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

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Message-ID: <20230511203646.ihljeknxni77uu5j@skbuf>
Date: Thu, 11 May 2023 23:36:46 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: 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, 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?

Which drivers provide DMA timestamps, and how do they currently signal
that they do this? Do they do this for all packets that get timestamped,
or for "some"?