linux-kernel - Re: [RFC 07/11] net: phy: reset invalid phy reads of 0 back to 0xffffffff

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Date:   Mon, 25 May 2020 09:20:45 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, andrew@...n.ch,
        f.fainelli@...il.com, hkallweit1@...il.com,
        madalin.bucur@....nxp.com, calvin.johnson@....nxp.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 07/11] net: phy: reset invalid phy reads of 0 back to
 0xffffffff

On Sun, May 24, 2020 at 11:20:01PM -0500, Jeremy Linton wrote:
> Hi,
> 
> On 5/23/20 1:44 PM, Russell King - ARM Linux admin wrote:
> > On Fri, May 22, 2020 at 04:30:55PM -0500, Jeremy Linton wrote:
> > > MMD's in the device list sometimes return 0 for their id.
> > > When that happens lets reset the id back to 0xfffffff so
> > > that we don't get a stub device created for it.
> > > 
> > > This is a questionable commit, but i'm tossing it out
> > > there along with the comment that reading the spec
> > > seems to indicate that maybe there are further registers
> > > that could be probed in an attempt to resolve some futher
> > > "bad" phys. It sort of comes down to do we want unused phy
> > > devices floating around (potentially unmatched in dt) or
> > > do we want to cut them off early and let DT create them
> > > directly.
> > 
> > I'm not sure what you mean "stub device" or "unused phy devices
> > floating around" - the individual MMDs are not treated as separate
> > "phy devices", but as one PHY device as a whole.
> > 
> 
> Well, I guess its clearer to say phy/mmd devices with a phy_id=0. Which is a
> problem if we don't have DT overriding the phy_id for a given address.
> Although AFAIK given a couple of the /sys/bus/mdio_bus/devices lists I've
> seen, and after studying this code for a while now, I think "bogus" phy's
> might be getting created*. I was far to easy, to upset the cart when I was
> hacking on this set, and end up with a directory chuck full of phys.
> 
> So this gets close to one of the questions I asked in the cover letter. This
> patch and 09/11 serve to cut off possibly valid phy's which are failing to
> identify themselves using the standard registers. Which per the 802.3 spec
> there is a blurb about 0 in the id registers for some cases. Its not really
> a critical problem for ACPI machines to have these phys around (OTOH, there
> might be issues with c22 phys on c45 electrical buses that respond to c45
> reg requests but don't set the c22 regs flag, I haven't seen that yet.).

If you have a classical clause 22 PHY on a clause 45 bus, it isn't
going to respond to clause 45 cycles, so it isn't going to respond to
a request to read the devices-in-package register, so there is no
"c22 regs" flag.

> I
> considered dropping this patch, and 9/11 was a last minute addition. I kept
> it because I was worried all those extra "reserved" MMDs would end up with
> id = 0's in there and break something.
> 
> * In places where there isn't actually a phy, likely a large part of the
> problem was clearing the c22 bit, which allowed 0xFFFFFFFF returns to slip
> through the devices list.

-- 
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