[Intel-gfx] [PATCH 3/8] drm/i915/cdclk: Remove the assumption that cd2x div==2 when using squashing

[Gustavo Sousa]

Quoting Ville Syrjala (2023-11-28 08:51:33-03:00)
>From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
>Currently we have a hardcoded assumption that the cd2x divider
>is always 2 when squahsing is used.

It seems the code was actually making the assumption that the
divider is always 1. With the current code, when squashing is used, we
have that bxt_cdclk_cd2x_div_sel(dev_priv, clock, vco) is equivalent to
bxt_cdclk_cd2x_div_sel(dev_priv, vco / 2, vco), meaning that the
returned value will always be BXT_CDCLK_CD2X_DIV_SEL_1.

> While that is true for all
>current platforms it might not hold in the future. So eliminate

Not sure about the other platforms, but for MTL, I have checked the
BSpec table and also did some calculations by hand, and it seems like
the cd2x divider is actually always 1.

Unless I'm missing something in my analysis above, I believe s/2/1/ in
the commit message is necessary. With that,

Reviewed-by: Gustavo Sousa <gustavo.sousa at intel.com>

>the assumption and calculate the correct divider from the other
>parameters.
>
>Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
>---
> drivers/gpu/drm/i915/display/intel_cdclk.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
>diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
>index 87d5e5b67c4e..d45071675629 100644
>--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
>+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
>@@ -1899,10 +1899,8 @@ static void _bxt_set_cdclk(struct drm_i915_private *dev_priv,
> 
>         waveform = cdclk_squash_waveform(dev_priv, cdclk);
> 
>-        if (waveform)
>-                clock = vco / 2;
>-        else
>-                clock = cdclk;
>+        clock = DIV_ROUND_CLOSEST(cdclk * cdclk_squash_len,
>+                                  cdclk_squash_divider(waveform));

Nit: maybe take the opportunity to rename "clock" to "unsquashed" for
clarity?

> 
>         if (HAS_CDCLK_SQUASH(dev_priv))
>                 dg2_cdclk_squash_program(dev_priv, waveform);
>-- 
>2.41.0
>