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

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Date:   Tue, 24 May 2022 02:52:40 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Lukas Wunner <lukas@...ner.de>
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 03:47:09PM +0200, Lukas Wunner wrote:
> 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 think some comments are needed. If i don't understand what is going
on, i'm sure others don't as well.

    Andrew