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

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Message-ID: <YfwGaD06/3W1UFQ+@shell.armlinux.org.uk>
Date:   Thu, 3 Feb 2022 16:44:24 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Yannick Vignon <yannick.vignon@....nxp.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Rayagond Kokatanur <rayagond@...avyalabs.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        sebastien.laveze@....nxp.com, Vladimir Oltean <olteanv@...il.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>, mingkai.hu@....com,
        Joakim Zhang <qiangqing.zhang@....com>,
        Yannick Vignon <yannick.vignon@....com>
Subject: Re: [PATCH net] net: stmmac: ensure PTP time register reads are
 consistent

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@....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@...linux.org.uk>

-- 
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