drm/i915/cdclk: Remove the hardcoded divider from cdclk_compute_crawl_and_squash_midpoint()

cdclk_compute_crawl_and_squash_midpoint() was still assuming
that cd2x divider == 1 (ie. full divider == 2). Remove that
assumption by computing the dividers properly.

We'll also toss in a WARN in case the divider somehow ends
up different between the old and new cdclk configs. That should
never happen given we have div==2 in all the cdclk table entries
for the affected platforms.

If in the future we need a config where the divider also needs
to be changed then we likely need to add an extra step into the
cdclk programming sequence to make sure things stay within
legal limits throughout the process.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240207013334.29606-4-ville.syrjala@linux.intel.comReviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>

drivers/gpu/drm/i915/display/intel_cdclk.c

@@ -1846,7 +1846,7 @@ static bool cdclk_compute_crawl_and_squash_midpoint(struct drm_i915_private *i91
 						    struct intel_cdclk_config *mid_cdclk_config)
 {
 	u16 old_waveform, new_waveform, mid_waveform;
-	int div = 2;
+	int old_div, new_div, mid_div;
 
 	/* Return if PLL is in an unknown state, force a complete disable and re-enable. */
 	if (cdclk_pll_is_unknown(old_cdclk_config->vco))
@@ -1865,6 +1865,18 @@ static bool cdclk_compute_crawl_and_squash_midpoint(struct drm_i915_private *i91
 	    old_waveform == new_waveform)
 		return false;
 
+	old_div = cdclk_divider(old_cdclk_config->cdclk,
+				old_cdclk_config->vco, old_waveform);
+	new_div = cdclk_divider(new_cdclk_config->cdclk,
+				new_cdclk_config->vco, new_waveform);
+
+	/*
+	 * Should not happen currently. We might need more midpoint
+	 * transitions if we need to also change the cd2x divider.
+	 */
+	if (drm_WARN_ON(&i915->drm, old_div != new_div))
+		return false;
+
 	*mid_cdclk_config = *new_cdclk_config;
 
 	/*
@@ -1877,15 +1889,17 @@ static bool cdclk_compute_crawl_and_squash_midpoint(struct drm_i915_private *i91
 
 	if (cdclk_squash_divider(new_waveform) > cdclk_squash_divider(old_waveform)) {
 		mid_cdclk_config->vco = old_cdclk_config->vco;
+		mid_div = old_div;
 		mid_waveform = new_waveform;
 	} else {
 		mid_cdclk_config->vco = new_cdclk_config->vco;
+		mid_div = new_div;
 		mid_waveform = old_waveform;
 	}
 
 	mid_cdclk_config->cdclk = DIV_ROUND_CLOSEST(cdclk_squash_divider(mid_waveform) *
 						    mid_cdclk_config->vco,
-						    cdclk_squash_len * div);
+						    cdclk_squash_len * mid_div);
 
 	/* make sure the mid clock came out sane */