linux-kernel - Re: [PATCH net v3] net: ti: icssg-prueth: Fix 1 PPS sync

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Message-ID: <20241104185031.0c843951@kernel.org>
Date: Mon, 4 Nov 2024 18:50:31 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: "Malladi, Meghana" <m-malladi@...com>
Cc: <vigneshr@...com>, <grygorii.strashko@...com>, <horms@...nel.org>,
 <jan.kiszka@...mens.com>, <diogo.ivo@...mens.com>, <pabeni@...hat.com>,
 <edumazet@...gle.com>, <davem@...emloft.net>, <andrew+netdev@...n.ch>,
 <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
 <linux-arm-kernel@...ts.infradead.org>, <srk@...com>, Roger Quadros
 <rogerq@...nel.org>, <danishanwar@...com>, Vadim Fedorenko
 <vadim.fedorenko@...ux.dev>
Subject: Re: [PATCH net v3] net: ti: icssg-prueth: Fix 1 PPS sync

On Mon, 4 Nov 2024 16:55:46 +0530 Malladi, Meghana wrote:
> On 11/1/2024 7:29 AM, Jakub Kicinski wrote:
> > On Mon, 28 Oct 2024 16:40:52 +0530 Meghana Malladi wrote:  
> >> The first PPS latch time needs to be calculated by the driver
> >> (in rounded off seconds) and configured as the start time
> >> offset for the cycle. After synchronizing two PTP clocks
> >> running as master/slave, missing this would cause master
> >> and slave to start immediately with some milliseconds
> >> drift which causes the PPS signal to never synchronize with
> >> the PTP master.  
> > 
> > You're reading a 64b value in chunks, is it not possible that it'd wrap
> > in between reads? This can be usually detected by reading high twice and
> > making sure it didn't change.
> > 
> > Please fix or explain in the commit message why this is not a problem..  
> Yes I agree that there might be a wrap if the read isn't atomic. As 
> suggested by Andrew I am currently not using custom read where I can 
> implement the logic you suggested

Right but I think Andrew was commenting on a patch which contained pure
re-implementation of read low / hi with no extra bells or whistles. 

> (reading high twice and making sure if 
> didn't change). Can you share me some references where this logic is 
> implemented in the kernel, so I can directly use that instead of writing 
> custom functions.

I think you need to write a custom one. Example:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/meta/fbnic/fbnic_time.c#n40