Russell,
On Sat, Aug 8, 2015 at 9:10 AM, Russell King rmk+kernel@arm.linux.org.uk wrote:
Given the TDMS clock, audio sample rate, and the N parameter, we can calculate the CTS value for the audio clock regenerator (ACR) using the following calculation given in the HDMI specification:
CTS = ftdms * N / (128 * fs)The specification says that the CTS value is an average value, which is true if the source hardware measures it. Where source hardware needs it to be programmed, it is particularly difficult to alternate it between two values correctly to ensure that we achieve a correct "average" fractional value at the sink.
Also, there's the problem that our "ftdms" is not a fully accurate value; it is rounded to a kHz value. This introduces an unnecessary (and harmless) fractional value into the above equation for combinations like 148.5MHz/1.001 for 44100Hz - we still calculate the correct CTS value.
Signed-off-by: Russell King rmk+kernel@arm.linux.org.uk
drivers/gpu/drm/bridge/dw_hdmi.c | 92 +++++++--------------------------------- 1 file changed, 16 insertions(+), 76 deletions(-)
If you take my feedback about your "drm: bridge/dw_hdmi: adjust pixel clock values in N calculation" patch [1], all this math just works out to "cts = pixel_clk / 1000". ...but doing the math does future proof us a bit, so it seems like a good idea.
Reviewed-by: Douglas Anderson dianders@chromium.org Tested-by: Douglas Anderson dianders@chromium.org