netdev - Re: [RFC PATCH net-next 03/11] spi: Add a PTP system timestamp to the transfer structure

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Message-ID: <20190820164937.GE4738@sirena.co.uk>
Date:   Tue, 20 Aug 2019 17:49:37 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Hubert Feurstein <h.feurstein@...il.com>, mlichvar@...hat.com,
        Richard Cochran <richardcochran@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-spi@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH net-next 03/11] spi: Add a PTP system timestamp to
 the transfer structure

On Tue, Aug 20, 2019 at 04:48:39PM +0300, Vladimir Oltean wrote:
> On Tue, 20 Aug 2019 at 15:55, Mark Brown <broonie@...nel.org> wrote:
> > On Fri, Aug 16, 2019 at 05:05:53PM +0300, Vladimir Oltean wrote:

> > > Maybe the SPI master driver should just report what sort of
> > > snapshotting capability it can offer, ranging from none (default
> > > unless otherwise specified), to transfer-level (DMA style) or
> > > byte-level.

> > That does then have the consequence that the majority of controllers
> > aren't going to be usable with the API which isn't great.

> Can we continue this discussion on this thread:
> https://www.spinics.net/lists/netdev/msg593772.html
> The whole point there is that if there's nothing that the driver can
> do, the SPI core will take the timestamps and record their (bad)
> precision.

I'm not on that thread.

> > I'm not 100% clear what the problem you're trying to solve is, or if
> > it's a sensible problem to try to solve for that matter.

> The problem can simply be summarized as: you're trying to read a clock
> over SPI, but there's so much timing jitter in you doing that, that
> you have a high degree of uncertainty in the actual precision of the
> readout you took.

That doesn't seem like a great idea...

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