Re: [PATCH v3 3/5] net: Let the active time stamping layer be selectable.

From: "Köry Maincent" <kory.maincent@bootlin.com>
To: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>,
	Horatiu Vultur <horatiu.vultur@microchip.com>,
	Michael Walle <michael@walle.cc>, Andrew Lunn <andrew@lunn.ch>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	netdev@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-omap@vger.kernel.org,
	Maxime Chevallier <maxime.chevallier@bootlin.com>,
	Richard Cochran <richardcochran@gmail.com>,
	thomas.petazzoni@bootlin.com,
	Russell King <linux@armlinux.org.uk>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Jay Vosburgh <j.vosburgh@gmail.com>,
	Veaceslav Falico <vfalico@gmail.com>,
	Andy Gospodarek <andy@greyhouse.net>,
	Claudiu Manoil <claudiu.manoil@nxp.com>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	UNGLinuxDriver@microchip.com,
	Minghao Chi <chi.minghao@zte.com.cn>,
	Jie Wang <wangjie125@huawei.com>,
	Oleksij Rempel <linux@rempel-privat.de>,
	Sean Anderson <sean.anderson@seco.com>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Wolfram Sang <wsa+renesas@sang-engineering.com>,
	Alexander Lobakin <alexandr.lobakin@intel.com>,
	Marco Bonelli <marco@mebeim.net>
Subject: Re: [PATCH v3 3/5] net: Let the active time stamping layer be selectable.
Date: Tue, 14 Mar 2023 12:02:54 +0100	[thread overview]
Message-ID: <20230314120254.76f8360c@kmaincent-XPS-13-7390> (raw)
In-Reply-To: <20230313084059.GA11063@pengutronix.de>

On Mon, 13 Mar 2023 09:40:59 +0100
Oleksij Rempel <o.rempel@pengutronix.de> wrote:

> > But, as you point out, with Köry's/Richard's proposal, the MAC driver
> > will be bypassed when the selected timestamping layer is the PHY, and
> > that's a problem currently.
> > 
> > May I suggest the following? There was another RFC which proposed the
> > introduction of a netdev notifier when timestamping is turned on/off:
> > https://lore.kernel.org/netdev/20220317225035.3475538-1-vladimir.oltean@nxp.com/
> >
> > If we have a notifier, then we can make switch drivers do things
> > differently. They can activate timestamping per se in the timestamping
> > NDO (which is only called when the MAC is the active timestamping layer),
> > and they can activate PTP traps in the netdev notifier (which is called
> > any time a timestamping status change takes place - the notifier info
> > should contain details about which net_device and timestamping layer
> > this is, for example).
> > 
> > It's just a proposal of how to create an alternative notification path
> > that doesn't disturb the goals of this patch set. 

I will take a look at your patch but indeed adding a timestamp notifier could
be a good idea.

> To make things even more complicated - I  have a project where the DSA master
> should be used for time stamping. Due to board specific limitations, we
> are forced to use FEC PHC instead of dsa KSZ switch PHC. So, it is not
> a choice between MAC and PHY, it is more the MAC before MAC and PHY.
> PTP sync in this case will have more jitter, but it still good enough
> for this project.
> Currently I use quick hack to do so, but mainlinamble solution working for
> most use cases will be nice.

In this case it is not a PHY/MAC PTP choice anymore but more a device
PTP choice which bring a lot more of complexity. Or maybe we could simply add a
"switch" timestamp layer later on. As Andrew said we will maybe increase the
number of timestamp layers in the future.

Regards,
Köry