linux-kernel - Re: [PATCH v4 4/5] drm/mipi-dbi: Add support for DRM_FORMAT

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Date: Tue, 4 Jun 2024 16:44:24 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: noralf@...nnes.org
Cc: Neil Armstrong <neil.armstrong@...aro.org>, 
	Jessica Zhang <quic_jesszhan@...cinc.com>, Sam Ravnborg <sam@...nborg.org>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	David Lechner <david@...hnology.com>, dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Kamlesh Gurudasani <kamlesh.gurudasani@...il.com>, 
	Tommaso Merciai <tommaso.merciai@...rulasolutions.com>
Subject: Re: [PATCH v4 4/5] drm/mipi-dbi: Add support for DRM_FORMAT_RGB888

On Tue, Jun 04, 2024 at 03:20:31PM +0200, Noralf Trønnes via B4 Relay wrote:
> From: Noralf Trønnes <noralf@...nnes.org>
> 
> DRM_FORMAT_RGB888 is 24 bits per pixel and it would be natural to send it
> on the SPI bus using a 24 bits per word transfer. The problem with this
> is that not all SPI controllers support 24 bpw.
> 
> Since DRM_FORMAT_RGB888 is stored in memory as little endian and the SPI
> bus is big endian we use 8 bpw to always get the same pixel format on the
> bus: b8g8r8.
> 
> The MIPI DCS specification lists the standard commands that can be sent
> over the MIPI DBI interface. The set_address_mode (36h) command has one
> bit in the parameter that controls RGB/BGR order. This means that the
> controller can be configured to receive the pixel as BGR.
> 
> RGB888 is rarely supported on these controllers but RGB666 is very common.
> All datasheets I have seen do at least support the pixel format option
> where each color is sent as one byte and the 6 MSB's are used.
> 
> All this put together means that we can send each pixel as b8g8r8 and an
> RGB666 capable controller sees this as b6x2g6x2r6x2.
> 
> v4:
> - s/emulation_format/pixel_format/ (Dmitry)
> 
> Signed-off-by: Noralf Trønnes <noralf@...nnes.org>
> ---
>  drivers/gpu/drm/drm_mipi_dbi.c | 29 +++++++++++++++++++++++++----
>  include/drm/drm_mipi_dbi.h     |  5 +++++
>  2 files changed, 30 insertions(+), 4 deletions(-)
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>


-- 
With best wishes
Dmitry