Re: [PATCH net-next v3 08/47] net: phylink: Support differing link speeds and interface speeds

From: Andrew Lunn <andrew@lunn.ch>
To: Sean Anderson <sean.anderson@seco.com>
Cc: "David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Madalin Bucur <madalin.bucur@nxp.com>,
	netdev@vger.kernel.org, Paolo Abeni <pabeni@redhat.com>,
	Eric Dumazet <edumazet@google.com>,
	linux-arm-kernel@lists.infradead.org,
	Russell King <linux@armlinux.org.uk>,
	linux-kernel@vger.kernel.org,
	Alexandru Marginean <alexandru.marginean@nxp.com>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Vladimir Oltean <olteanv@gmail.com>
Subject: Re: [PATCH net-next v3 08/47] net: phylink: Support differing link speeds and interface speeds
Date: Sun, 17 Jul 2022 03:26:52 +0200	[thread overview]
Message-ID: <YtNlXA4lBeG+gRXH@lunn.ch> (raw)
In-Reply-To: <984fec49-4c08-9d5a-d62f-c59f106f8fe5@seco.com>

> > This seem error prone when new PHY_INTERFACE_MODES are added. I would
> > prefer a WARN_ON_ONCE() in the default: so we get to know about such
> > problems.
> 
> Actually, this is the reason I did not add a default: clause to the
> switch (and instead listed everything out). If a new interface mode is
> added, there will be a warning (as I discovered when preparing this
> patch).

Ah, the compiler produces a warning. O.K. that is good. Better than an
WARN_ON_ONCE at runtime.

> > Bike shedding a bit, but would it be better to use host_side_speed and
> > line_side_speed? When you say link_speed, which link are your
> > referring to? Since we are talking about the different sides of the
> > PHY doing different speeds, the naming does need to be clear.
> When I say "link" I mean the thing that the PMD speaks. That is, one of
> the ethtool link mode bits. I am thinking of a topology like
> 
> 
> MAC (+PCS) <-- phy interface mode (MII) --> phy <-- link mode --> far-end phy
> 
> The way it has been done up to now, the phy interface mode and the link
> mode have the same speed. For some MIIs, (such as MII or GMII) this is
> actually the case, since the data clock changes depending on the data
> speed. For others (SGMII/USXGMII) the data is repeated, but the clock
> rate stays the same. In particular, the MAC doesn't care what the actual
> link speed is, just what configuration it has to use (so it selects the
> right clock etc).
> 
> The exception to the above is when you have no phy (such as for
> 1000BASE-X):
> 
> MAC (+PCS) <-- MDI --> PMD <-- link mode --> far-end PMD
> 
> All of the phy interface modes which can be used this way are
> "non-adaptive." That is, in the above case they have a fixed speed.
> 
> That said, I would like to keep the "phy interface mode speed" named
> "speed" so I don't have to write up a semantic patch to rename it in all
> the drivers.

So you want phydev->speed to be the host side speed. That leaves the
line side speed as a new variable, so it can be called line_side_speed?

I just find link_speed ambiguous, and line_side_speed less so.

The documentation for phydev->speed needs updating to make it clear it
is the host side speed.

   Andrew