linux-kernel - Re: [RFC] ARM: kernel: irq: Simplify allocation of stack frame

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Message-ID: <20131006224150.GA25647@n2100.arm.linux.org.uk>
Date:	Sun, 6 Oct 2013 23:41:50 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Joel Fernandes <joelf@...com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, Nicolas Pitre <nico@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Jonathan Austin <jonathan.austin@....com>,
	André Hentschel <nerv@...ncrow.de>
Subject: Re: [RFC] ARM: kernel: irq: Simplify allocation of stack frame

On Sun, Oct 06, 2013 at 05:30:47PM -0500, Joel Fernandes wrote:
> On receiving IRQ exception in SVC mode, all the SVC mode registers are saved
> onto the stack very early on.
> 
> The stack frame allocation code for IRQ entry during SVC mode (svc_entry) is
> hard to read as 4-less is allocated initially only to be allocated later
> implicity using the mov r3, [sp, #-4]! instruction. We make code easier to read
> by allocating the 4 bytes on the stack frame in the beginning itself and remove
> all instances where 4 bytes is adjusted.

You omit to say that this results in saving one additional register
unnecessarily in the stmia.  We could use a stmib there instead which
would avoid that issue while keeping the rest of the change.
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