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

[Christian König]

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/4b21ce1b4b5d262e7d4656b8ececc891fc3cb806
>
> 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);