netdev - Re: [PATCH net-next v2 1/6] net: dsa: add support for phylink mac_select

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Date:   Mon, 21 Feb 2022 16:55:40 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Ansuel Smith <ansuelsmth@...il.com>, Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>
Subject: Re: [PATCH net-next v2 1/6] net: dsa: add support for phylink
 mac_select_pcs()

On Mon, Feb 21, 2022 at 02:44:34PM +0000, Russell King (Oracle) wrote:
> On Mon, Feb 21, 2022 at 04:32:54PM +0200, Vladimir Oltean wrote:
> > Alternatively, phylink_create() gets the initial PHY interface mode
> > passed to it, I wonder, couldn't we call mac_select_pcs() with that in
> > order to determine whether the function is a stub or not?
> 
> That would be rather prone to odd behaviour depending on how
> phylink_create() is called, depending on the initial interface mode.
> If the initial interface mode causes mac_select_pcs() to return NULL
> but it actually needed to return a PCS for a different interface mode,
> then we fail.

I agree. I just wanted to make it clear that if you have a better idea
than a pointer-encoded -EOPNOTSUPP, I'm not bent on going with -EOPNOTSUPP.

> > *and as I haven't considered, to be honest. When phylink_major_config()
> > gets called after a SGMII to 10GBaseR switchover, and mac_select_pcs is
> > called and returns NULL, the current behavior is to keep working with
> > the PCS for SGMII. Is that intended?
> 
> It was not originally intended, but as a result of the discussion
> around this patch which didn't go anywhere useful, I dropped it as
> a means to a path of least resistance.
> 
> https://patchwork.kernel.org/project/linux-arm-kernel/patch/E1mpSba-00BXp6-9e@rmk-PC.armlinux.org.uk/

Oh, but that patch didn't close exactly this condition that we're
talking about here, did it? It allows phylink_set_pcs() to be called
with NULL, but phylink_major_config() still has the non-NULL check,
which prevents it from having any effect in this scenario:

	/* If we have a new PCS, switch to the new PCS after preparing the MAC
	 * for the change.
	 */
	if (pcs)
		phylink_set_pcs(pl, pcs);

I re-read the conversation and I still don't see this argument being
given, otherwise I wouldn't have opposed...