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

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Message-ID: <e5c5c9ba-fca1-4fa3-a416-1fc972ebd258@ti.com>
Date: Tue, 5 Nov 2024 17:31:13 +0530
From: "Malladi, Meghana" <m-malladi@...com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <vigneshr@...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 11/5/2024 8:20 AM, Jakub Kicinski wrote:
> 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
Ok thank you. I will add custom function for this and update the patch.