PROBLEM: EDID regression results in color banding on Lenovo G50 series laptops [fix proposition]

[Jani Nikula]

On Mon, 08 May 2017, Tomasz Papież <tmszpp at gmail.com> wrote:
> Since the introduction of kernel 4.8 I've been experiencing color 
> banding on my Lenovo G50-80 notebook. I also had reports of the same 
> symptoms on the G50-70 model.
>
> I figured out that the problem had been introduced by commit 
> 210a021dab639694600450c14b877bf3e3240adc

commit 210a021dab639694600450c14b877bf3e3240adc
Author: Mario Kleiner <mario.kleiner.de at gmail.com>
Date:   Wed Jul 6 12:05:48 2016 +0200

    drm/edid: Set 8 bpc color depth for displays with "DFP 1.x compliant TMDS".

Cc: Mario.


>
> The G50-80's LCD panel supports 6bpc, but is now set to 8bpc,
> and the system probably falls back to 6bpc
> without any dithering, which result in heavily banded gradients.
>
> I fixed it for myself by adding an EDID quirk for that particular LCD 
> panel (see the attached patch). It's unknown to me, however, if there 
> are any other panels affected by the issue, so maybe a more general
> solution might be desirable.
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index fad3d44e4642..ee3124bcae87 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -147,6 +147,9 @@ static const struct edid_quirk {
>  	/* Medion MD 30217 PG */
>  	{ "MED", 0x7b8, EDID_QUIRK_PREFER_LARGE_75 },
>  
> +	/* Fix for Lenovo G50 */
> +	{ "SDC", 18514, EDID_QUIRK_FORCE_6BPC },
> +
>  	/* Panel in Samsung NP700G7A-S01PL notebook reports 6bpc */
>  	{ "SEC", 0xd033, EDID_QUIRK_FORCE_8BPC },
>  
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-- 
Jani Nikula, Intel Open Source Technology Center