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

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Message-Id: <D9I3OWQF8T3Y.1Q5U9E2RI5YZX@fairphone.com>
Date: Mon, 28 Apr 2025 09:41:10 +0200
From: "Luca Weiss" <luca.weiss@...rphone.com>
To: "Konrad Dybcio" <konrad.dybcio@....qualcomm.com>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Rob Herring" <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Felipe Balbi" <balbi@...nel.org>, "Srinivas Kandagatla"
 <srini@...nel.org>, "Liam Girdwood" <lgirdwood@...il.com>, "Mark Brown"
 <broonie@...nel.org>, "Jaroslav Kysela" <perex@...ex.cz>, "Takashi Iwai"
 <tiwai@...e.com>, "Bjorn Andersson" <andersson@...nel.org>, "Konrad Dybcio"
 <konradybcio@...nel.org>, "Wesley Cheng" <quic_wcheng@...cinc.com>,
 "Stephan Gerhold" <stephan.gerhold@...aro.org>
Cc: <~postmarketos/upstreaming@...ts.sr.ht>, <phone-devel@...r.kernel.org>,
 <linux-usb@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski@...aro.org>, <linux-arm-msm@...r.kernel.org>,
 <linux-sound@...r.kernel.org>
Subject: Re: [PATCH 6/6] arm64: dts: qcom: sm7225-fairphone-fp4: Enable USB
 audio offload support

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@...rphone.com>
>> ---
> [...]
>
>> +&sound {
>> +	compatible = "fairphone,fp4-sndcard";
>> +	model = "Fairphone 4";
>> +
>> +	mm1-dai-link {
>> +		link-name = "MultiMedia1";
>> +
>> +		cpu {
>> +			sound-dai = <&q6asmdai MSM_FRONTEND_DAI_MULTIMEDIA1>;
>> +		};
>> +	};
>> +
>> +	usb-dai-link {
>> +		link-name = "USB Playback";
>> +
>> +		cpu {
>> +			sound-dai = <&q6afedai USB_RX>;
>> +		};
>> +
>> +		codec {
>> +			sound-dai = <&q6usbdai USB_RX>;
>> +		};
>
> 'co'dec < 'cp'u> +

Aah, I thought I checked for that already. Will fix.

>> +		platform {
>> +			sound-dai = <&q6routing>;
>> +		};
>> +	};
>> +};
>> +
>>  &tlmm {
>>  	gpio-reserved-ranges = <13 4>, <56 2>;
>>  
>> @@ -1178,6 +1214,7 @@ &usb_1 {
>>  &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.

Regards
Luca

>
> Konrad