Re: [PATCH net] net: stmmac: ensure PTP time register reads are consistent

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Yannick Vignon <yannick.vignon@oss.nxp.com>
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 16:44:24 +0000	[thread overview]
Message-ID: <YfwGaD06/3W1UFQ+@shell.armlinux.org.uk> (raw)
In-Reply-To: <13dc6f72-8ef4-6990-1c67-2b92c6894e87@oss.nxp.com>

On Thu, Feb 03, 2022 at 05:38:10PM +0100, Yannick Vignon wrote:
> 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.

Okay, thanks.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

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