Re: Issue with commit 33c133cc7598e60976a phy: IRQ cannot be shared

[Sergei Shtylyov]

Hello.

On 08/15/2014 01:10 PM, christophe leroy wrote:

> > > I have an hardware with two ethernet interfaces, and with the two PHYs inside
> > > the same component INTEL LXT973 which has only one interrupt.
> > > I also have another hardware with two ethernet interfaces and two independant
> > > PHYs. But the two PHYs are wired to the same interrupt.
> > > This is working perfectly up to Linux 3.12.

> >    Hm, I'm surprised it works. Are you sure you're getting interrupts from
> > both PHYs? Because if both Ethernet controllers are active simultaneously,
> > only the first registered PHY IRQ handler should get all the interrupts.

> Yes it works. Why should only the first one get the interrupts ?
> handle_irq_event_percpu() (from kernel/irq/handle.c, extract below) calls all
> handlers regardless of whether they answer IRQ_NONE or IRQ_HANDLED. The break
> applies to the switch(), not to the while(). So all handlers are called.

   Indeed, my reasoning seems obsolete now, if ever valid at all. :-/
   I couldn't yet remember other reasons that caused me to do that patch last 
December. Perhaps it was also connected to the "rude" behaviour of the 
phylib's IRQ handler, which calls disable_irq_nosync()...

[...]

> > > Reading the commit log, I can't really understand the reason for the change.

> > > Is it really worth it, and therefore how shall my case be handled ?

> >    PHY IRQs are not necessary for the phylib state machine.

> However, polling is less efficient than IRQs. It wastes CPU and link loss
> detection is slower.

   Yes, but you can't avoid it even with valid IRQ, the way phylib is 
written: the state workqueue is activated once a second even in the absence of 
interrupts.

   What can also be done is getting rid of the IRQ workqueue and using 
threaded IRQs,

> BR
> Christophe

WBR, Sergei

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