Re: [PATCH 6/6] arm64: dts: qcom: sm7225-fairphone-fp4: Enable USB audio offload support

["Luca Weiss" <luca.weiss@fairphone.com>]

On Mon Apr 28, 2025 at 11:43 PM CEST, Konrad Dybcio wrote:
> On 4/28/25 9:41 AM, Luca Weiss wrote:
>> On Fri Apr 25, 2025 at 11:06 PM CEST, Konrad Dybcio wrote:
>>> On 4/25/25 12:44 PM, Luca Weiss wrote:
>>>> Enable USB audio offloading which allows to play audio via a USB-C
>>>> headset with lower power consumption and enabling some other features.
>>>>
>>>> This can be used like the following:
>>>>
>>>>   $ amixer -c0 cset name='USB_RX Audio Mixer MultiMedia1' On
>>>>   $ aplay --device=plughw:0,0 test.wav
>>>>
>>>> Compared to regular playback to the USB sound card no interrupts should
>>>> appear on the xhci-hcd interrupts during playback, instead the ADSP will
>>>> be handling the playback.
>>>
>>> "should" isn't very optimistic - I assume this works for you? > 
>> 
>> Yes it does!
>> 
>> With 'should' I meant to describe the expected behavior from using this
>> since most people are probably not familiar with how this works.
>> 
>>>> Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
>>>> ---
>
> [...]
>
>>>>  &usb_1_dwc3 {
>>>>  	maximum-speed = "super-speed";
>>>>  	dr_mode = "otg";
>>>> +	num-hc-interrupters = /bits/ 16 <3>;
>>> Where does this number come from?
>> 
>> I'm honestly not 100% sure. As far as I understand it, with
>> 'qcom,usb-audio-intr-idx = /bits/ 16 <2>;' in the qcom,q6usb node (which
>> I've checked against downstream) we declare which "XHCI interrupter
>> number to use". Without the num-hc-interrupters property we get an error
>> that not enough interrupters are available (I assume only 1 is then), so
>> this value practically needs to be higher than the <2> from earlier.
>> 
>> Why it's this value and not a higher value e.g. 4 I'm not really sure.
>> Downstream code looks somewhat different and "max_interrupters" in
>> drivers/usb/ doesn't come from a dt property. I'd need to check more in
>> details what this code does - or maybe Wesley can help.
>
> I got word that it's simply hw specific - please move it over to the
> soc dt with the value of 3

Will do, thanks for checking!

Would you have a reference how to get the correct value for it based on
downstream or the running kernel on the hw?

Regards
Luca

>
> Konrad