC PATCH v3 0/3] Support for Solid Fill Planes

[Dmitry Baryshkov]

On Fri, 6 Jan 2023 at 20:41, Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Fri, Jan 06, 2023 at 05:43:23AM +0200, Dmitry Baryshkov wrote:
> > On Fri, 6 Jan 2023 at 02:38, Jessica Zhang <quic_jesszhan at quicinc.com> wrote:
> > >
> > >
> > >
> > > On 1/5/2023 3:33 AM, Daniel Vetter wrote:
> > > > On Wed, Jan 04, 2023 at 03:40:33PM -0800, Jessica Zhang wrote:
> > > >> Introduce and add support for a solid_fill property. When the solid_fill
> > > >> property is set, and the framebuffer is set to NULL, memory fetch will be
> > > >> disabled.
> > > >>
> > > >> In addition, loosen the NULL FB checks within the atomic commit callstack
> > > >> to allow a NULL FB when the solid_fill property is set and add FB checks
> > > >> in methods where the FB was previously assumed to be non-NULL.
> > > >>
> > > >> Finally, have the DPU driver use drm_plane_state.solid_fill and instead of
> > > >> dpu_plane_state.color_fill, and add extra checks in the DPU atomic commit
> > > >> callstack to account for a NULL FB in cases where solid_fill is set.
> > > >>
> > > >> Some drivers support hardware that have optimizations for solid fill
> > > >> planes. This series aims to expose these capabilities to userspace as
> > > >> some compositors have a solid fill flag (ex. SOLID_COLOR in the Android
> > > >> hardware composer HAL) that can be set by apps like the Android Gears
> > > >> app.
> > > >>
> > > >> Userspace can set the solid_fill property to a blob containing the
> > > >> appropriate version number and solid fill color (in RGB323232 format) and
> > > >> setting the framebuffer to NULL.
> > > >>
> > > >> Note: Currently, there's only one version of the solid_fill blob property.
> > > >> However if other drivers want to support a similar feature, but require
> > > >> more than just the solid fill color, they can extend this feature by
> > > >> creating additional versions of the drm_solid_fill struct.
> > > >>
> > > >> Changes in V2:
> > > >> - Dropped SOLID_FILL_FORMAT property (Simon)
> > > >> - Switched to implementing solid_fill property as a blob (Simon, Dmitry)
> > > >> - Changed to checks for if solid_fill_blob is set (Dmitry)
> > > >> - Abstracted (plane_state && !solid_fill_blob) checks to helper method
> > > >>    (Dmitry)
> > > >> - Removed DPU_PLANE_COLOR_FILL_FLAG
> > > >> - Fixed whitespace and indentation issues (Dmitry)
> > > >
> > > > Now that this is a blob, I do wonder again whether it's not cleaner to set
> > > > the blob as the FB pointer. Or create some kind other kind of special data
> > > > source objects (because solid fill is by far not the only such thing).
> > > >
> > > > We'd still end up in special cases like when userspace that doesn't
> > > > understand solid fill tries to read out such a framebuffer, but these
> > > > cases already exist anyway for lack of priviledges.
> > > >
> > > > So I still think that feels like the more consistent way to integrate this
> > > > feature. Which doesn't mean it has to happen like that, but the
> > > > patches/cover letter should at least explain why we don't do it like this.
> > >
> > > Hi Daniel,
> > >
> > > IIRC we were facing some issues with this check [1] when trying to set
> > > FB to a PROP_BLOB instead. Which is why we went with making it a
> > > separate property instead. Will mention this in the cover letter.
> >
> > What kind of issues? Could you please describe them?
>
> We switched from bitmask to enum style for prop types, which means it's
> not possible to express with the current uapi a property which accepts
> both an object or a blob.
>
> Which yeah sucks a bit ...
>
> But!
>
> blob properties are kms objects (like framebuffers), so it should be
> possible to stuff a blob into an object property as-is. Of course you need
> to update the validation code to make sure we accept either an fb or a
> blob for the internal representation. But that kind of split internally is
> required no matter what I think.

I checked your idea and notes from Jessica. So while we can pass blobs
to property objects, the prop_fb_id is created as an object property
with the type DRM_MODE_OBJECT_FB. Passing DRM_MODE_OBJECT_BLOB would
fail a check in drm_property_change_valid_get() ->
__drm_mode_object_find(). And I don't think that we should break the
existing validation code for this special case.

If you insist on using FB_ID for passing solid_fill information, I'd
ask you to reconsider using a 1x1 framebuffer. It would be fully
compatible with the existing userspace, which can then treat it
seamlessly.

> -Daniel
>
> >
> > >
> > > [1]
> > > https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/drm_property.c#L71

-- 
With best wishes
Dmitry