netdev - Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping layers to user space.

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Message-ID: <ZF1WS4a2bbUiTLA0@shell.armlinux.org.uk>
Date: Thu, 11 May 2023 21:55:39 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Vladimir Oltean <vladimir.oltean@....com>,
	Jakub Kicinski <kuba@...nel.org>,
	Köry Maincent <kory.maincent@...tlin.com>,
	netdev@...r.kernel.org, glipus@...il.com,
	maxime.chevallier@...tlin.com, vadim.fedorenko@...ux.dev,
	richardcochran@...il.com, gerhard@...leder-embedded.com,
	thomas.petazzoni@...tlin.com, krzysztof.kozlowski+dt@...aro.org,
	robh+dt@...nel.org
Subject: Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping
 layers to user space.

On Thu, May 11, 2023 at 10:50:30PM +0200, Andrew Lunn wrote:
> On Thu, May 11, 2023 at 11:36:46PM +0300, Vladimir Oltean wrote:
> > On Thu, Apr 06, 2023 at 06:46:46PM -0700, Jakub Kicinski wrote:
> > > > +/* Hardware layer of the SO_TIMESTAMPING provider */
> > > > +enum timestamping_layer {
> > > > +	SOF_MAC_TIMESTAMPING = (1<<0),
> > > > +	SOF_PHY_TIMESTAMPING = (1<<1),
> > > 
> > > We need a value for DMA timestamps here.
> > 
> > What's a DMA timestamp, technically? Is it a snapshot of the PHC's time
> > domain, just not at the MII pins?
> 
> I also wounder if there is one definition of DMA timestampting, or
> multiple. It could simply be that the time stamp is in the DMA
> descriptor,

Surely that is equivalent to MAC timestamping? Whether the MAC
places it in a DMA descriptor, or whether it places it in some
auxiliary information along with the packet is surely irrelevant,
because the MAC has to have the timestamp available to it in some
manner. Where it ends up is just a function of implementation surely?

I'm just wondering what this would mean for mvpp2, where the
timestamps are in the descriptors. If we have a "DMA timestamp"
is that a "DMA timestamp" or a "MAC timestamp"? The timestamp comes
from the MAC in this case.

-- 
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