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

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Message-Id: <2356AD12-9631-4707-8EA5-385E9D6A5716@geanix.com>
Date:   Wed, 6 Sep 2023 09:03:02 +0200
From:   Sean Nyekjaer <sean@...nix.com>
To:     Andi Shyti <andi.shyti@...nel.org>, Wolfram Sang <wsa@...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 Andi,

> On 6 Sep 2023, at 01.08, Andi Shyti <andi.shyti@...nel.org> wrote:
> 
> Hi Sean,
> 
> On Mon, Sep 04, 2023 at 07:29:59AM +0200, Sean Nyekjaer wrote:
>> 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.
> 
> What about the opposite? I.e. a non atomic and later an atomic,
> for very late tardive communications :)

Sure it’s the opposite? Normal scenario is “non atomic” transfers going on and under shutdown it switches to “atomic”.
From i2c_in_atomic_xfer_mode() it can’t go from “atomic” -> “non atomic”.

extern enum system_states {
SYSTEM_BOOTING,
SYSTEM_SCHEDULING,
SYSTEM_FREEING_INITMEM,
SYSTEM_RUNNING,
SYSTEM_HALT,
SYSTEM_POWER_OFF,
SYSTEM_RESTART,
SYSTEM_SUSPEND,
} system_state;

If you are asking what happens if a “non atomic” transfer is ongoing and irq’s is disabled, IDK.

Let’s get Wolfram in the loop (Sorry I forgot to add you) :)

/Sean