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

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Message-ID: <20230519132250.35ce4880@kernel.org>
Date: Fri, 19 May 2023 13:22:50 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: "Russell King (Oracle)" <linux@...linux.org.uk>, Andrew Lunn
 <andrew@...n.ch>, 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, Jacob Keller <jacob.e.keller@...el.com>, "Zulkifli,
 Muhammad Husaini" <muhammad.husaini.zulkifli@...el.com>
Subject: Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping
 layers to user space.

On Fri, 19 May 2023 16:28:02 +0300 Vladimir Oltean wrote:
> > I may have lost track of what the argument is. There are devices
> > which will provide a DMA stamp for Tx and Rx. We need an API that'll
> > inform the user about it. 
> > 
> > To be clear I'm talking about drivers which are already in the tree,
> > not opening the door for some shoddy new HW in.  
> 
> So this is circling back to my original question (with emphasis on the
> last part):
> 
> | 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"?
> 
> https://lore.kernel.org/netdev/20230511203646.ihljeknxni77uu5j@skbuf/
> 
> If only "some" packets (unpredictable which) get DMA timestamps when
> a MAC timestamp isn't available, what UAPI can satisfactorily describe
> that? And who would want that?

The short answer is "I don't know". There's been a lot of talk about
time stamps due to Swift/BBRv2, but I don't have first hand experience
with it. That'd require time stamping "all" packets but the precision 
is really only required to be in usec.

Maybe Muhammad has a clearer use case in mind.