linux-kernel - Re: [RFC PATCH] i2c: remove use of in

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Date:   Thu, 28 Mar 2019 09:10:02 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     Linux I2C <linux-i2c@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] i2c: remove use of in_atomic()

Hi Wolfram,

On Wed, Mar 27, 2019 at 10:13 PM Wolfram Sang
<wsa+renesas@...g-engineering.com> wrote:
> Commit cea443a81c9c ("i2c: Support i2c_transfer in atomic contexts")
> added in_atomic() to the I2C core. However, the use of in_atomic()
> outside of core kernel code is discouraged and was already[1] when this
> code was added in early 2008. The above commit was a preparation for
> b7a3670131c7 ("i2c-pxa: Add polling transfer"). Its commit message says
> explicitly it was added "for cases where I2C transactions have to occur
> at times interrups are disabled". So, the intention was 'disabled
> interrupts'. This matches the use cases for atomic I2C transfers I have
> seen so far: very late communication (mostly to a PMIC) to powerdown or
> reboot the system. For those cases, interrupts are disabled then. It
> doesn't seem that in_atomic() adds value.
>
> Note that only ~10 out of ~120 bus master drivers support atomic
> transfers, mostly by polling always when no irq is supplied. A generic
> I2C client driver cannot assume support for atomic transfers. This is
> currently a platform-dependent corner case.
>
> The I2C core will soon gain an extra callback into bus drivers
> especially for atomic transfers to make them more generic. The code
> deciding which transfer to use (atomic/non-atomic) should mimic the
> behaviour which locking to use (trylock/lock). Because I don't want to
> add more in_atomic() to the I2C core, this patch simply removes it.
>
> [1] https://lwn.net/Articles/274695/
>
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>

LGTM, so:
Ackeded-by: Geert Uytterhoeven <geert+renesas@...der.be>

But please wait for Peter and/or Frederic to give their fiat, too.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds