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

Sergei Shtylyov
2014-08-15T13:39:11Z

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