Note that if your driver doesn't care, it doesn't have to ask. If your driver
does, you pay a penalty, but it's hardly a penalty that everyone would pay.

-matt


On Fri, 7 Sep 2001, der Mouse wrote:

> >> [enough state to do a "may_i_tsleep()" call]
> 
> > Basically, it's a single instruction in the interrupt stub (increment
> > interrupt nesting level) and another in the return (decrement),
> 
> On CISC architectures, perhaps (i386, vax, 68k).  SPARC, I can't see
> how to get it down below four instructions (sethi, ld, add/sub, st),
> and as I recall it, the PowerPC is similar.  On some such
> architectures, there's also a classic race between the ld-add/sub-st
> sequence and other interrupts occurring, a race which if it goes wrong
> will corrupt the count; on the SPARC, ISTR something about double error
> halts which would mean it's not an issue, but can't recall whether that
> applies to interrupts or just traps.  If you have to splhigh() around
> that, the instruction count goes up: rd, nop, nop, nop, or, wr, nop,
> nop, nop, sethi, ld, add/sub, st, wr, nop, nop, nop.
> 
> Of course, that's still no more than 17 instructions, only two of which
> access memory (which the SPARC, at least, hates to do).  The details
> will vary on other architectures, of course, but even 34 instructions
> and two four references (per interrupt, naturally) may end up being
> judged worth it....
> 
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