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

From: Daniel Vetter <daniel@ffwll.ch>
To: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: quic_abhinavk@quicinc.com, dri-devel@lists.freedesktop.org,
	swboyd@chromium.org, daniel.vetter@ffwll.ch,
	seanpaul@chromium.org, laurent.pinchart@ideasonboard.com,
	linux-arm-msm@vger.kernel.org,
	Jessica Zhang <quic_jesszhan@quicinc.com>,
	freedreno@lists.freedesktop.org
Subject: Re: [RFC PATCH 1/3] drm: Introduce color fill properties for drm plane
Date: Wed, 9 Nov 2022 14:59:21 +0100	[thread overview]
Message-ID: <Y2uyOcVbadRwr9/O@phenom.ffwll.local> (raw)
In-Reply-To: <ee755c43-434a-a990-0efa-ed5c6baa237e@linaro.org>

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@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.
- 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