From: Yannick Vignon <yannick.vignon@oss.nxp.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>,
Alexandre Torgue <alexandre.torgue@foss.st.com>,
Jose Abreu <joabreu@synopsys.com>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Maxime Coquelin <mcoquelin.stm32@gmail.com>,
Rayagond Kokatanur <rayagond@vayavyalabs.com>,
netdev@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, sebastien.laveze@oss.nxp.com,
Vladimir Oltean <olteanv@gmail.com>,
Xiaoliang Yang <xiaoliang.yang_1@nxp.com>,
mingkai.hu@nxp.com, Joakim Zhang <qiangqing.zhang@nxp.com>,
Yannick Vignon <yannick.vignon@nxp.com>
Subject: Re: [PATCH net] net: stmmac: ensure PTP time register reads are consistent
Date: Thu, 3 Feb 2022 17:38:10 +0100 [thread overview]
Message-ID: <13dc6f72-8ef4-6990-1c67-2b92c6894e87@oss.nxp.com> (raw)
In-Reply-To: <YfwCnV2TV8fznZ33@shell.armlinux.org.uk>
On 2/3/2022 5:28 PM, Russell King (Oracle) wrote:
> On Thu, Feb 03, 2022 at 05:00:25PM +0100, Yannick Vignon wrote:
>> From: Yannick Vignon <yannick.vignon@nxp.com>
>>
>> Even if protected from preemption and interrupts, a small time window
>> remains when the 2 register reads could return inconsistent values,
>> each time the "seconds" register changes. This could lead to an about
>> 1-second error in the reported time.
>
> Have you checked whether the hardware protects against this (i.o.w. the
> hardware latches the PTP_STSR value when PTP_STNSR is read, or vice
> versa? Several PTP devices I've looked at do this to allow consistent
> reading.
>
It doesn't. I was able to observe inconsistent values doing reads in
either order, and we had already observed the issue with that same IP on
another device (Cortex-M based, not running Linux). It's not easy to
reproduce, the time window is small, but it's there.
next prev parent reply other threads:[~2022-02-03 16:38 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-03 16:00 Yannick Vignon
2022-02-03 16:28 ` Russell King (Oracle)
2022-02-03 16:38 ` Yannick Vignon [this message]
2022-02-03 16:44 ` Russell King (Oracle)
2022-02-03 22:10 ` patchwork-bot+netdevbpf
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