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    SubjectRe: [PATCH] video: limit stack usage of ir-kbd-i2c.c
    On Tue, Feb 26, 2008 at 11:23:20PM +0100, Jean Delvare wrote:
    > Hi Marcin,
    Hi

    > 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.
    That won't work. Variables at beginning of function take only ~600 bytes,
    so the rest must be from inner blocks and inlines (probably).

    > * 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.
    There's "noinline" attribute in linux/compiler.h (compiler-gcc.h actually)
    for these situations.

    > * 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.
    Nice idea.

    I'll try 2nd and 4th approaches.

    Marcin Slusarz


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