Re: [PATCH] ASoC: meson: aiu: fifo-spdif: soft reset the S/PDIF datapath on start/stop

[Jerome Brunet <jbrunet@baylibre.com>]

On ven. 26 juin 2026 at 22:41, Martin Blumenstingl <martin.blumenstingl@googlemail.com> wrote:

> On Fri, Jun 26, 2026 at 10:04 AM Christian Hewitt
> <christianshewitt@gmail.com> wrote:
>>
>> The I2S FIFO soft-resets its fast domain on start (AIU_RST_SOFT bit 0 +
>> AIU_I2S_SYNC read in aiu_fifo_i2s_trigger), mirroring the downstream
>> vendor driver's audio_out_i2s_enable(). The S/PDIF FIFO has no equivalent:
>> it only toggles the IEC958 DCU, so a stale datapath FIFO can be replayed,
>> producing the "machine gun noise" buffer underrun - on start when switching
>> outputs, and on stop when playback ends. The latter is audible on devices
>> with an always-on S/PDIF-fed DAC (e.g. the ES7144 on the WeTek Play2).
>>
>> The vendor driver resets the IEC958 fast domain (AIU_RST_SOFT bit 2) on
>> both enable and disable (audio_hw_958_enable), and when reconfiguring
>> (audio_hw_958_reset clears AIU_958_DCU_FF_CTRL then resets). Do the same:
>> reset before enabling the DCU on start, and after disabling it on stop.
>>
>> Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
> Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
>
> This matches the vendor driver, references:
> - fast-reset SPDIF is triggered on enable and disable: [0]
> - fast-reset SPDIF is triggered after all of the configuration is
> written, then DCU_FF_CTRL is enabled: [1]

Take what the vendor driver does with a grain of salt, especially when
it comes to audio

>
> [...]
>>         case SNDRV_PCM_TRIGGER_SUSPEND:
>>         case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
>>         case SNDRV_PCM_TRIGGER_STOP:
>>                 fifo_spdif_dcu_enable(component, false);
>> +               snd_soc_component_write(component, AIU_RST_SOFT,
>> +                                       AIU_RST_SOFT_958_FAST);
> It doesn't seem to make any difference, so just for the record:
> The vendor driver first triggers AIU_RST_SOFT_958_FAST then disables DCU: [2]
>

One could even wonder if there is any point in applying a reset when
everything is stopped, if the enable path will apply this same
reset before anything else is started again ? Does it really changes
anything to the reported issue ?

>
> Best regards,
> Martin
>
>
> [0] https://github.com/endlessm/linux-s905x/blob/c1391ba1116aed801aaabfd9c7bbb76fda9801ec/sound/soc/aml/m8/aml_audio_hw.c#L1016
> [1] https://github.com/endlessm/linux-s905x/blob/c1391ba1116aed801aaabfd9c7bbb76fda9801ec/sound/soc/aml/m8/aml_audio_hw.c#L979-L983
> [2] https://github.com/endlessm/linux-s905x/blob/c1391ba1116aed801aaabfd9c7bbb76fda9801ec/sound/soc/aml/m8/aml_audio_hw.c#L1016-L1018

-- 
Jerome