linux-kernel - Re: [PATCH] i2c: core: Fix atomic xfer check for non-preempt config

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Message-ID: <ZZblCO9li-TMSOKV@shikoro>
Date: Thu, 4 Jan 2024 18:04:08 +0100
From: Wolfram Sang <wsa@...nel.org>
To: Benjamin Bara <bbara93@...il.com>
Cc: Lee Jones <lee@...nel.org>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	peterz@...radead.org, mwalle@...nel.org,
	Tor Vic <torvic9@...lbox.org>,
	Erhard Furtner <erhard_f@...lbox.org>, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Benjamin Bara <benjamin.bara@...data.com>, stable@...r.kernel.org
Subject: Re: [PATCH] i2c: core: Fix atomic xfer check for non-preempt config

On Thu, Jan 04, 2024 at 09:17:08AM +0100, Benjamin Bara wrote:
> From: Benjamin Bara <benjamin.bara@...data.com>
> 
> Since commit aa49c90894d0 ("i2c: core: Run atomic i2c xfer when
> !preemptible"), the whole reboot/power off sequence on non-preempt kernels
> is using atomic i2c xfer, as !preemptible() always results to 1.
> 
> During device_shutdown(), the i2c might be used a lot and not all busses
> have implemented an atomic xfer handler. This results in a lot of
> avoidable noise, like:
> 
> [   12.687169] No atomic I2C transfer handler for 'i2c-0'
> [   12.692313] WARNING: CPU: 6 PID: 275 at drivers/i2c/i2c-core.h:40 i2c_smbus_xfer+0x100/0x118
> ...
> 
> Fix this by allowing non-atomic xfer when the interrupts are enabled, as
> it was before.
> 
> Fixes: aa49c90894d0 ("i2c: core: Run atomic i2c xfer when !preemptible")
> Cc: stable@...r.kernel.org # v5.2+
> Signed-off-by: Benjamin Bara <benjamin.bara@...data.com>

Thanks! The code looks what I also would have suggested reading the bug
reports. So:

Applied to for-current, thanks!

> +	/*
> +	 * non-atomic xfers often use wait_for_completion*() calls to wait
> +	 * efficiently (schedule out voluntarily) on the completion of the xfer,
> +	 * which are then "completed" by an IRQ. If the constraints are not
> +	 * satisfied, fall back to an atomic xfer.
> +	 */
> +	return system_state > SYSTEM_RUNNING &&
> +	       (IS_ENABLED(CONFIG_PREEMPT_COUNT) ? !preemptible() : irqs_disabled());

I removed the comment, though. I don't think it explains the following
code well enough, i.e. why we have a decision based on a Kconfig
symbol. We can (and should) fix this incrementally, though. I hope this
is OK with everyone.

Thanks to everyone putting work into this. Much appreciated!


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