netdev - Re: [PATCH net-next v4 3/4] sfc: support unicast PTP

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Message-ID: <Y/jem/Z+AxDEScK8@hoboy.vegasvil.org>
Date:   Fri, 24 Feb 2023 07:58:19 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Íñigo Huguet <ihuguet@...hat.com>,
        Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
        ecree.xilinx@...il.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
        Yalin Li <yalli@...hat.com>, habetsm.xilinx@...il.com
Subject: Re: [PATCH net-next v4 3/4] sfc: support unicast PTP

On Fri, Feb 24, 2023 at 07:43:35AM -0800, Richard Cochran wrote:
> On Fri, Feb 24, 2023 at 09:01:59AM +0000, Martin Habets wrote:
> > On Wed, Feb 22, 2023 at 08:52:45AM -0800, Richard Cochran wrote:
> > > The user space PTP stack must be handle out of order messages correct
> > > (which ptp4l does do BTW).
> > 
> > This takes CPU time. If it can be avoided that is a good thing, as
> > it puts less pressure on the host. It is not just about CPU load, it
> > is also about latency.
> 
> It neither takes more CPU nor induces additional latency to handle
> messages out of order.  The stack simply uses an event based state
> machine.  In between events, the stack is sleeping on input.

If you are talking about CPU cycles and latency *in the driver* then,
by all means, choose your queues to implement the best solution.

But that wasn't the argument given in the original post.

Thanks,
Richard