[PATCH v2] drm/format-helper: Only advertise supported formats for conversion

[Hector Martin]

On 28/10/2022 17.07, Thomas Zimmermann wrote:
> In yesterday's discussion on IRC, it was said that several devices 
> advertise ARGB framebuffers when the hardware actually uses XRGB? Is 
> there hardware that supports transparent primary planes?

ARGB hardware probably exists in the form of embedded systems with
preconfigured blending. For example, one could imagine an OSD-type setup
where there is a hardware video scaler controlled entirely outside of
DRM/KMS (probably by a horrible vendor driver), and the overlay
framebuffer is exposed via simpledrm as a dumb memory region, and
expects ARGB to work. So ideally, we wouldn't expose XRGB8888 on
ARGB8888 systems.

But there is this problem:

arch/arm64/boot/dts/qcom/msm8998-oneplus-common.dtsi:
   format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sdm630-sony-xperia-nile.dtsi:
   format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sdm660-xiaomi-lavender.dts:
   format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sdm845-shift-axolotl.dts:
format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sdm850-samsung-w737.dts:
format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm6125-sony-xperia-seine-pdx201.dts:
           format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm6350-sony-xperia-lena-pdx213.dts:
           format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts:
format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm8150-sony-xperia-kumano.dtsi:
   format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm8250-sony-xperia-edo.dtsi:
   format = "a8r8g8b8";
arch/arm64/boot/dts/qcom/sm8350-sony-xperia-sagami.dtsi:
   format = "a8r8g8b8";
arch/arm64/boot/dts/socionext/uniphier-ld20-akebi96.dts:
format = "a8r8g8b8";

I'm pretty sure those phones don't have transparent screens, nor
magically put video planes below the firmware framebuffer. If there are
12 device trees for phones in mainline which lie about having alpha
support, who knows how many more exist outside? If we stop advertising
pretend-XRGB8888 on them, I suspect we're going to break a lot of
software...

Of course, there is one "correct" solution here: have an actual
xrgb8888->argb8888 conversion helper that just clears the high byte.
Then those platforms lying about having alpha and using xrgb8888 from
userspace will take a performace hit, but they should arguably just fix
their device tree in that case. Maybe this is the way to go in this
case? Note that there would be no inverse conversion (no advertising
argb8888 on xrgb8888 backends), so that one would be dropped vs. what we
have today. This effectively keeps the "xrgb8888 helpers and nothing
else" rule while actually supporting it for argb8888 backend
framebuffers correctly. Any platforms actually wanting to use argb8888
framebuffers with meaningful alpha should be configuring their userspace
to preferentially render directly to argb8888 to avoid the perf hit anyway.

- Hector