On Wed, Feb 27, 2019 at 07:48:33PM +0200, Jyri Sarha wrote: > On 27/02/2019 13:47, Russell King - ARM Linux admin wrote: > > On Mon, Feb 25, 2019 at 10:58:34PM +0200, Jyri Sarha wrote: > >> I think there are other options too. The obvious one would be passing > >> the blck_ratio callback trough HDMI-codec to HDMI-bridge-driver, but > >> yes, I do not like that either. > >> > >> The other would be changing the implicit bclk_ratio to 2 x sample-width, > >> and accepting blck_ratios of 36 and 40 in tda998x, and set CTS_N_M = 3 > >> and CTS_N_K = 2 for them too. > > To put a bit of further information out there... > > > > I really don't like that idea - what if we have a transmitter that > > really does use a bclk_ratio of 36 or 40? That would mess up the > > CTS calculation in the TDA998x. > > > > The equation I've come up to fit what we know for TDA998x is: > > > > k = m * bclk_ratio / 128 > > > > where k and m are parameters that we values we select for TDA998x. > > This reflects the entire clock regeneration system including the sink. > > The possible values of k are 1, 2, 3, 4, or 8. m are 1, 2, 4, or 8. > > I also have this equation which defines the fs regenerated at the sink: > > > > fso = bclk_ratio * fsi * m / (128 * k) > > > > If we did have a ratio of 36 or 40, the first equation above fails > > since k is not one of the possible integers. If we round down and use > > e.g. 2 for the 36fs case, then we'd actually end up with a sample > > frequency on the sink of 1.125x faster than it should be, and the sink > > would suffer starvation of audio samples. If we rounded up to 3, then > > the sample frequency will be 3/4 of it's true value, so the sink will > > overflow and would have to discard audio samples. > > > > We know this happens, because that's the root of the problem at hand: > > wrongly setting the m and k values for the bclk_ratio being supplied > > to the TDA998x causes audio to be corrupted when reproduced by the > > sink. > > > > Given that, we do need some way to validate the bclk_ratio when it is > > set, and not during hw_params which would (a) lead to ALSA devices > > failing when userspace is negotiating the parameters and (b) gives no > > way in the kernel for set_bclk_ratio() to discover whether a particular > > ratio is supported by the codec. > > > > So, I think there's three possible ways around this: > > 1. adding a set_bclk_ratio() method to hdmi_codec_ops > > 2. calling hw_params() when our set_bclk_ratio() method is called > > (what if the rest of the format isn't set or incompatible, which > > may cause the hw_params() method to fail?) > > 3. adding a list of acceptable bclk_ratio values to hdmi_codec_pdata > > Or leaving the bclk_ratio field zero in struct hdmi_codec_params if > set_bclk_ratio() is not called and leaving the bridge driver to decide > what to do in such a situation. > > And in tda998x_audio_hw_params() doing something like this: > > if (audio.bclk_ratio == 0) > audio.bclk_ratio = DIV_ROUND_UP(params->sample_width, 8) * 8; > > But then again I think it would be quite sane option to set bclk_ration > in hdmi-codec to 2*sample_width and simply refuse the ratios of 36 or 40 > in tda998x. ... which then leads to breakage of userspace if 18 or 20 bit formats were attempted, making the bridge driver's capabilities undiscoverable. Returning an error from hw_params causes hard failures. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx https://mailman.alsa-project.org/mailman/listinfo/alsa-devel
