Improve documentation by giving an overview of the components involved in the generation of the CDCLK. Signed-off-by: Gustavo Sousa <gustavo.sousa@xxxxxxxxx> --- drivers/gpu/drm/i915/display/intel_cdclk.c | 25 ++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c index 30dae4fef6cb..ef1660f94e46 100644 --- a/drivers/gpu/drm/i915/display/intel_cdclk.c +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c @@ -63,6 +63,31 @@ * 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: * - Full PLL disable + re-enable with new VCO frequency. Pipes must be inactive. -- 2.43.0
