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

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Thu, May 11, 2023 at 09:55:39PM +0100, Russell King (Oracle) wrote:
> 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.

No, a MAC timestamp carried through a DMA descriptor is still a MAC
timestamp (better said: timestamp taken at the MAC).

DMA timestamps probably have to do with this igc patch set, which I
admit to not having had the patience to follow along all the way and
understand what is its status and if it was ever accepted in that form,
or a different form, or if Vinicius' work for multiple in-flight TX
timestamps is an alternate solution to the same problem as DMA
timestamps, or whatever:
https://lore.kernel.org/netdev/20221018010733.4765-1-muhammad.husaini.zulkifli@intel.com/

So I just asked.