15.01.2014 13:27, Jyri Sarha kirjoitti:
On 12/31/2013 03:25 PM, Mark Brown wrote:
On Fri, Dec 20, 2013 at 12:39:38PM +0200, Jyri Sarha wrote:
support. The only supported sample format is SNDRV_PCM_FORMAT_S32_LE. The 8 least significant bits are ignored.
Where does this constraint come from?
From driver/gpu/drm/i2c/tda998x_drv.c. The driver configures CTS_N register statically to a value that works only with 4 byte samples. According to my tests it is possible to support 3 and 2 byte samples too by changing the CTS_N register value, but I am not sure if the configuration can be changed on the fly. My data sheet of the nxp chip is very vague about the register definitions, but I suppose the register configures some clock divider on the chip. HDMI supports only upto 24bit audio and the data sheet states that any extraneous least significant bits are ignored.
That sounds strange, CTS/N values only depend on audio sample rate and TMDS/video clock, not on the audio format or the size of samples (HDMI spec sec 7.2 - Audio Sample Clock Capture and Regeneration).
Sure there isn't anything more going on (like the used HDMI sink being more tolerant to wrong CTS/N with 4-byte samples, or more likely something else I can't immediately think of)?
BTW, radeon driver has some code that calculates static CTS/N values since the hw autogeneration is not reliable there: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers... Not that it directly helps now, but just for the record...