Re: [PATCH v2 08/16] docs: Add XZ_EXTERN to c_id_attributes

[Lasse Collin]

On 2024-07-21 Jonathan Corbet wrote:
> I spent a little while trying to figure out why we need XZ_EXTERN at
> all but lost in the #includes...

This is a good question. I looked at it and now I think that it's not
actually needed. Thus, this patch to Documentation/conf.py should be
dropped from this series, and I will submit a new patch to remove
XZ_EXTERN.

Preboot code on several archs has "#define STATIC static", for example:

    arch/x86/boot/compressed/misc.c
    arch/arm/boot/compressed/decompress.c
    arch/mips/boot/compressed/decompress.c
    drivers/firmware/efi/libstub/zboot.c

These files #include one of lib/decompress_*.c files. The STATIC macro
is used to detect if the code is being built for preboot code instead
of initramfs decompression. The STATIC macro is also used to make a few
functions static in lib/decompress_*.c files (and also in
lib/inflate.c).

Note that even if STATIC isn't initially defined, the
lib/decompress_*.c files have

    #include <linux/decompress/mm.h>

which will then "#define STATIC" (empty value).

lib/decompress_unxz.c makes all XZ functions static in preboot code via
the XZ_EXTERN macro. I'm not sure why I have done so. The commit
message from 2009 in my upstream tree isn't very specific.

STATIC is used also in lib/inflate.c to make functions static. However,
that file *seems* to be used only on alpha and nios2; it's *not* used
by lib/decompress_inflate.c which uses lib/zlib_inflate/inflate.c with
its extern functions in preboot code. But lib/inflate.c might have made
me think that there's a need to make functions static in some cases.

lib/decompress_unzstd.c is newer. It doesn't attempt to make all
functions static in preboot use.

Omitting XZ_EXTERN doesn't produce any warnings or make any difference
in x86 or ARM64 (CONFIG_EFI_ZBOOT=y) kernel sizes. (I'm ignoring a few
dozen bytes of noise on ARM64 between repeated builds.)

The boot code on PowerPC is special and it touches the XZ_EXTERN macro
in its xz_config.h. Relevant files:

    arch/powerpc/boot/xz_config.h
    arch/powerpc/boot/decompress.c

The "#undef XZ_EXTERN" can be confusing but in the end all XZ_EXTERN
uses become "static" still. Comparing to zlib usage on PowerPC, it
seems that decompressor functions aren't required to be static (zlib's
files are pre-processed with a sed script but it doesn't make anything
static). So, even without testing, it seems quite clear that removing
XZ_EXTERN would be fine on PowerPC too.

Thus, let's drop this patch to Documentation/conf.py, and I submit a
patch to remove XZ_EXTERN.

Thanks!

-- 
Lasse Collin