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

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Message-ID: <Z8VfKYMGEKhvluJV@smile.fi.intel.com>
Date: Mon, 3 Mar 2025 09:50:01 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Richard Cochran <richardcochran@...il.com>
Cc: Arnd Bergmann <arnd@...nel.org>,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	Arnd Bergmann <arnd@...db.de>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Tianfei Zhang <tianfei.zhang@...el.com>,
	Jonathan Lemon <jonathan.lemon@...il.com>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Thomas Weißschuh <linux@...ssschuh.net>,
	Calvin Owens <calvin@...nvd.org>,
	Philipp Stanner <pstanner@...hat.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fpga@...r.kernel.org
Subject: Re: [PATCH] RFC: ptp: add comment about register access race

On Sun, Mar 02, 2025 at 12:55:08PM -0800, Richard Cochran wrote:
> 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@...db.de>
> > > 
> > > 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.

Right.

> > > 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.

Perhaps it's still good to have a comment, but rephrase it that the code is
questionable depending on the HW behaviour that needs to be checked.

-- 
With Best Regards,
Andy Shevchenko