netdev - Re: [PATCH net-next v3 1/6] net: phy: mscc: migrate to phy_select/restore

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Message-ID: <20181119154005.GA8367@piout.net>
Date:   Mon, 19 Nov 2018 16:40:05 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Andreas Schwab <schwab@...e.de>,
        Quentin Schulz <quentin.schulz@...tlin.com>,
        davem@...emloft.net, f.fainelli@...il.com,
        allan.nielsen@...rochip.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, thomas.petazzoni@...tlin.com,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH net-next v3 1/6] net: phy: mscc: migrate to
 phy_select/restore_page functions

On 19/11/2018 16:28:30+0100, Andrew Lunn wrote:
> On Mon, Nov 19, 2018 at 04:13:10PM +0100, Andreas Schwab wrote:
> > On Nov 19 2018, Andrew Lunn <andrew@...n.ch> wrote:
> > 
> > > On Mon, Nov 19, 2018 at 03:57:17PM +0100, Andreas Schwab wrote:
> > >> On Okt 08 2018, Quentin Schulz <quentin.schulz@...tlin.com> wrote:
> > >> 
> > >> > The Microsemi PHYs have multiple banks of registers (called pages).
> > >> > Registers can only be accessed from one page, if we need a register from
> > >> > another page, we need to switch the page and the registers of all other
> > >> > pages are not accessible anymore.
> > >> >
> > >> > Basically, to read register 5 from page 0, 1, 2, etc., you do the same
> > >> > phy_read(phydev, 5); but you need to set the desired page beforehand.
> > >> >
> > >> > In order to guarantee that two concurrent functions do not change the
> > >> > page, we need to do some locking per page. This can be achieved with the
> > >> > use of phy_select_page and phy_restore_page functions but phy_write/read
> > >> > calls in-between those two functions shall be replaced by their
> > >> > lock-free alternative __phy_write/read.
> > >> >
> > >> > Let's migrate this driver to those functions.
> > >> 
> > >> This has some serious locking problem.
> > >
> > > Hi Andreas
> > >
> > > Could you be more specific. Are you getting a deadlock? A WARN_ON?
> > 
> > See the stack trace.  That's where it hangs.
> 
> So you never said it hangs. The stacktrace helps, but a description of
> what actually happens also helps. And i expect Quentin has booted this
> code lots of times and not had a hang. So some hits how to reproduce
> it would also help. Maybe your kernel config?
> 
> I'm interested because he is using the core mdio locking
> primitives. If those are broken, i want to know.
> 

My first intuition is that he mac driver quentin is using does
phy_connect when the interface is opened while macb is doing it at probe
time. I didn't investigate but maybe this can help :)

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com