[PATCH] ARM: vfp: Fix up exception location in Thumb mode
Russell King - ARM Linux
linux at arm.linux.org.uk
Fri Jan 14 11:52:23 EST 2011
On Fri, Jan 14, 2011 at 10:24:52AM -0600, Dave Martin wrote:
> Is the comment preceding __und_usr_unknown causing some confusion here?
> * The FP module is called with these registers set:
> * r0 = instruction
> * r2 = PC+4
> That reflects the ARM case only: for Thumb, r2 is always PC+2 (?)
Actually, referring to 'PC' here is confusing (and yes, I probably wrote
it) - does 'PC' refer to the address of the faulting instruction or the
current PC value...
Your '(?)' there is exactly the problem I'm referring to - I don't think
there's much of a clear idea really what's going on here...
> The comment at the start of do_undefinstr() (which receives these
> registers) is correct though:
> * According to the ARM ARM, PC is 2 or 4 bytes ahead,
> * depending whether we're in Thumb mode or not.
> * Correct this offset.
The ARM ARM says that in order to return to the instruction which
generated the exception, subtract 2 bytes for thumb or 4 bytes for
So, in order to point at the instruction which generated the exception,
we have to subtract this value from the PC value we were passed. I
suggest changing this comment to:
* According to the ARM ARM, the PC is 2 or 4 bytes ahead
* depending on Thumb mode. Correct this offset so that
* regs->ARM_pc points at the faulting instruction.
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