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List:       linux-arm-kernel
Subject:    Re: pending interrupts
From:       Nicolas Pitre <nico () cam ! org>
Date:       2004-09-15 19:47:48
Message-ID: Pine.LNX.4.61.0409151531550.19910 () xanadu ! home
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On Wed, 15 Sep 2004, Matthias Welwarsky wrote:

> On Tuesday 14 September 2004 19:04, Nicolas Pitre wrote:
> > On Tue, 14 Sep 2004, Matthias Welwarsky wrote:
> > > Also, it's a little difficult to
> > > write the FIQ code in C, since you lack a stack :)
> >
> > FIQ handlers are always written in assembly, of course.
> 
> Well, yes, however the badness of writing them in C is highly overrated. If 
> you can trick your compiler into not using a stack and only the banked 
> registers, it is entirely possible.

But thein it isn't C anymore.

And what's the point?

If you want to use FIQs that's for a single reason: speed. And normally 
FIQ handlers can be written in only a _few_ assembly instructions to 
read a value, maybe do little math and store it in memory then return 
ASAP, with all required pointers already set in the FIQ banked regs.
Writing C code for this is rather overkill.

Also if you want to use C you'll need to preserve/restore r0 through r7 
anyway since the compiler needs the low registers for function call 
argument passing.

In other words, you may arrange to be able to use C for FIQ handlers of 
course, but if you do that you simply missed the very reason for using a 
FIQ handler in the first place.


Nicolas

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