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

From: Marc Zyngier <[email-masked]>
Date: Fri, 06 May 2022 11:58:43 +0100

On Thu, 05 May 2022 19:53:28 +0100,
Lukas Wunner <[email-masked]> wrote:
> 
> 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/[email-masked]/
> 
> 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().

Please don't directly use __irq_enter_raw() and similar things
directly in driver code (it doesn't do anything related to RCU, for
example, which could be problematic if used in arbitrary contexts).
Given how infrequent this interrupt is, I'd rather you use something
similar to what lan78xx is doing, and be done with it.

And since this is a construct that seems to be often repeated, why
don't you simply make the phy interrupt handling available over a
smp_call_function() interface, which would always put you in the
correct context and avoid faking things up?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.