linux-kernel - Re: [PATCH v3 2/4] i2c: core: run atomic i2c xfer when !preemptible

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Date:   Sun, 2 Apr 2023 12:04:48 +0200
From:   Benjamin Bara <bbara93@...il.com>
To:     Wolfram Sang <wsa@...nel.org>, Benjamin Bara <bbara93@...il.com>,
        Lee Jones <lee@...nel.org>, rafael.j.wysocki@...el.com,
        dmitry.osipenko@...labora.com, jonathanh@...dia.com,
        richard.leitner@...ux.dev, treding@...dia.com,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-tegra@...r.kernel.org,
        Benjamin Bara <benjamin.bara@...data.com>,
        stable@...r.kernel.org
Cc:     peterz@...radead.org
Subject: Re: [PATCH v3 2/4] i2c: core: run atomic i2c xfer when !preemptible

On Wed, 29 Mar 2023 at 21:50, Wolfram Sang <wsa@...nel.org> wrote:
> Could you make sure please?

Sure, I'll try. The check before bae1d3a was:
in_atomic() || irqs_disabled()
which boils down to:
(preempt_count() != 0) || irqs_disabled()
preemptible() is defined as:
(preempt_count() == 0 && !irqs_disabled())

so this patch should behave the same as pre-v5.2, but with the
additional system state check. From my point of view, the additional
value of the in_atomic() check was that it activated atomic i2c xfers
when preemption is disabled, like in the case of panic(). So reverting
that commit would also re-activate atomic i2c transfers during emergency
restarts. However, I think considering the system state makes sense
here.

>From my understanding, non-atomic i2c transfers require enabled IRQs,
but atomic i2c transfers do not have any "requirements". So the
irqs_disabled() check is not here to ensure that the following atomic
i2c transfer works correctly, but to use non-atomic i2c xfer as
long/often as possible.

Unfortunately, I am not sure yet about !CONFIG_PREEMPTION. I looked into
some i2c-bus implementations which implement both, atomic and
non-atomic. As far as I saw, the basic difference is that the non-atomic
variants usually utilize the DMA and then call a variant of
wait_for_completion(), like in i2c_imx_dma_write() [1]. However, the
documentation of wait_for_completion [2] states that:
"wait_for_completion() and its variants are only safe in process context
(as they can sleep) but not (...) [if] preemption is disabled".
Therefore, I am not quite sure yet if !CONFIG_PREEMPTION uses the
non-atomic variant at all or if this case is handled differently.

> Asking Peter Zijlstra might be a good idea.
> He helped me with the current implementation.

Thanks for the hint! I wrote an extra email to him and added him to CC.

Thanks & best regards,
Benjamin

[1] drivers/i2c/busses/i2c-imx.c
[2] https://www.kernel.org/doc/Documentation/scheduler/completion.txt