[alsa-devel] [PATCH 0/2] Introduce dmic mode switch delay parameter

[Pierre-Louis Bossart]

On 10/23/18 12:11 PM, Tc, Jenny wrote:
>> Allowing some timing adjustments for the clock transitions is a good
>> thing. The way it's done is questionable and raises a number of concerns.
>>
>> First, there was an Intel internal discussion before my extended break
>> on why this 'dmic-codec' is needed on Intel platforms. To the best of
>> my knowledge we don't control the mics with GPIOs, which was the
>> initial purpose of this driver. We have experimental evidence on
>> ApolloLake and GeminiLake that using the soc-dummy/soc-dummy-dai
>> definitions are enough, and it may be a good thing to agree on the
>> direction here. If you want a parameter, you can still use a machine
>> driver DMI-based kernel quirk and/or pass a kernel parameter, the need
>> to extend this dmic-codec is far from obvious to me.
> The driver already exposes another parameter (wakeup-delay-ms) using device tree.
> Enabling ACPI device enumeration provides a way to pass existing parameter
> and also cover the new parameter(modeswitch_delay_ms) introduced in this patch set.
> Isn't it good to adopt ACPI enumeration if the driver has multiple parameters to handle?

ACPI enumeration has nothing to do with multiple parameters. You use 
ACPI enumeration when you want the devices to be created based on a set 
of descriptors exposed by the DSDT instead of hard-coding the device 
support in the kernel.

>   
>> Assuming you still want to use this codec, then there are still major
>> concerns about the ACPI directions. As Mark noted it, "DMIC" does not
>> follow any of the guidelines or accepted definitions with an
>> unambiguous vendor and part ID. We know we already have conflicts
>> between Intel-defined ACPI IDs, e.g. for RT298 on multiple platforms,
>> let's be careful here, shall we?
> Agree, need to find proper ACPI ID for the device.

ok, but not sure what to define. You don't want too many identifiers 
either, this generated lots of patches for no good reason. What are the 
needs here? You probably don't want to identify the DMIC vendor so this 
could be an Intel-defined ID. But I wonder if this might be reused on 
AMD platforms?

>   
>> And I am coming to my last point. The Skylake driver already contains
>> code to create the dmic devices by hand (see below the git grep
>> results). So I wonder what happens if you use both ACPI-based
>> enumeration AND manually create the dmic device - I view these
>> solutions as mutually incompatible. Either you have not tested against
>> the upstream code or something is missing from your patchset. What am
>> I missing?
> Skl driver already registers a DMIC (dmic-codec) device and with ACPI enumeration
> one more device (DMIC:00) gets registered. The snd_soc_dai_link  structure populated
> in the machine driver decides which codec device to be used in the capture path and
> there by handles the compatibility issue you pointed out.

Wow. What's the point of having two devices? I am not against your 
solution but at the very least there should be something in the SKL 
driver to detect the presence of DMIC ACPI identifiers and only manually 
register the dmic-codec if no identifier was found.

Changing the dailink to point to one device instead of another is not a 
good idea, the machine driver should be independent from all this, and 
be reusable between the SKL driver or SOF drivers. The last thing you 
want is a hack in there.

>   
>
>> I forgot to add another open on ACPI support: what would be the scope of the "DMIC" device?
>> With ACPI we typically have a parent-child relationship, e.g. we put audio codec below the relevant
>> I2C/SPI controller in the DSDT definitions. In the absence of a DMIC bus, you need to be careful how
>> the DMIC device is added in the DSDT - it'd likely need to be below the scope of the HDaudio controller.
> In DSDT, the device is added under the Intel HDA (1f.3 for SKL/KBL) parent device.

ok, makes sense. Do you think it'd be possible to use ACPI initrd 
overlays to add support for those parameters if they don't natively 
exist in the BIOS?