linux-kernel - Re: [PATCH v4] Fix freeze in lm8333 i2c keyboard driver

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Message-ID: <CAH2-hcJYVpBNwnMS6qUp4=MW8kSryDAz7G5cNA8R00QabC9bjg@mail.gmail.com>
Date:   Wed, 3 May 2023 10:54:37 +0200
From:   Tomáš Mudruňka <tomas.mudrunka@...il.com>
To:     Jeff LaBundy <jeff@...undy.com>
Cc:     dmitry.torokhov@...il.com, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] Fix freeze in lm8333 i2c keyboard driver

> Just to clarify, my stance is that this call should go _before_ the handler
> is registered.

Ok, i will fix the patch later today.

> Any events that come before then are off the table, and this is OK because
> user space isn't going to start consuming key events until well after this
> driver has probed anyway.

Well, that was never my point. I don't care about capturing events
that happen before driver was properly loaded.
My only concern was to limit possibility of deadlock which happened previously.
Because that makes device unusable till the IC is power cycled.
Which might be especially annoying on devices that have power button
implemented using this exact IC :-)

> The reason behind my assertion is that as a matter of best practice, you
> should not have two asynchronous threads that can in theory access the same
> register.

Yeah, this makes bit more sense now. Didn't realized IRQ might
interrupt that lm8333_irq_thread() call immediately.
While not very likely to cause problems like deadlock of the driver, i
think it's a valid point.

After all this is what happens with IRQF_ONESHOT anyway right? Each
time the IRQ is triggered it's disabled, lm8333_irq_thread() is run
and then it's enabled immediately after that. So i guess the behaviour
on each keypress is very similar to calling lm8333_irq_thread() before
registering the IRQ handler, which gives me some confidence there
might not be huge chance for deadlock under normal circumstances.

Though i wonder what would happen if some EMI burst would create rapid
train of randomly timed keypresses, that might just hit the
unfortunate sweetspot after while... Might test that later in the lab,
since i really need the software to remain operational after such
condition had passed.