Re: [PATCH] RFC: ptp: add comment about register access race

[Andy Shevchenko]

On Thu, Feb 27, 2025 at 03:17:27PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> While reviewing a patch to the ioread64_hi_lo() helpers, I noticed
> that there are several PTP drivers that use multiple register reads
> to access a 64-bit hardware register in a racy way.
> 
> There are usually safe ways of doing this, but at least these four
> drivers do that.  A third register read obviously makes the hardware
> access 50% slower. If the low word counds nanoseconds and a single
> register read takes on the order of 1µs, the resulting value is
> wrong in one of 4 million cases, which is pretty rare but common
> enough that it would be observed in practice.

...

> Sorry I hadn't sent this out as a proper patch so far. Any ideas
> what we should do here?

Actually this reminds me one of the discussion where it was some interesting
HW design that latches the value on the first read of _low_ part (IIRC), but
I might be mistaken with the details.

That said, it's from HW to HW, it might be race-less in some cases.

-- 
With Best Regards,
Andy Shevchenko