[PATCH] drm/mgag200: Fix PLL setup for G200_SE_A rev >=4

[Thomas Zimmermann]

Hi

Am 13.10.22 um 10:29 schrieb Jocelyn Falempe:
> For G200_SE_A, PLL M setting is wrong, which leads to blank screen,
> or "signal out of range" on VGA display.
> previous code had "m |= 0x80" which was changed to
> m |= ((pixpllcn & BIT(8)) >> 1);
> 
> Tested on G200_SE_A rev 42
> 
> This line of code was moved to another file with
> commit 85397f6bc4ff ("drm/mgag200: Initialize each model in separate
> function") but can be easily backported before this commit.
> 
> Fixes: 2dd040946ecf ("drm/mgag200: Store values (not bits) in struct mgag200_pll_values")
> Cc: stable at vger.kernel.org
> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
> ---
>   drivers/gpu/drm/mgag200/mgag200_g200se.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/mgag200/mgag200_g200se.c b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> index be389ed91cbd..4ec035029b8b 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_g200se.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_g200se.c
> @@ -284,7 +284,7 @@ static void mgag200_g200se_04_pixpllc_atomic_update(struct drm_crtc *crtc,
>   	pixpllcp = pixpllc->p - 1;
>   	pixpllcs = pixpllc->s;
>   
> -	xpixpllcm = pixpllcm | ((pixpllcn & BIT(8)) >> 1);
> +	xpixpllcm = pixpllcm | BIT(7);

Thanks for figuring this out. G200SE apparently is special compared to 
the other models. The old MGA docs only list this bit as <reserved>. 
Really makes me wonder why this is different.

Please write it as

   BIT(7) | pixpllcm

so that bit settings are ordered MSB-to-LSB and include a one-line 
comment that says that G200SE needs to set this bit unconditionally.

Best regards
Thomas



>   	xpixpllcn = pixpllcn;
>   	xpixpllcp = (pixpllcs << 3) | pixpllcp;
>   

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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