On Wed, 11 May 2011 22:53:55 +0530
Rabin Vincent [off-list ref] wrote:

On Tue, May 10, 2011 at 13:40, Martin Schwidefsky
[off-list ref] wrote:

quoted

That leaves arm as the last remaining architecture with a non trivial
ftrace_call_adjust function. There the least significant bit is removed
from the address with an and operation. The comment says this is done
for Thumb-2. This implies that for Thumb-1 the offset is 0 and for
Thumb-2 the offset is -1, correct?

ARM supports building the kernel using either the ARM instruction set or
the Thumb-2 instruction set.  The kernel cannot be built with the
"Thumb-1" instruction set (btw usually referred to as just Thumb).

Thumb-2 via recordmcount.pl needs the clearing of the lsb because the
relocation (R_ARM_ABS32) that gets used for the assembly file
that recordmcount.pl generates and assembles dictates that the lsb be
set if the target symbol is Thumb/Thumb-2 function.  mcount_adjust would
not help here since the ORing is done later, when the relocation is
applied.

Hmm, from what I can make out the C version of recordmcount uses R_ARM_ABS32
as well. 
 

Thumb-2 via recordmcount.c does not need the clearing of the lsb in
ftrace_call_adjust.

So the clearing of the lsb is only required if the recordmcount.pl script
is used?

Building with the ARM instruction set also does not need the clearing
of the lsb.

Who does the ORing? I can't find anything in recordmount.pl/recordmcount.c
which looks like doing an OR, does the assembler do that based on the
symbol type?

quoted

Thumb-2 the offset is -1, correct? If there is a way to distinguish
the two targets in recordmcount at compile time we could convert arm
as well. Which would allow us to remove the ftrace_call_adjust function.

To remove ftrace_call_adjust, we could either deprecate the
recordmcount.pl usage for ARM (you already have to edit the Kconfig to
use it) or modify it to generate specific relocations explicitly instead
of using the assembler data directives.

Hmm, it would be a desirable property if the C version and the pearl
version of recordmcount would do the same. Or we could remove the arm
support from the pearl script, the C version is faster anyway.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.