On Mon, 21 Nov 2005, Matthew Wilcox wrote:
> >
> > if (!dev->irq)
> > return;
> >
> > (whatever, made up). And that having NO_IRQ be anything but 0 is thus
> > fundamentally broken.
>
> The idea was to give them something better to use instead of this.
Why?
The above is obvious even to a non-programmer. There _is_ no better thing
from a clarity standpoint.
> The only relevant thing I found with google was
> http://www.microsoft.com/whdc/archive/pciirq.mspx
Look at, for example, any cardbus controller. The way to disable
generation of the legacy interrupt? Write an irq value of 0 to the
interrupt control register.
Or check out the irq routers in arch/i386/pci/irq.c. Every _single_ one of
them does the same. Zero means disabled.
The point I'm trying to make is that there already _is_ a special bit
pattern that means "no irq", and it's what
- the kernel has used for the last 14 years
- is the easiest and most logical one to test for
- real hardware uses
so why not just use it?
Anybody who says that
if (!dev->irq)
return;
isn't logical is just on drugs. It's logical _and_ readable. More so than
any alternative, especially as any alternative will always have the
problem that some people will write the above _anyway_.
If you make NO_IRQ be something else than 0, for safety you should also
make the irq thing be a structure so that the test against zero (which is
special, as _defined_ by the C language) won't work.
At which point you might as well just do something like
struct interrupt_descriptor {
unsigned int nr:31;
unsigned int valid:1;
};
and then people can just say
if (!dev->irq.valid)
return;
instead, which is also readable, and where you simply cannot do the old
"if (!dev->irq)" at all.
The fact is, 0 _is_ special. Not just for hardware, but because 0 has a
magical meaning as "false" in the C language.
Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
