Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS

[Wu Zhangjin <wuzhangjin@gmail.com>]

On Thu, 2009-10-22 at 11:34 -0700, David Daney wrote:
> Wu Zhangjin wrote:
> > On Wed, 2009-10-21 at 11:24 -0400, Steven Rostedt wrote:
> [...]
> >>> +
> >>> +NESTED(_mcount, PT_SIZE, ra)
> >>> +	RESTORE_SP_FOR_32BIT
> >>> +	PTR_LA	t0, ftrace_stub
> >>> +	PTR_L	t1, ftrace_trace_function /* please don't use t1 later, safe? */
> >> Is t0 and t1 safe for mcount to use? Remember, mcount does not follow
> >> the dynamics of C function ABI.
> > 
> > So, perhaps we can use the saved registers(a0,a1...) instead.
> > 
> 
> a0..a7 may not always be saved.
> 
> You can use at, v0, v1 and all the temporary registers.  Note that for 
> the 64-bit ABIs sometimes the names t0-t4 shadow a4-a7.  So for a 64-bit 
> kernel, you can use: $1, $2, $3, $12, $13, $14, $15, $24, $25, noting 
> that at == $1 and contains the callers ra.  For a 32-bit kernel you can 
> add $8, $9, $10, and $11

before the profiling source code(jal _mcount), there are only necessary
stack operations, nobody have touched the GPRs, so, I think we are safe
to use any of them, that's why the t0,t1 I have used not fail. and
'Cause the a0,a1 is used as the arguments to the real profiling
function, so, I will keep back to use t0, t1, as David Daney said $12,
$13 in 64bit and $8, $9 in 32bit.

Thanks!

Regards,
	Wu Zhangjin