linux-kernel - Re: [PULL REQUEST] i2c-for-6.7-rc2

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Date:   Mon, 20 Nov 2023 17:22:29 +0100
From:   Wolfram Sang <wsa@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Rosin <peda@...ntia.se>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Andi Shyti <andi.shyti@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PULL REQUEST] i2c-for-6.7-rc2

Hi Will,

thanks a lot for this summary!

> and I think the high-level problem was something like:
> 
> 1. CPU x writes some stuff to memory (I think one example was i2c_dw_xfer()
>    setting 'dev->msg_read_idx' to 0)
> 2. CPU x writes to an I/O register on this I2C controller which generates
>    an IRQ (end of i2c_dw_xfer_init())
> 3. CPU y takes the IRQ
> 4. CPU y reads 'dev->msg_read_idx' and doesn't see the write from (1)
> 
> (i2c folks: please chime in if I got this wrong)

I admit that I didn't dive into this specific discussion. But we had
this kind of re-ordering problem in the past in i2c, so avoiding the
relaxed_* where possible came to be a good thing in my book. So, I
recommended removing it for all writes, not only the one causing
problems here. relaxed_* should only be used when really needed. So,
this is why I applied the patch, plus I trust the people giving their
tags after the in-depth discussion. But yeah, if somebody more
experienced with this driver could double-check against the potential
locking problem, this would be good.

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