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

[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.

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);
-- 
2.25.1