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);
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