linux-kernel - Re: [PATCH v4 5/6] ASoC: codecs: wcd938x: add mux control support for hp audio mux

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Message-ID: <8fbc8cd7-f8e7-e33b-74df-cdea389ac9a4@axentia.se>
Date: Mon, 24 Mar 2025 17:33:59 +0100
From: Peter Rosin <peda@...ntia.se>
To: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
 Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: broonie@...nel.org, andersson@...nel.org, krzk+dt@...nel.org,
 ivprusov@...utedevices.com, luca.ceresoli@...tlin.com,
 zhoubinbin@...ngson.cn, paulha@...nsource.cirrus.com, lgirdwood@...il.com,
 robh@...nel.org, conor+dt@...nel.org, konradybcio@...nel.org,
 perex@...ex.cz, tiwai@...e.com, linux-sound@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, johan+linaro@...nel.org,
 Christopher Obbard <christopher.obbard@...aro.org>
Subject: Re: [PATCH v4 5/6] ASoC: codecs: wcd938x: add mux control support for
 hp audio mux

Hi!

2025-03-24 at 16:58, Srinivas Kandagatla wrote:
> 
> 
> On 24/03/2025 15:18, Dmitry Baryshkov wrote:
>> On Mon, Mar 24, 2025 at 01:58:06PM +0000, Srinivas Kandagatla wrote:
>>>
>>>
>>> On 24/03/2025 13:50, Dmitry Baryshkov wrote:
>>>> On Mon, 24 Mar 2025 at 15:01, <srinivas.kandagatla@...aro.org> wrote:
>>>>>
>>>>> From: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>

*snip*

>>>>> +       int ret = mux_control_try_select(wcd938x->us_euro_mux, state);
>>>>
>>>> Hmm. Does this really work? You have selected the mux in probe
>>>> function, now you are trying to select it again. If I'm reading the
>>>> code correctly, you will get -EBUSY here.
>>>
>>> On successful selection of mux state, the mux will be kept available
>>> (mux_control_deselect) for any new callers.
>>>
>>> So we will not get EBUSY for the second caller.
>>
>> No. wcd938x_populate_dt_data() selects the state by calling
>> wcd938x_select_mux_state().
> 
> At this point we also released it (both in success and error case).
> 
> This will hold on to the previous state unless we have defined a fallback idle-state.
> 
> 
>  Then you call mux_control_try_select() here.
>> As far as I understand, it will return -EBUSY as the sempahore is > already taken. Moreover, this is not how the MUX API is supposed to be
>> used. The driver is supposed to hold a state while it is still in use.

Dmitry is correct. A mux consumer is supposed to keep the mux selected
while it needs the mux to remain in a certain state. Relying on details
such as idle as-is and that no other consumer butts in and clobbers the
state is fragile. Mux access is not exclusive, at least not until a
mux state is selected.

Cheers,
Peter