Re: [PATCH 03/12] powerpc/module: Create a special stub for ftrace_caller()

From: Torsten Duwe
Date: Thu Feb 25 2016 - 08:31:42 EST

On Thu, Feb 25, 2016 at 11:08:54AM +1100, Balbir Singh wrote:
> How about some comments on r2
> r2 is still pointing to the module's toc, will be saved by ftrace_caller and restored by the instruction following bl ftrace_caller (after patching _mcount/nop)

To be precise: ftrace_caller needs to save _and_ restore r2 in case of -mprofile-kernel.

> > + /* Stub uses address relative to kernel_toc */
> > + reladdr = (unsigned long)ftrace_caller - get_paca()->kernel_toc;

kernel_toc is a compile time constant; do you really want to look it up in
memory at runtime each time? It's a bit tricky to get the +- 0x8000 right

I wrote:
extern unsigned long __toc_start;
reladdr = addr - ((unsigned long)(&__toc_start) + 0x8000UL);

looks a bit odd, but evaluates to a constant for ftrace_caller.

Either way is fine with me:

Signed-off-by: Torsten Duwe <duwe@xxxxxxx>
Reviewed-by: Torsten Duwe <duwe@xxxxxxx>

> Reviewed-by: Balbir Singh <bsingharora@xxxxxxxxx>