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

[Christian Hewitt]

> On 29 Jun 2026, at 4:04 pm, Jerome Brunet <jbrunet at baylibre.com> wrote:
> 
> On ven. 26 juin 2026 at 22:41, Martin Blumenstingl <martin.blumenstingl at googlemail.com> wrote:
> 
>> On Fri, Jun 26, 2026 at 10:04 AM Christian Hewitt
>> <christianshewitt at 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 at gmail.com>
>> Reviewed-by: Martin Blumenstingl <martin.blumenstingl at 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 ?

Using disable/reset or reset/disable didn’t make any difference in my
testing, but without any reset playback stop on the WeTek Play2 board
with hardwired DAC always triggers the buffer underrun.

CH.