[PATCH v3 1/8] drm/blend: Add a generic alpha property

[Stefan Schake]

On Fri, Feb 16, 2018 at 7:20 PM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> On Fri, Feb 16, 2018 at 06:39:29PM +0100, Maxime Ripard wrote:
>> Some drivers duplicate the logic to create a property to store a per-plane
>> alpha.
>>
>> This is especially useful if we ever want to support extra protocols for
>> Wayland like:
>> https://lists.freedesktop.org/archives/wayland-devel/2017-August/034741.html
>>
>> Let's create a helper in order to move that to the core.
>>
>> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>> Reviewed-by: Boris Brezillon <boris.brezillon at bootlin.com>
>> Signed-off-by: Maxime Ripard <maxime.ripard at bootlin.com>
>> ---
>>  Documentation/gpu/kms-properties.csv |  2 +-
>>  drivers/gpu/drm/drm_atomic.c         |  4 ++++-
>>  drivers/gpu/drm/drm_atomic_helper.c  |  4 ++++-
>>  drivers/gpu/drm/drm_blend.c          | 32 +++++++++++++++++++++++++++++-
>>  include/drm/drm_blend.h              |  1 +-
>>  include/drm/drm_plane.h              |  6 +++++-
>>  6 files changed, 48 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/gpu/kms-properties.csv b/Documentation/gpu/kms-properties.csv
>> index 927b65e14219..25ad3503d663 100644
>> --- a/Documentation/gpu/kms-properties.csv
>> +++ b/Documentation/gpu/kms-properties.csv
>> @@ -99,5 +99,5 @@ radeon,DVI-I,“coherent”,RANGE,"Min=0, Max=1",Connector,TBD
>>  ,,"""underscan vborder""",RANGE,"Min=0, Max=128",Connector,TBD
>>  ,Audio,“audio”,ENUM,"{ ""off"", ""on"", ""auto"" }",Connector,TBD
>>  ,FMT Dithering,“dither”,ENUM,"{ ""off"", ""on"" }",Connector,TBD
>> -rcar-du,Generic,"""alpha""",RANGE,"Min=0, Max=255",Plane,TBD
>> +,,"""alpha""",RANGE,"Min=0, Max=Driver dependant",Plane,Opacity of the plane from transparent (0) to fully opaque (MAX). If this property is set to a value different than max, and that the pixel will define an alpha component, the property will have precendance and the pixel value will be ignored.
>
> Those semantics don't seem particularly good to me. I think we would want the
> per-pixel alpha and global alpha both to be effecive at the same time. You can
> always decide to ignore the per-pixel alpha by using a pixel format without
> alpha.
>
> Also, where's the userspace that wants this feature?

drm_hwcomposer uses an 8-bit per-plane alpha property, and from what I
can tell the semantics are that both pixel and plane alpha apply
simultaneously if present.
Here is what I think was the kernel-side for this:

https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/306122

I've added Sean Paul, he might be able to give a more definitive answer.

Regards,
Stefan