RE: [PATCH 0/3] ASoC/soundwire: remove ghost peripherals from the mach table

[Liao, Bard]

> -----Original Message-----
> From: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxx>
> Sent: Wednesday, September 16, 2026 3:09 AM
> To: Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>; linux-
> sound@xxxxxxxxxxxxxxx; vkoul@xxxxxxxxxx; broonie@xxxxxxxxxx;
> tiwai@xxxxxxx
> Cc: vinod.koul@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> peter.ujfalusi@xxxxxxxxxxxxxxx; Liao, Bard <bard.liao@xxxxxxxxx>
> Subject: Re: [PATCH 0/3] ASoC/soundwire: remove ghost peripherals from the
> mach table
> 
> On 9/15/26 15:13, Bard Liao wrote:
> > ACPI may report a ghost SoundWire peripheral. It will cause unexpected
> > error like duplicated links, codec driver can't probe, etc. This series
> > check the presence of SoundWire peripherals and skip the non-existing
> > peripherals.
> 
> I am afraid this raises quite a few opens, such as the 'mockup' support
> and delayed enumeration.

The mockup support will not change. We will only check the existance if
no mach item matched. And we can always add the mochup configurations to
the mach table.
And indeed, it will add a little delay in some cases. However, based on
our test, only about 1 ms delay will be added.

> 
> A better way to only deal with actual codecs would be to only probe
> codec drivers when the codecs report as ATTACHED and get enumerated,
> instead of during the ACPI parsing stage.

But this doesn't solve our issue. The DAI link will still be created
and the sound card will not probe because the codec driver doesn't probe.
Our target is that the ghost device should not be added in the DAI link.
So that the sound card can probe properly.

> This is a solution that was discussed a ong time ago, probably circa
> 2016, during one of the LPC miniconferences, and the direction from
> maintainers was that the probe could be used to enable resources (power,
> gpio, clocks) that might be required for the hardware codec to become
> functional and report as ATTACHED. That's the reason why the probe is
> done on all codecs exposed in ACPI, even 'ghost' ones, with an
> update_status() callback to the codec driver when the presence of that
> codec is detected on the bus.
> 
> In practice I am not aware of any codec drivers doing anything with
> power/gpio/clocks in the probe stages, at least for ACPI platforms, so
> it may be a good time to revisit this direction. SDCA class drivers do
> exactly what I described, the subdevices are registered only upon
> enumeration, not during ACPI parsing. It's a much simpler design with a
> lot fewer potential races.
> 
> Problems:
> - this would be a very invasive change to sdw_slave_add(), with the
> device_register() skipped and moved to the enumeration stage. It'd have
> to be opt-in and used only a newer platforms to avoid breaking the
> 'legacy' devices.

I didn't change sdw_slave_add(). All the change in SoundWire driver is to
add a flag and a completion to share the information of whether the
enumeration of the bus is completed.

> - there is still *nothing* that would tell you that all codec hardware
> on a given platform completed the enumeration. You could have a fixed
> delay but this would need to be large enough to cover all cases and that
> could make the platform boot slower than the current solution - not ideal.

Yeah, I set a 3 sec timeout. I believe that is large enough. And the flag
and the completion is to avoid the meaningless waiting. With the
is_present flag, we can ensure a bus will not be waiting for more than 1
time. And with the enumeration_complete completion, we won't wait for the
ghost device to be enumerated.

> 
> Another option would be to parse the ACPI0018 device, which describes
> audio endpoints and makes references to codecs, which could be used to
> filter out 'ghost' codecs that don't provide any endpoints. The spec for
> this ACPI0018 is not public but could be reverse-engineered from Windows
> platforms. The main benefit is that this filtering could be done in the
> ACPI parsing stages and not change anything in the probe and startup
> sequences.

Is ACPI0018 reliable? Will it still list the ghost device? The ghost rt722
has all endpoints listed in its DisCo table