Re: [PATCH] mod_devicetable.h: reduce sizeof(struct of_device_id) by 80 bytes

From: Jeff Mahoney
Date: Thu May 02 2019 - 08:30:24 EST

On 5/2/19 5:41 AM, Rasmus Villemoes wrote:
On 26/04/2019 11.27, Arnd Bergmann wrote:
On Thu, Apr 25, 2019 at 10:31 PM Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> wrote:

For an arm imx_v6_v7_defconfig kernel, .rodata becomes 70K smaller; shrinks by another ~13K, making the whole kernel image
about 83K, or 0.3%, smaller.

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

The space savings are nice, but I wonder if the format of these
structures is part of the ABI or not. I have some vague recollection
of that, but it's possible that it's no longer true in this century.

scripts/mod/file2alias.c processes the structures into a different
format and seems to be written specifically to avoid problems
with changes like the one you did. Can anyone confirm that
this is true before we apply the patch?

I can't confirm it, of course, but I did do some digging around and
couldn't find anything other than file2alias, which as you mention is
prepared for such a change. I also couldn't find any specific reason for
the 128 (it's not a #define, so at least originally it didn't seem to be
tied to some external consumer) - Jeff, do you remember why you chose
that back when you did 5e6557722e69?

I had been wondering why I'd been included on this thread. I completely forgot that I wrote this code nearly 15 years ago. :)

It was probably as simple as there not being a real limit for how long the compatible string could be and wanting to make it flexible. I was targetting a powerpc mac notebook I had at the time -- not tight memory embedded systems, so sorry for that.

But we cannot really know whether there is some userspace tool that
parses the .ko ELF objects the same way that file2alias does, doing
pattern matching on the symbol names etc. I cannot see why anybody would
_do_ that (the in-tree infrastructure already generates the
MODULE_ALIAS() from which modules.alias gets generated), but the only
way of knowing, I think, is to try to apply the patch and see if anybody

The size is part of the ABI, though. module-init-tools has a copy of the same struct and uses that size to walk an array of of_device_id when a module as more than one. If you shrink it, that will certainly break.

file2alias does the right things only because it's tightly coupled to the kernel version it's being used with. It still directly accesses the structure definitions in the headers.


Jeff Mahoney