linux-kernel - Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

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Date:	Tue, 26 Feb 2008 23:23:20 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	i2c@...sensors.org, video4linux-list@...hat.com
Subject: Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

Hi Marcin,

On Tue, 26 Feb 2008 22:03:16 +0100, Marcin Slusarz wrote:
> Do you have an idea (or patch :D) how to solve this:
> 0x00000234 v4l_compat_translate_ioctl [v4l1-compat]:    1376
> ? That's on top of my make checkstack output

Random ideas (but I am in no way a specialist of this exercise):

* You could try moving the structures to the blocks where they are used,
in the case a given structure is used for only one ioctl. I'm not too
sure how gcc handles local variables declared inside blocks with
regards to stack reservation though. I thought it would work but my
experiments today seem to suggest it doesn't.

* You can move the handling of some ioctls to dedicated functions, just
like I did in i2c-dev:
http://lists.lm-sensors.org/pipermail/i2c/2008-February/003010.html
However there is a risk that gcc will inline these functions (that's
what happened to me...) Not sure how to prevent gcc from inlining.

* You can allocate the structures dynamically, as you originally wanted
to do for ir-kbd-i2c. However this has a performance penalty and will
fragment the memory, so it's not ideal.

* If each ioctl uses only one of the structures, you may define a union
of all the structures. The size of the union will be the size of the
biggest structure, so you save a lot of space on the stack.

-- 
Jean Delvare
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