netdev - Re: [RFC PATCH net-next 0/6] ptp: Support hardware clocks with additional free running time

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Message-ID: <20220308005523.GB6994@hoboy.vegasvil.org>
Date:   Mon, 7 Mar 2022 16:55:23 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Gerhard Engleder <gerhard@...leder-embedded.com>
Cc:     yangbo.lu@....com, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mlichvar@...hat.com,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH net-next 0/6] ptp: Support hardware clocks with
 additional free running time

On Mon, Mar 07, 2022 at 06:54:19PM +0100, Gerhard Engleder wrote:

> ktime_to_cycles uses hwtstamp as key for the cache lookup. As long as
> the PHC is monotonic, the key is unique. If the time of the PHC is set, then
> the cache would be invalidated. I'm afraid that setting the PHC could lead to
> wrong or missing timestamps. Setting the PHC in hardware, timestamp
> generation in hardware, and cache invalidation in software would need to
> be synchronized somehow.

You can avoid errors even with a time jump:

Make a variant (union) of skb_shared_hwtstamps to allow driver to
provide an address or cookie instead of ktime_t.  Set a flag in the
skbuff to signal this.

Let the Rx path check the flag and fetch the time stamp by callback
with the address/cookie.

Thanks,
Richard