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

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Message-ID: <54e14000-3fd7-47fa-aec3-ffc2bab2e991@lunn.ch>
Date: Thu, 11 May 2023 22:50:30 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: 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, linux@...linux.org.uk
Subject: Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping
 layers to user space.

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, but the sample of the PHC was taken as the SOF was
received. It could be the sample of the PHC at the point the DMA
descriptor was handed back to the host. It could be the PHC was
sampled when the host reads the descriptor, etc.

I also wounder if there is sufficient documentation to be able to tell
them apart for devices which support it. So maybe it does not even
matter what the exact definition is?

	Andrew