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/4b21ce1b4b5d262e7d4656b8ececc891fc3cb806
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);
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