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

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Message-ID: <20220729085036.2b180478@pc-10.home>
Date:   Fri, 29 Jul 2022 08:50:36 +0200
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, davem@...emloft.net,
        Rob Herring <robh+dt@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        thomas.petazzoni@...tlin.com,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, Horatiu.Vultur@...rochip.com,
        Allan.Nielsen@...rochip.com, UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH net-next 3/4] net: phy: Add helper to derive the number
 of ports from a phy mode

Hello Florian, Andrew,

On Thu, 28 Jul 2022 14:44:47 -0700
Florian Fainelli <f.fainelli@...il.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