Re: [PATCH v2] i2c: designware: Fix corrupted memory seen in the ISR

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 19, 2023 at 02:38:22PM +0200, Yann Sionneau wrote:
> Hi,
> 
> On 9/19/23 12:19, Wolfram Sang wrote:
> > > I also agree that a wmb() in the i2c driver is not the more elegant fix.
> > > For similar reasons, we hid barriers in the write*() macros, drivers
> > > need to stay architecture-agnostic as much as possible.
> > Exactly my thinking. I wanted to read this patch discussion later this
> > week. But from glimpsing at it so far, I already wondered why there
> > isn't a memory barrier in the final accessor to the register.
> 
> The regmap accessors used by the designware driver end up calling
> writel_relaxed() and readl_relaxed() : https://elixir.bootlin.com/linux/v6.6-rc2/source/drivers/i2c/busses/i2c-designware-common.c#L71

OK, since it ends up with the *_relaxed() accessors, there are no
barriers here. I wonder whether the regmap API should have both standard
and relaxed variants. If a regmap driver does not populate the
.reg_write_relaxed etc. members, a regmap_write_relaxed() would just
fall back to regmap_write().

We went through similar discussions many years ago around the I/O
accessors and decided to add the barriers to readl/writel() even if they
become more expensive, correctness should be first. The relaxed variants
were added as optimisations if specific memory ordering was not
required. I think the regmap API should follow the same semantics, go
for correctness first as you can't tell what the side-effect of a
regmap_write() is (e.g. kicking off DMA or causing an interrupt on
another CPU).

> In those cases I would say the smp_* barriers are what we are supposed to
> use, isn't it?

While smp_* is ok, it really depends on what the regmap_write() does. Is
it a write to a shared peripheral (if not, you may need a DSB)? Does the
regmap_write() caller know this? That's why I think having the barrier
in dw_reg_write() is better.

If you do want to stick to a fix in i2c_dw_xfer_init(), you could go for
dma_wmb(). While this is not strictly DMA, it's sharing data with
another coherent agent (a different CPU in this instance). The smp_wmb()
is more about communication via memory not involving I/O. But this still
assumes that the caller knows regmap_write() ends up with an I/O
write*() (potentially relaxed).

-- 
Catalin