drm/i915/cdclk: Document CDCLK components

Improve documentation by giving an overview of the components involved
in the generation of the CDCLK.

v2: Fix htmldoc error because of missing blank line at the start of
    bulleted list.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221185131.287302-2-gustavo.sousa@intel.com

1 files changed, 26 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
index 77a25caacc16..22473c55b899 100644
--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
@@ -63,6 +63,32 @@
  * DMC will not change the active CDCLK frequency however, so that part
  * will still be performed by the driver directly.
  *
+ * There are multiple components involved in the generation of the CDCLK
+ * frequency:
+ *
+ * - We have the CDCLK PLL, which generates an output clock based on a
+ *   reference clock and a ratio parameter.
+ * - The CD2X Divider, which divides the output of the PLL based on a
+ *   divisor selected from a set of pre-defined choices.
+ * - The CD2X Squasher, which further divides the output based on a
+ *   waveform represented as a sequence of bits where each zero
+ *   "squashes out" a clock cycle.
+ * - And, finally, a fixed divider that divides the output frequency by 2.
+ *
+ * As such, the resulting CDCLK frequency can be calculated with the
+ * following formula:
+ *
+ *     cdclk = vco / cd2x_div / (sq_len / sq_div) / 2
+ *
+ * , where vco is the frequency generated by the PLL; cd2x_div
+ * represents the CD2X Divider; sq_len and sq_div are the bit length
+ * and the number of high bits for the CD2X Squasher waveform, respectively;
+ * and 2 represents the fixed divider.
+ *
+ * Note that some older platforms do not contain the CD2X Divider
+ * and/or CD2X Squasher, in which case we can ignore their respective
+ * factors in the formula above.
+ *
  * Several methods exist to change the CDCLK frequency, which ones are
  * supported depends on the platform:
  *