Re: [PATCH net-next 3/4] net: phy: Add helper to derive the number of ports from a phy mode

From: Maxime Chevallier <maxime.chevallier@bootlin.com>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>,
	davem@davemloft.net, Rob Herring <robh+dt@kernel.org>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	devicetree@vger.kernel.org, thomas.petazzoni@bootlin.com,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	linux-arm-kernel@lists.infradead.org,
	Horatiu.Vultur@microchip.com, Allan.Nielsen@microchip.com,
	UNGLinuxDriver@microchip.com
Subject: Re: [PATCH net-next 3/4] net: phy: Add helper to derive the number of ports from a phy mode
Date: Fri, 29 Jul 2022 08:50:36 +0200	[thread overview]
Message-ID: <20220729085036.2b180478@pc-10.home> (raw)
In-Reply-To: <df39cfc9-6fd8-e277-870b-67059dcebb2b@gmail.com>

Hello Florian, Andrew,

On Thu, 28 Jul 2022 14:44:47 -0700
Florian Fainelli <f.fainelli@gmail.com> wrote:

> On 7/28/22 14:32, Andrew Lunn wrote:
> >> +int phy_interface_num_ports(phy_interface_t interface)
> >> +{
> >> +	switch (interface) {
> >> +	case PHY_INTERFACE_MODE_NA:
> >> +	case PHY_INTERFACE_MODE_INTERNAL:
> >> +		return 0;  
> > 
> > I've not yet looked at how this is used. Returning 0 could have
> > interesting effects i guess? INTERNAL clearly does have some sort of
> > path between the MAC and the PHY, so i think 1 would be a better
> > value. NA is less clear, it generally means Don't touch. But again,
> > there still needs to be a path between the MAC and PHY, otherwise
> > there would not be any to touch.
> > 
> > Why did you pick 0?  

My reasonning was that PHY_INTERNAL is likely a custom solution to link
IPs existing on the same die, so nothing prevents vendors from
multiplexing links on that interface. But it's a far-fetched
reasonning, so 1 can be good, as other interfaces that are meant to be
used on-die like XGMII.

> I would agree that returning 1 is a more sensible default to avoid
> breaking users of that function. However this makes me wonder, in
> what case will we break the following common meaning:
> 
> - Q -> quad
> - P -> penta
> - O -> octal
> 
> Is the helper really needed in the sense that the phy_interface_t
> enumeration is explicit enough thanks to or because of its name? --
> Florian

Good question actually ! It started as a point from Russell proposing a
helper to get the number of serdes lanes for a given interface, but
this sisn't quite fit the use-case, which was simply to simplify

	if (interface == PHY_INTERFACE_MODE_QSGMII ||
	    interface == PHY_INTERFACE_MODE_QUSGMII)

into

	if (phy_interface_num_ports(interface) == 4)

But this a slim simplification at the cost of a new helper to maintain,
so I can repove that if you want.

Thanks for the reviews,

Maxime

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