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

[Jocelyn Falempe]

On 13/10/2022 11:05, Thomas Zimmermann wrote:
> 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.

I think it might be because of the "clock * 2" trick for this model.
(so N parameter is half of what it should be, and doesn't have BIT(8) 
set). But I don't have the G200SE A specific hardware spec either.


> 
> 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.

Thanks, I will send a v2 shortly.

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