linux-kernel - Re: Regression: spi: core: avoid waking pump thread from spi

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Message-ID: <20190115212539.GK5522@sirena.org.uk>
Date:   Tue, 15 Jan 2019 21:25:39 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Martin Sperl <kernel@...tin.sperl.org>
Cc:     Jon Hunter <jonathanh@...dia.com>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-spi@...r.kernel.org
Subject: Re: Regression: spi: core: avoid waking pump thread from spi_sync
 instead run teardown delayed

On Tue, Jan 15, 2019 at 09:58:55PM +0100, Martin Sperl wrote:

> Maybe a bigger change to the reduce the complexity of
> the state machine would solve that problem and also
> reduce code complexity... 

Yeah, that's where I was getting to with that test patch I posted.

> I may find some time over the weekend if no solution
> has been found until then.

Thanks for volunteering :)

> The way I would envision it it would have a “state”
> as a level (0=shutdown, 1=hw enabled, 2=in pump, 
> 3=in transfer, 4=in hw-mode,...) and a complete
> to allow waking the shutdown thread (and by this
> avoiding the busy wait loop we have now).
> This would replace those idling, busy, and running flags.

That's a good idea, yes - a single enum much more reflects what we can
actually do in terms of transitions.

> Drawback: it is invasive, but let us see what it
> really looks like...

I think we need to either drop your change (which would be bad since it
is a big performance improvement, I'd punted it for later when I did the
original refactoring to push the work into the caller threads then never
got around to it) or have a invasive changes to make the new situation
clearer.  Right now things are just far too complex to reason about
which isn't helping anyone.

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