On 27/02/2019 20:00, Russell King - ARM Linux admin wrote: > 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 >> The suggestion below would be my first choice. Packing the same information that we already have in sample_width to bclk_ratio does not make too much sense. >> 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. > Knowing how limited the support to those formats is in Linux userspace, I would say that it is terribly unlikely anybody uses those formats. Before the currently worked on bclk_ratio, those formats would only have worked if the CPU DAI would implicitly use 48-bit bclk_ratio. I would say that is quite unlikely too. I such a case exists or emerges later, then they just need start using the bclk_ratio callback in their machine driver. But still, I think leaving the bclk_ratio to zero if it is not set is a better choice. Best regards, Jyri -- Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx https://mailman.alsa-project.org/mailman/listinfo/alsa-devel
