Re: [PATCH v4] i2c: designware: Fix corrupted memory seen in the ISR

[Andy Shevchenko]

On Thu, Nov 09, 2023 at 03:19:27AM +0000, Jan Bottorff wrote:
> When running on a many core ARM64 server, errors were
> happening in the ISR that looked like corrupted memory. These
> corruptions would fix themselves if small delays were inserted
> in the ISR. Errors reported by the driver included "i2c_designware
> APMC0D0F:00: i2c_dw_xfer_msg: invalid target address" and
> "i2c_designware APMC0D0F:00:controller timed out" during
> in-band IPMI SSIF stress tests.
> 
> The problem was determined to be memory writes in the driver were not
> becoming visible to all cores when execution rapidly shifted between
> cores, like when a register write immediately triggers an ISR.
> Processors with weak memory ordering, like ARM64, make no
> guarantees about the order normal memory writes become globally
> visible, unless barrier instructions are used to control ordering.
> 
> To solve this, regmap accessor functions configured by this driver
> were changed to use non-relaxed forms of the low-level register
> access functions, which include a barrier on platforms that require
> it. This assures memory writes before a controller register access are
> visible to all cores. The community concluded defaulting to correct
> operation outweighed defaulting to the small performance gains from
> using relaxed access functions. Being a low speed device added weight to
> this choice of default register access behavior.

...

> v3->v4: add missing changelog

Side note: Usually it's enough to just reply to the patch with the changelog.

...

> -	*val = swab32(readl_relaxed(dev->base + reg));
> +	*val = swab32(readl(dev->base + reg));

> -	writel_relaxed(swab32(val), dev->base + reg);
> +	writel(swab32(val), dev->base + reg);

I'm wondering why ioread32be() / iowrite32be() can't be used here...

Probably it would require to switch entire IO to use ioreadXX() /
iowriteXX() APIs and since we touch all of them (?) may be it makes
sense convert to use them at the same time. Dunno.

-- 
With Best Regards,
Andy Shevchenko