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

Wolfram Sang
2023-11-20T16:22:32Z

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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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