[PATCH] drm/amdgpu: clip the ref divider max value at 100

[Christian König]

Am 04.11.20 um 11:40 schrieb Sharma, Shashank:
> [AMD Public Use]
>
> Hello Christian,
> Yes, that 100 is hardcoded in Radeon, and git blame says it was one of your patches which made it 100 from 128 😊.
> Would you mind having a look at commit id: 4b21ce1b4b5d262e7d4656b8ececc891fc3cb806 ?

Ah, yes that one :)

Yeah the background is that this was just an educated guess because I 
couldn't find anybody which could tell me what the real limits of the 
PLL is.

Looks like we just forgot to apply that patch to amdgpu.

Regards,
Christian.

>
> Regards
> Shashank
> -----Original Message-----
> From: Koenig, Christian <Christian.Koenig at amd.com>
> Sent: Wednesday, November 4, 2020 3:35 PM
> To: Sharma, Shashank <Shashank.Sharma at amd.com>; amd-gfx at lists.freedesktop.org
> Cc: Deucher, Alexander <Alexander.Deucher at amd.com>; Qin, Eddy <Eddy.Qin at amd.com>
> Subject: Re: [PATCH] drm/amdgpu: clip the ref divider max value at 100
>
> Am 03.11.20 um 18:13 schrieb Shashank Sharma:
>> This patch limits the ref_div_max value to 100, during the calculation
>> of PLL feedback reference divider. With current value (128), the
>> produced fb_ref_div value generates unstable output at particular
>> frequencies. Radeon driver limits this value at 100.
> Mhm, is that 100 hard coded in radeon? I have no idea where that is coming from.
>
> Best would probably to grab a hardware engineer and try to figure out what the real maximums for the PLL is to still produce a stable signal.
>
> Christian.
>
>> On Oland, when we try to setup mode 2048x1280 at 60 (a bit weird, I
>> know), it demands a clock of 221270 Khz. It's been observed that the
>> PLL calculations using values 128 and 100 are vastly different, and
>> look like this:
>>
>> +------------------------------------------+
>> |Parameter    |AMDGPU        |Radeon       |
>> |             |              |             |
>> +-------------+----------------------------+
>> |Clock feedback              |             |
>> |divider max  |  128         |   100       |
>> |cap value    |              |             |
>> |             |              |             |
>> |             |              |             |
>> +------------------------------------------+
>> |ref_div_max  |              |             |
>> |             |  42          |  20         |
>> |             |              |             |
>> |             |              |             |
>> +------------------------------------------+
>> |ref_div      |  42          |  20         |
>> |             |              |             |
>> +------------------------------------------+
>> |fb_div       |  10326       |  8195       |
>> +------------------------------------------+
>> |fb_div       |  1024        |  163        |
>> +------------------------------------------+
>> |fb_dev_p     |  4           |  9          |
>> |frac fb_de^_p|              |             |
>> +----------------------------+-------------+
>>
>> With ref_div_max value clipped at 100, AMDGPU driver can also drive
>> videmode 2048x1280 at 60 (221Mhz) and produce proper output without any
>> blanking and distortion on the screen.
>>
>> PS: This value was changed from 128 to 100 in Radeon driver also, here:
>> https://github.com/freedesktop/drm-tip/commit/4b21ce1b4b5d262e7d4656b8
>> ececc891fc3cb806
>>
>> Cc: Alex Deucher <Alexander.Deucher at amd.com>
>> Cc: Christian König <christian.koenig at amd.com>
>> Cc: Eddy Qin <Eddy.Qin at amd.com>
>>
>> Signed-off-by: Shashank Sharma <shashank.sharma at amd.com>
>> ---
>>    drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c | 2 +-
>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c
>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c
>> index 1f2305b7bd13..23a2e1ebf78a 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c
>> @@ -85,7 +85,7 @@ static void amdgpu_pll_get_fb_ref_div(unsigned nom, unsigned den, unsigned post_
>>    				      unsigned *fb_div, unsigned *ref_div)
>>    {
>>    	/* limit reference * post divider to a maximum */
>> -	ref_div_max = min(128 / post_div, ref_div_max);
>> +	ref_div_max = min(100 / post_div, ref_div_max);
>>    
>>    	/* get matching reference and feedback divider */
>>    	*ref_div = min(max(DIV_ROUND_CLOSEST(den, post_div), 1u),
>> ref_div_max);