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

[Richard Cochran]

On Thu, Feb 27, 2025 at 05:23:48PM +0200, Andy Shevchenko wrote:
> 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.

If the hardware does NOT latch the registers together, then the driver must do:

  1. hi1 = read hi
  2. low = read lo
  3. hi2 = read h1
  4. if (hi2 == hi1 return (hi1 << 32) | low;
  5. goto step 1.

This for correctness, and correctness > performance.

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

Need to have driver authors check the data sheet because ...

> 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.

... of this.

Thanks,
Richard