From mboxrd@z Thu Jan  1 00:00:00 1970
From: u.kleine-koenig@pengutronix.de (Uwe =?iso-8859-1?Q?Kleine-K=F6nig?=)
Date: Tue, 23 Feb 2010 21:27:59 +0100
Subject: [PATCH 02/10] ARM: ftrace: document mcount formats
In-Reply-To: <20100223194253.GB6325@debian>
References: <1266090518-31120-1-git-send-email-rabin@rab.in>
	<1266090518-31120-3-git-send-email-rabin@rab.in>
	<20100213203748.GC16528@pengutronix.de>
	<20100222180624.GA4061@debian>
	<20100222192009.GA13527@pengutronix.de>
	<20100223194253.GB6325@debian>
Message-ID: <20100223202759.GC20792@pengutronix.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |