Re: [RFC 1/2] irq: record edge-level setting

[Stephen Hemminger <shemminger@osdl.org>]

On Mon, 24 Apr 2006 15:34:20 -0700 (PDT)
Linus Torvalds <torvalds@osdl.org> wrote:

> 
> 
> On Mon, 24 Apr 2006, Stephen Hemminger wrote:
> > 
> > Maybe that's why it never was done in the past, too much work and historical
> > baggage.
> 
> It's messy. That whole ELCR register was mis-designed: you can change the 
> edge/level detection with it, but since it _also_ changes the polarity of 
> the signal, you can't actually do so from a sw angle, and it has to match 
> the hardware. So you can't say "I want to treat this interrupt as level 
> triggered", and just set the bit ;^/
> 
> To make matters worse, I wouldn't be in the least surprised if the ELCR 
> register is totally ignored by many south-bridges for the internally 
> generated interrupts (ie devices that are embedded in the SB), since the 
> register really doesn't matter for them.
> 
> And it doesn't help that Intel mis-designed the edge-detection logic on 
> the IO-APIC. On the old i8259, if you masked an interrupt and unmasked it, 
> an active interrupt would always be seen as an edge, because the 
> edge-detection was done _after_ masking. On the IO-APIC crap, the masking 
> is done after edge-detection, so if you mask the APIC hardware level, and 
> an edge happens, you'll never ever learn of it ever again.

That is the kind of crap that makes NAPI difficult.
See Documentation/networking/NAPI_HOWTO.txt for rotting packet..

> I'm sure other system architectures have similar problems, but it's 
> irritating.
> 
> 			Linus