[alsa-devel] [PATCH 1/8] soundwire: intel: filter SoundWire controller device search

[Pierre-Louis Bossart]

On 5/7/19 7:26 AM, Vinod Koul wrote:
> On 03-05-19, 19:29, Pierre-Louis Bossart wrote:
>> The convention is that the SoundWire controller device is a child of
>> the HDAudio controller. However there can be more than one child
>> exposed in the DSDT table, and the current namespace walk returns the
>> last device.
>>
>> Add a filter and terminate early when a valid _ADR is provided,
>> otherwise keep iterating to find the next child.
> 
> So what are the other devices in DSDT here..

this is what I see:

Scope (HDAS)
         {
             Device (IDA)
             {
                 Name (_ADR, 0x00020001)  // _ADR: Address
             }
         }

I thought this was nonsense but your question triggered me to look into 
the Intel SST ACPI specs (not public I am afraid but shared with the OS 
who shall not be named).
Using the same source of information as below, I *believe* this is 
HDaudio related, bits 31..16 mean HDaudio with codec SDI 2, and NodeId 1 
for the function group. This would make sense as I believe there are two 
codecs on the board that can be pin-strapped to boot either in HDaudio 
or SoundWire mode- but this is a conjecture only.

At any rate, we need a hardware rework and mutual exclusion between 
HDaudio and SoundWire, so we have to ignore this one when SoundWire is 
enabled.

>> +
>> +	/*
>> +	 * On some Intel platforms, multiple children of the HDAS
>> +	 * device can be found, but only one of them is the SoundWire
>> +	 * controller. The SNDW device is always exposed with
>> +	 * Name(_ADR, 0x40000000) so filter accordingly
>> +	 */
>> +	if (adr != 0x40000000)
> 
> I do not recall if 4 corresponds to the links you have or soundwire
> device type, is this number documented somewhere is HDA specs?

I thought it was a magic number, but I did check and for once it's 
documented and the values match the spec :-)
I see in the ACPI docs bits 31..28 set to 4 indicate a SoundWire Link 
Type and bits 3..0 indicate the SoundWire controller instance, the rest 
is reserved to zero.

> 
> Also it might good to create a define for this

I will respin this one to add the documentation above, and only filter 
on the 4 ms-bits. Thanks for forcing me to RTFM :-)