netdev - Re: [PATCH net-next v2 4/5] ptp: Support late timestamp determination

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Date:   Wed, 13 Apr 2022 22:51:54 +0200
From:   Gerhard Engleder <gerhard@...leder-embedded.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>, yangbo.lu@....com,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mlichvar@...hat.com,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 4/5] ptp: Support late timestamp determination

> > I'm thinking about why there should be a slow path with lookup. If the
> > address/cookie
> > points to a defined data structure with two timestamps, then no lookup
> > for the phc or
> > netdev is necessary. It should be possible for every driver to
> > allocate a skbuff with enough
> > space for this structure in front of the received Ethernet frame.
>
> Adding 16 bytes for every allocated skbuff is going to be a tough
> sell.  Most people don't want/need this.

Most people are not affected because they use drivers which do not
support cycles. Those drivers stay the same, no 16 bytes are added.
For TX those 16 bytes are not added, because SKBTX_HW_TSTAMP_USE_CYCLES
is used to fill in the right time stamp.

For igc and tsnep the 16 bytes in front of the RX frame exist anyway.
So this would be a minimal solution with best performance as a first
step. A lookup for netdev/phc can be added in the future if there is
a driver, which needs that.

Is it worth posting an implementation in that direction?

Thanks!

Gerhard