linux-kernel - Re: [PATCH v5 5/6] ASoC: codecs: wcd938x: add mux control support for hp audio mux

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Message-ID: <b993c7a4-ff3e-4e79-bde8-2b5bdf3f2fff@linaro.org>
Date: Tue, 25 Mar 2025 21:39:04 +0000
From: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To: Peter Rosin <peda@...ntia.se>,
 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 v5 5/6] ASoC: codecs: wcd938x: add mux control support for
 hp audio mux

Thanks Peter,

On 25/03/2025 20:13, Peter Rosin wrote:
> Hi!
> 
> 2025-03-25 at 19:04, Srinivas Kandagatla wrote:
>> I wish we could be taken care in mux-core or even in the deselect api
> 
> It is not easily done. A mux is a shared resource. How can the mux core
> know if it is consumer A or consumer B that deselects the mux if both
> ignore failures when calling select? Mux select is backed by a semaphore
> and there is no guarantee that a consumer selects/deselects from the
> same thread or anything like that. The onus is on the consumer to get
> this right and only deselect when select is successful.

Should deselect fail if there was no previous mux selected?

> 
> I believe the documentation is clear on this topic: "do not call
> mux_control_deselect() if mux_control_select() fails".

True, the documentation is pretty clear about this behavior.
> 
> One thing can be done from the mux core, and that is to provide a new
> API where consumers get a mux that is exclusive so that the consumer
> can call select/deselect without involving a lock in the core. There
> need not even be a requirement to call deselect between selects in that
> case. Such an API is what many consumers want, I think, but it is of
> course not really compatible with the existing API, which is totally
> focused on the need to share a mux among multiple consumers.
> 
exclusive apis would simplify the consumer side of code for sure.

> And, of course, someone has to do it.

Yes, I can give it a go and see how it will turn out.

--srini
> 
> Cheers,
> Peter