From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: "Richard Cochran" <richardcochran@gmail.com>,
"Thomas Weißschuh" <thomas.weissschuh@linutronix.de>,
"Arnd Bergmann" <arnd@arndb.de>,
"Andrew Lunn" <andrew+netdev@lunn.ch>,
"David S. Miller" <davem@davemloft.net>,
"Eric Dumazet" <edumazet@google.com>,
"Jakub Kicinski" <kuba@kernel.org>,
"Paolo Abeni" <pabeni@redhat.com>,
"Tianfei Zhang" <tianfei.zhang@intel.com>,
"Jonathan Lemon" <jonathan.lemon@gmail.com>,
"Vadim Fedorenko" <vadim.fedorenko@linux.dev>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Thomas Weißschuh" <linux@weissschuh.net>,
"Calvin Owens" <calvin@wbinvd.org>,
"Philipp Stanner" <pstanner@redhat.com>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-fpga@vger.kernel.org
Subject: Re: [PATCH] RFC: ptp: add comment about register access race
Date: Thu, 27 Feb 2025 17:23:48 +0200 [thread overview]
Message-ID: <Z8CDhIN5vhcSm1ge@smile.fi.intel.com> (raw)
In-Reply-To: <20250227141749.3767032-1-arnd@kernel.org>
On Thu, Feb 27, 2025 at 03:17:27PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.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.
...
> 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
next prev parent reply other threads:[~2025-02-27 15:23 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-27 14:17 [PATCH] RFC: ptp: add comment about register access race Arnd Bergmann
2025-02-27 15:23 ` Andy Shevchenko [this message]
2025-03-02 20:55 ` Richard Cochran
2025-03-03 7:50 ` Andy Shevchenko
2025-03-04 4:09 ` Richard Cochran
2025-03-04 8:24 ` Arnd Bergmann
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