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

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Message-ID: <20230906073247.hjzjywivxzt3lcxw@zenone.zhora.eu>
Date:   Wed, 6 Sep 2023 09:32:47 +0200
From:   Andi Shyti <andi.shyti@...nel.org>
To:     Sean Nyekjaer <sean@...nix.com>
Cc:     Wolfram Sang <wsa@...nel.org>,
        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 Sean,

> >>>> @@ -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”.

well at some point we move from non atomic to atomic and we
preempt whatever is non atomic in order to go atomic, including
non atomic transfers.

A "global" variable thrown there without protection is a bit weak
and we need to be sure to be covering all possible scenarios when
this variable is used.

> 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) :)

Nah, it's OK... I am thinking aloud here and trying to cover
possible scenarios. I also think that setting up a spinlock might
be too much paranoiac and not necessary.

I'm going to ack it... but I will keep a few thoughts on thinking
what can happen wrong here.

Acked-by: Andi Shyti <andi.shyti@...nel.org> 

Andi