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

[Jocelyn Falempe]

On 13/10/2022 12:36, Ville Syrjälä wrote:
> On Thu, Oct 13, 2022 at 11:05:19AM +0200, 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.
> 
> Could measure eg. the vblank interval with and without that bit set
> and see what effect it has. Assuming the PLL locks without the bit
> of course.
> 

Thanks for the pointer, but I don't have physical access to the system, 
so I'm not able to check that.
I think reverting to the previous setting that used to work is the safer 
approach here.

-- 

Jocelyn