linux-kernel - Re: [PATCH 1/7] soundwire: bus: use sdw_update_no

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Date:   Tue, 8 Dec 2020 10:26:00 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Bard Liao <yung-chuan.liao@...ux.intel.com>,
        alsa-devel@...a-project.org, tiwai@...e.de,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        ranjani.sridharan@...ux.intel.com, hui.wang@...onical.com,
        broonie@...nel.org, srinivas.kandagatla@...aro.org,
        jank@...ence.com, sanyog.r.kale@...el.com,
        rander.wang@...ux.intel.com, bard.liao@...el.com
Subject: Re: [PATCH 1/7] soundwire: bus: use sdw_update_no_pm when
 initializing a device

On 07-12-20, 09:31, Pierre-Louis Bossart wrote:
> On 12/6/20 10:43 PM, Vinod Koul wrote:
> > On 05-12-20, 08:59, Pierre-Louis Bossart wrote:
> > > Thanks for the review Vinod.
> > > 
> > > On 12/5/20 1:45 AM, Vinod Koul wrote:
> > > > On 03-12-20, 04:46, Bard Liao wrote:
> > > > > From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
> > > > > 
> > > > > When a Slave device is resumed, it may resume the bus and restart the
> > > > > enumeration. During that process, we absolutely don't want to call
> > > > > regular read/write routines which will wait for the resume to
> > > > > complete, otherwise a deadlock occurs.
> > > > > 
> > > > > Fixes: 60ee9be25571 ('soundwire: bus: add PM/no-PM versions of read/write functions')
> > > > 
> > > > Change looks okay, but not sure why this is a fix for adding no pm
> > > > version?
> > > 
> > > when we added the no_pm version, we missed the two cases below where
> > > sdw_update() was used and that creates a deadlock. To me that's a conceptual
> > > bug, we didn't fully use the no_pm versions, hence the Fixes tag.
> > 
> > Documentation says:
> > "A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
> > is used to make it easy to determine where a bug originated, which can help
> > review a bug fix. This tag also assists the stable kernel team in determining
> > which stable kernel versions should receive your fix. This is the preferred
> > method for indicating a bug fixed by the patch. See :ref:`describe_changes`
> > for more details."
> > 
> > I do not this this helps here as this does not help distros etc
> > I would say this is incremental development which improved a case not
> > done properly earlier, unless you would like this to be backported.. In
> > that case it helps folks...
> 
> IMHO the changes in the series are absolutely required to have a reliable
> suspend-resume operation and will need to be back-ported. When I said
> 'conceptual bug', I didn't mean a hypothetical case, I really meant that a
> proven race condition and timeouts will occur. That's not good... We will
> provide the list of this patches to distros that are known to support
> SoundWire as a 'must apply'.
> 
> If you look at the series, we provided Fixes tags for all patches except
> 'cosmetic' ones which don't fundamentally change the behavior (Patch 3,
> patch 7) or when the code has not reached Linus' tree (patch 5).
> 
> That said, 5.10 was the first release where SoundWire started to be
> functional so there's no need to apply these patches to earlier versions of
> the stable tree.
> 
> Does this help?

Yes, that helps, thanks

-- 
~Vinod