linux-kernel - Re: [PATCH 1/8] soundwire: fix enumeration completion

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Date:   Wed, 5 Jul 2023 16:30:57 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Mark Brown <broonie@...nel.org>, Vinod Koul <vkoul@...nel.org>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Banajit Goswami <bgoswami@...cinc.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Rander Wang <rander.wang@...ux.intel.com>
Subject: Re: [PATCH 1/8] soundwire: fix enumeration completion

On Wed, Jul 05, 2023 at 02:53:17PM +0200, Pierre-Louis Bossart wrote:
> On 7/5/23 14:30, Johan Hovold wrote:
> > The soundwire subsystem uses two completion structures that allow
> > drivers to wait for soundwire device to become enumerated on the bus and
> > initialised by their drivers, respectively.
> > 
> > The code implementing the signalling is currently broken as it does not
> > signal all current and future waiters and also uses the wrong
> > reinitialisation function, which can potentially lead to memory
> > corruption if there are still waiters on the queue.
> 
> That change sounds good, but I am not following the two paragraphs below.
> 
> > Not signalling future waiters specifically breaks sound card probe
> > deferrals as codec drivers can not tell that the soundwire device is
> > already attached when being reprobed. 
> 
> What makes you say that? There is a test in the probe and the codec
> driver will absolutely be notified, see bus_type.c
> 
> 	if (drv->ops && drv->ops->update_status) {
> 		ret = drv->ops->update_status(slave, slave->status);
> 		if (ret < 0)
> 			dev_warn(dev, "%s: update_status failed with status %d\n", __func__,
> ret);
> 	}

I'm talking about signalling the codec driver using the soundwire device
via the completion structs. Unless the underling device is detached and
reattached, trying to wait for completion a second time will currently
timeout instead of returning immediately.

This affects codecs like rt5682, which wait for completion in component
probe (see rt5682_probe()).

> > Some codec runtime PM
> > implementations suffer from similar problems as waiting for enumeration
> > during resume can also timeout despite the device already having been
> > enumerated.
> 
> I am not following this either. Are you saying the wait_for_completion()
> times out because of the init_completion/reinit_completion confusion, or
> something else.

It times out because the completion counter is not saturated unless you
use complete_all().

Drivers that wait unconditionally in resume, will time out the second
time they are runtime resumed unless the underlying device has been
detached and reattached in the meantime (e.g. wsa881x_runtime_resume()).

Johan