linux-kernel - Re: [PATCH RFC net-next 4/5] net: phy: introduce is_c45_over

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Message-ID: <YkWUGwi0tbWFyUy/@shell.armlinux.org.uk>
Date:   Thu, 31 Mar 2022 12:44:27 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Michael Walle <michael@...le.cc>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Xu Liang <lxu@...linear.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 4/5] net: phy: introduce is_c45_over_c22 flag

On Thu, Mar 24, 2022 at 06:18:14PM +0100, Michael Walle wrote:
> Am 2022-03-24 17:23, schrieb Andrew Lunn:
> > > Isn't it safe to assume that if a PHY implements the indirect
> > > registers for c45 in its c22 space that it will also have a valid
> > > PHY ID and then the it's driver will be probed?
> > 
> > See:
> > https://elixir.bootlin.com/linux/latest/source/drivers/net/phy/phy_device.c#L895
> > 
> > No valid ID in C22 space.
> 
> I actually looked at the datasheet and yes, it implements the
> registers 13 and 14 in c22 to access the c45 space. I couldn't
> find any descriptions of other c22 registers though.

I'm not sure which PHY you're referring to here, but iirc, the later
hardware revisions of the 88x3310 implement the indirect access, but
earlier revisions do not.

> > In general, if the core can do something, it is better than the driver
> > doing it. If the core cannot reliably figure it out, then we have to
> > leave it to the drivers. It could well be we need the drivers to set
> > has_c45. I would prefer that drivers don't touch c45_over_c22 because
> > they don't have the knowledge of what the bus is capable of doing. The
> > only valid case i can think of is for a very oddball PHY which has C45
> > register space, but cannot actually do C45 transfers, and so C45 over
> > C22 is the only option.
> 
> And how would you know that the PHY has the needed registers in c22
> space? Or do we assume that every C45 PHY has these registers?

That's the problem. Currently C22 PHY drivers that do not support the
C45 register space have to set the .read_mmd and .write_mmd methods to
genphy_read_mmd_unsupported/genphy_write_mmd_unsupported which
effectively disables access to the C45 register space. In order for
that to happen, we must have read the C22 PHY ID and bound the driver.

That doesn't help with reading the PHY ID though.

-- 
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