linux-kernel - Re: [PATCH v3] i2c: stm32f7: Add atomic

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Message-Id: <6A1B6BF0-C298-43D3-9B63-0FB1EC9E902B@geanix.com>
Date:   Mon, 4 Sep 2023 07:29:59 +0200
From:   Sean Nyekjaer <sean@...nix.com>
To:     Andi Shyti <andi.shyti@...nel.org>
Cc:     Pierre-Yves MORDRET <pierre-yves.mordret@...s.st.com>,
        Alain Volmat <alain.volmat@...s.st.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-i2c@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] i2c: stm32f7: Add atomic_xfer method to driver

Hi Andy,

> On 3 Sep 2023, at 14.46, Andi Shyti <andi.shyti@...nel.org> wrote:
> 
> Hi Pierre-Yves, Alain,
> 
> mind taking a look here?
> 
> [...]
> 
>> @@ -357,6 +357,7 @@ struct stm32f7_i2c_dev {
>> u32 dnf_dt;
>> u32 dnf;
>> struct stm32f7_i2c_alert *alert;
>> + bool atomic;
> 
> this smells a bit racy here, this works only if the xfer's are
> always sequential.
> 
> What happens when we receive at the same time two xfer's, one
> atomic and one non atomic?

From the include/i2c.h:
 * @master_xfer_atomic: same as @master_xfer. Yet, only using atomic context
 *   so e.g. PMICs can be accessed very late before shutdown. Optional.

So it’s only used very late in the shutdown.

It’s implemented the same way as in:
drivers/i2c/busses/i2c-imx.c
drivers/i2c/busses/i2c-meson.c
drivers/i2c/busses/i2c-mv64xxx.c
drivers/i2c/busses/i2c-tegra.c
… etc…


In drivers/i2c/i2c-core.h it’s determined whether it’s atomic transfer or not:

/*
 * We only allow atomic transfers for very late communication, e.g. to access a
 * PMIC when powering down. Atomic transfers are a corner case and not for
 * generic use!
 */
static inline bool i2c_in_atomic_xfer_mode(void)
{
        return system_state > SYSTEM_RUNNING && irqs_disabled();
}

So you would not have an atomic transfer and later an non atomic.

/Sean