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

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 6 Jan 2023 23:49:34 +0200
Dmitry Baryshkov <dmitry.baryshkov@linaro.org> wrote:

> On Fri, 6 Jan 2023 at 20:41, Daniel Vetter <daniel@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@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.

Hi,

indeed, what about simply using a 1x1 framebuffer for real? Why was that
approach rejected?

Is there some problem with drivers just special-casing 1x1 framebuffers
and hitting the solid-fill hardware path instead of
framebuffer-with-scaling hardware path?

If needed, the KMS plane could have a property that tells userspace
that if you set a 1x1 RGB FB here without any color ops, I am able to
scale that to *any* size without any limits very efficiently.


Thanks,
pq

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


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