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@xxxxxxx>
> Sent: Wednesday, November 4, 2020 3:35 PM
> To: Sharma, Shashank
<Shashank.Sharma@xxxxxxx>;
amd-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: Deucher, Alexander
<Alexander.Deucher@xxxxxxx>; Qin, Eddy
<Eddy.Qin@xxxxxxx>
> 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@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@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@xxxxxxx>
>> Cc: Christian König
<christian.koenig@xxxxxxx>
>> Cc: Eddy Qin
<Eddy.Qin@xxxxxxx>
>>
>> Signed-off-by: Shashank Sharma
<shashank.sharma@xxxxxxx>
>> ---
>> 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);
