linux-kernel - Re: [PATCH 02/10] ARM: ftrace: document mcount formats

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Message-ID: <20100223202759.GC20792@pengutronix.de>
Date:	Tue, 23 Feb 2010 21:27:59 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Rabin Vincent <rabin@....in>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Abhishek Sagar <sagar.abhishek@...il.com>
Subject: Re: [PATCH 02/10] ARM: ftrace: document mcount formats

Hi Rabin,

> OK.  How about this for the last paragraph:
> 
>  mcount can be thought of as a function called in the middle of a
>  subroutine call.  As such, it needs to be transparent for both the
>  caller and the callee: the original lr needs to be restored when
>  leaving mcount, and no registers should be clobbered.  (In the
>  __gnu_mcount_nc implementation, we clobber the ip register.  This
>  is OK because the ARM calling convention allows it to be clobbered
>  in subroutines and doesn't use it to hold parameters.)
I rechecked the document describing the calling convention.  It writes
about ip:

Both the ARM- and Thumb-state BL instructions are unable to address the
full 32-bit address space, so it may be necessary for the linker to
insert a veneer between the calling routine and the called subroutine.
Veneers may also be needed to support ARM-Thumb inter-working or dynamic
linking.  Any veneer inserted must preserve the contents of all
registers except IP (r12) and the condition code flags; a conforming
program must assume that a veneer that alters IP may be inserted at any
branch instruction that is exposed to a relocation that supports inter-
working or long branches.

So the last sentence isn't completly accurate.  But describing the
actual situation is a bit overkill, so I'm OK with your suggestion.

Best regards and thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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