netdev - Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time stamping layer be selectable

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Message-ID: <20231120181736.058fd6e2@kmaincent-XPS-13-7390>
Date: Mon, 20 Nov 2023 18:17:36 +0100
From: Köry Maincent <kory.maincent@...tlin.com>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: Jakub Kicinski <kuba@...nel.org>, Florian Fainelli
 <florian.fainelli@...adcom.com>, Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, Andrew Lunn <andrew@...n.ch>,
 Heiner Kallweit <hkallweit1@...il.com>, Russell King
 <linux@...linux.org.uk>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Richard
 Cochran <richardcochran@...il.com>, Radu Pirea
 <radu-nicolae.pirea@....nxp.com>, Jay Vosburgh <j.vosburgh@...il.com>, Andy
 Gospodarek <andy@...yhouse.net>, Nicolas Ferre
 <nicolas.ferre@...rochip.com>, Claudiu Beznea <claudiu.beznea@...on.dev>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, Jonathan Corbet
 <corbet@....net>, Horatiu Vultur <horatiu.vultur@...rochip.com>,
 UNGLinuxDriver@...rochip.com, Simon Horman <horms@...nel.org>, Thomas
 Petazzoni <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, Maxime Chevallier
 <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time
 stamping layer be selectable

On Mon, 20 Nov 2023 18:10:04 +0200
Vladimir Oltean <vladimir.oltean@....com> wrote:

> On Mon, Nov 20, 2023 at 03:53:44PM +0100, Köry Maincent wrote:
> > I did thought about it but I got stuck by the case of hardware timestamping
> > without PHC. Richard explained the reason of its existence here:
> > https://lore.kernel.org/netdev/ZS3MKWlnPqTe8gkq@hoboy.vegasvil.org/#t
> > 
> > Maybe I got a bit stuck in my implementation and should investigate more
> > your proposition and how to deal with this case. Do you have an idea on how
> > to solve it?  
> 
> I would take what Richard said with a grain of salt, and interpret as
> "there exists hardware with hwts but w/o PHC, and that may work for
> marginal use cases", not that "we should design having that as a
> first-class citizen in mind".

> When elected as master, it kinda works, and does synchronize with a
> slave, even if ptp4l gets confused about the phc_index being -1.
> 
> But when elected as a slave by the BMCA, ptp4l gets confused and thinks
> that phc_index == -1 means that it's supposed to use software timestamping.
> 
> So, I guess the only thing we need to do to this kind of setup is not do
> too much harm to it.
> 
> We break nothing if we make the phc_index the central aspect of hwts
> layer selection - except for the fact that such a MAC won't be able to
> change its timestamping layer to be a PHY.
> 
> If we wanted to add such a capability to that MAC driver, the obvious
> way to solve the lack of a PHC is to create a PHC that returns
> -EOPNOTSUPP for all of its ptp_clock_info operations (gettime, settime
> etc). It may possibly be that, in the worst case, ptp4l needs to probe
> for each syscall on the NIC's PHC being operational before deciding what
> can be done with it. But that's already an improvement over the current
> handling to make it more graceful, it's not to keep things on par.

Ok, you convinced me.
Thanks for your arguments and spending time explaining it.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com