linux-kernel - Re: [PATCH 1/2] drm: Add CRTC background color property

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Message-ID: <2a8ab8c3-36ae-46bd-8f98-6b3760532cb5@collabora.com>
Date: Tue, 2 Sep 2025 19:26:01 +0300
From: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
To: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Sandy Huang <hjc@...k-chips.com>, Heiko Stübner
 <heiko@...ech.de>, Andy Yan <andy.yan@...k-chips.com>, kernel@...labora.com,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org,
 Matt Roper <matthew.d.roper@...el.com>
Subject: Re: [PATCH 1/2] drm: Add CRTC background color property

On 9/2/25 4:36 PM, Ville Syrjälä wrote:
> On Tue, Sep 02, 2025 at 12:27:56PM +0300, Cristian Ciocaltea wrote:
>> Some display controllers can be hardware programmed to show non-black
>> colors for pixels that are either not covered by any plane or are
>> exposed through transparent regions of higher planes.  This feature can
>> help reduce memory bandwidth usage, e.g. in compositors managing a UI
>> with a solid background color while using smaller planes to render the
>> remaining content.
>>
>> To support this capability, introduce the BACKGROUND_COLOR standard DRM
>> mode property, which can be attached to a CRTC through the
>> drm_crtc_attach_background_color_property() helper function.
>>
>> Additionally, define a 64-bit ARGB format value to be built with the
>> help of a dedicated drm_argb64() utility macro.  Individual color
>> components can be extracted with desired precision using the
>> corresponding DRM_ARGB64_*() macros.
>>
>> Co-developed-by: Matt Roper <matthew.d.roper@...el.com>
>> Signed-off-by: Matt Roper <matthew.d.roper@...el.com>
>> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
>> ---
>>  drivers/gpu/drm/drm_atomic_state_helper.c |  1 +
>>  drivers/gpu/drm/drm_atomic_uapi.c         |  4 ++++
>>  drivers/gpu/drm/drm_blend.c               | 37 +++++++++++++++++++++++++++----
>>  drivers/gpu/drm/drm_mode_config.c         |  6 +++++
>>  include/drm/drm_blend.h                   |  4 +++-
>>  include/drm/drm_crtc.h                    | 12 ++++++++++
>>  include/drm/drm_mode_config.h             |  5 +++++
>>  include/uapi/drm/drm_mode.h               | 30 +++++++++++++++++++++++++
>>  8 files changed, 94 insertions(+), 5 deletions(-)

[...]

>> +/*
>> + * Put 16-bit ARGB values into a standard 64-bit representation that
>> + * can be used for ioctl parameters, inter-driver communication, etc.
>> + */
>> +static inline __u64
>> +drm_argb64(__u16 alpha, __u16 red, __u16 green, __u16 blue)
>> +{
>> +	return (__u64)alpha << 48 | (__u64)red << 32 | (__u64)green << 16 | blue;
>> +}
>> +
>> +/*
>> + * Extract the specified number of least-significant bits of a specific
>> + * color component from a standard 64-bit ARGB value.
> 
> Why would you ever want the least significant bits?

Right, that's useless - will replace with proper helpers dealing with custom
precision.

Thanks,
Cristian

>> + */
>> +#define DRM_ARGB64_COMP(c, shift, numlsb) \
>> +	((__u16)(((c) >> (shift)) & ((1UL << (numlsb) % 17) - 1)))
>> +#define DRM_ARGB64_ALPHA_LSB(c, numlsb) DRM_ARGB64_COMP(c, 48, numlsb)
>> +#define DRM_ARGB64_RED_LSB(c, numlsb)   DRM_ARGB64_COMP(c, 32, numlsb)
>> +#define DRM_ARGB64_GREEN_LSB(c, numlsb) DRM_ARGB64_COMP(c, 16, numlsb)
>> +#define DRM_ARGB64_BLUE_LSB(c, numlsb)  DRM_ARGB64_COMP(c, 0, numlsb)
>> +
>> +/*
>> + * Convenience wrappers to extract all 16 bits of a specific color
>> + * component from a standard 64-bit ARGB value.
>> + */
>> +#define DRM_ARGB64_ALPHA(c)		DRM_ARGB64_ALPHA_LSB(c, 16)
>> +#define DRM_ARGB64_RED(c)		DRM_ARGB64_RED_LSB(c, 16)
>> +#define DRM_ARGB64_GREEN(c)		DRM_ARGB64_GREEN_LSB(c, 16)
>> +#define DRM_ARGB64_BLUE(c)		DRM_ARGB64_BLUE_LSB(c, 16)
>> +
>>  #if defined(__cplusplus)
>>  }
>>  #endif
>>
>> -- 
>> 2.51.0
>