netdev - Re: [PATCH net-next v2 5/7] usbnet: smsc95xx: Forward PHY interrupts to PHY driver to avoid polling

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Date:   Thu, 5 May 2022 20:53:28 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.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>,
        Andrew Lunn <andrew@...n.ch>,
        Russell King <linux@...linux.org.uk>,
        Ferry Toth <fntoth@...il.com>
Subject: Re: [PATCH net-next v2 5/7] usbnet: smsc95xx: Forward PHY interrupts
 to PHY driver to avoid polling

On Thu, May 05, 2022 at 11:32:07AM -0700, Jakub Kicinski wrote:
> On Tue, 3 May 2022 15:15:05 +0200 Lukas Wunner wrote:
> > @@ -608,11 +618,20 @@ static void smsc95xx_status(struct usbnet *dev, struct urb *urb)
> >  	intdata = get_unaligned_le32(urb->transfer_buffer);
> >  	netif_dbg(dev, link, dev->net, "intdata: 0x%08X\n", intdata);
> >  
> > +	/* USB interrupts are received in softirq (tasklet) context.
> > +	 * Switch to hardirq context to make genirq code happy.
> > +	 */
> > +	local_irq_save(flags);
> > +	__irq_enter_raw();
> > +
> >  	if (intdata & INT_ENP_PHY_INT_)
> > -		;
> > +		generic_handle_domain_irq(pdata->irqdomain, PHY_HWIRQ);
> >  	else
> >  		netdev_warn(dev->net, "unexpected interrupt, intdata=0x%08X\n",
> >  			    intdata);
> > +
> > +	__irq_exit_raw();
> > +	local_irq_restore(flags);
> 
> IRQ maintainers could you cast your eyes over this?
> 
> Full patch:
> 
> https://lore.kernel.org/all/c6b7f4e4a17913d2f2bc4fe722df0804c2d6fea7.1651574194.git.lukas@wunner.de/

This is basically identical to what drivers/net/usb/lan78xx.c does
in lan78xx_status(), except I'm passing the hw irq instead of the
linux irq to genirq code, thereby avoiding the overhead of a
radix_tree_lookup().

generic_handle_domain_irq() warns unconditionally on !in_irq(),
unlike handle_irq_desc(), which constrains the warning to
handle_enforce_irqctx() (i.e. x86 APIC, arm GIC/GICv3).
Perhaps that's an oversight in generic_handle_domain_irq(),
unless __irq_resolve_mapping() becomes unsafe outside in_irq()
for some reason...

In any case the unconditional in_irq() necessitates __irq_enter_raw()
here.

And there's no _safe variant() of generic_handle_domain_irq()
(unlike generic_handle_irq_safe() which was recently added by
509853f9e1e7), hence the necessity of an explicit local_irq_save().

Thanks,

Lukas