netdev - Re: [PATCH net-next v3 5/7] usbnet: smsc95xx: Forward PHY interrupts to PHY driver to avoid polling

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Date:   Mon, 23 May 2022 15:47:09 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        linux-usb@...r.kernel.org,
        Steve Glendinning <steve.glendinning@...well.net>,
        UNGLinuxDriver@...rochip.com, Oliver Neukum <oneukum@...e.com>,
        Andre Edich <andre.edich@...rochip.com>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Martyn Welch <martyn.welch@...labora.com>,
        Gabriel Hojda <ghojda@...urs.ro>,
        Christoph Fritz <chf.fritz@...glemail.com>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Philipp Rosenberger <p.rosenberger@...bus.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Ferry Toth <fntoth@...il.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        'Linux Samsung SOC' <linux-samsung-soc@...r.kernel.org>
Subject: Re: [PATCH net-next v3 5/7] usbnet: smsc95xx: Forward PHY interrupts
 to PHY driver to avoid polling

On Mon, May 23, 2022 at 02:34:49PM +0200, Andrew Lunn wrote:
> > --- a/drivers/net/phy/phy_device.c
> > +++ b/drivers/net/phy/phy_device.c
> > @@ -283,8 +283,11 @@ static __maybe_unused int mdio_bus_phy_suspend(struct device *dev)
> >  	 * may call phy routines that try to grab the same lock, and that may
> >  	 * lead to a deadlock.
> >  	 */
> > -	if (phydev->attached_dev && phydev->adjust_link)
> > +	if (phydev->attached_dev && phydev->adjust_link) {
> > +		if (phy_interrupt_is_valid(phydev))
> > +			synchronize_irq(phydev->irq);
> >  		phy_stop_machine(phydev);
> > +	}
> 
> What is this hunk trying to achieve? As far as i know, interrupts have
> not been disabled. So as soon as the call to synchronize_irq()
> finishes, could well be another interrupt happens.

That other interrupt would bail out of phy_interrupt() because
the is_prepared flag is set on the PHY's struct device, see
first hunk of the patch.

The problem is that an interrupt may occur before the system
sleep transition commences.  phy_interrupt() will notice that
is_prepared is not (yet) set, hence invokes drv->handle_interrupt().
Let's say the IRQ thread is preempted at that point, the system
sleep transition is started and mdio_bus_phy_suspend() is run.
It calls phy_stop_machine(), so the state machine is now stopped.
Now phy_interrupt() continues, and the PHY driver's ->handle_interrupt()
callback starts the state machine.  Boom, that's not what we want.

So the synchronize_irq() ensures that any already running
phy_interrupt() runs to completion before phy_stop_machine()
is called.  It doesn't matter if another interrupt occurs
because then is_prepared will have been set and therefore
phy_interrupt() won't call drv->handle_interrupt().

Let me know if I haven't explained it in sufficient clarity,
I'll be happy to try again. :)

I'm more concerned about the first hunk of the patch because I'm
not sure I got the wakeup stuff right...

Thanks,

Lukas