Re: [patch 1/4] Make usb_hcd_irq available for multi-role USB controllers with a shared interrupt line

On Thu, 26 Apr 2007 14:42:39 -0400 (EDT), Alan Stern <[email protected]> wrote:
> On Thu, 26 Apr 2007, Peter Korsgaard wrote:
> 
> > Some multi-role (host/peripheral) USB controllers use a shared interrupt
> > line for all parts of the chip. Export usb_hcd_irq so drivers can call it
> > from their interrupt handler instead of duplicating code.
> > Drivers pass an irqnum of 0 to usb_add_hcd to signal that the interrupt handler
> > shouldn't be registerred by the core.
> 
> There was some controversy about this sort of thing in the past, and I 
> don't remember what the outcome was.  The difficulty is that 0 is 
> potentially a valid IRQ number, so it shouldn't be used to mean "don't 
> register the IRQ handler".

For ohci-hcd, we used -1. The problem here is, while IRQ 0 is definitely
not valid for PCI devices (as Alan Cox reminded me a couple of weeks ago),
USB controllers are found in odd embedded configurations often, so we
cannot just use 0 as invalid everywhere. I don't know if -1 is good though.
Seems like it should be. Alan Cox also cautioned against using NO_IRQ,
because it's not universally defined by all platforms, and in general is
an invention of old IDE. Looks like -1 is the way to go.

-- Pete

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