# git rev-parse -q --verify d652d5f1eeeb06046009f4fcb9b4542249526916^{commit}
# git fetch -q -n -f git://fs.ozlabs.ibm.com/kernel/linus master
warning: The last gc run reported the following. Please correct the root cause
and remove .git/gc.log.
Automatic cleanup will not be performed until the file is removed.

warning: There are too many unreachable loose objects; run 'git prune' to remove them.

# git rev-parse -q --verify d652d5f1eeeb06046009f4fcb9b4542249526916^{commit}
d652d5f1eeeb06046009f4fcb9b4542249526916
# git checkout -q -f -B kisskb d652d5f1eeeb06046009f4fcb9b4542249526916
# git clean -qxdf
# < git log -1
# commit d652d5f1eeeb06046009f4fcb9b4542249526916
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Thu Dec 17 09:27:57 2020 -0800
# 
#     drm/edid: fix objtool warning in drm_cvt_modes()
#     
#     Commit 991fcb77f490 ("drm/edid: Fix uninitialized variable in
#     drm_cvt_modes()") just replaced one warning with another.
#     
#     The original warning about a possibly uninitialized variable was due to
#     the compiler not being smart enough to see that the case statement
#     actually enumerated all possible cases.  And the initial fix was just to
#     add a "default" case that had a single "unreachable()", just to tell the
#     compiler that that situation cannot happen.
#     
#     However, that doesn't actually fix the fundamental reason for the
#     problem: the compiler still doesn't see that the existing case
#     statements enumerate all possibilities, so the compiler will still
#     generate code to jump to that unreachable case statement.  It just won't
#     complain about an uninitialized variable any more.
#     
#     So now the compiler generates code to our inline asm marker that we told
#     it would not fall through, and end end result is basically random.  We
#     have created a bridge to nowhere.
#     
#     And then, depending on the random details of just exactly what the
#     compiler ends up doing, 'objtool' might end up complaining about the
#     conditional branches (for conditions that cannot happen, and that thus
#     will never be taken - but if the compiler was not smart enough to figure
#     that out, we can't expect objtool to do so) going off in the weeds.
#     
#     So depending on how the compiler has laid out the result, you might see
#     something like this:
#     
#         drivers/gpu/drm/drm_edid.o: warning: objtool: do_cvt_mode() falls through to next function drm_mode_detailed.isra.0()
#     
#     and now you have a truly inscrutable warning that makes no sense at all
#     unless you start looking at whatever random code the compiler happened
#     to generate for our bare "unreachable()" statement.
#     
#     IOW, don't use "unreachable()" unless you have an _active_ operation
#     that generates code that actually makes it obvious that something is not
#     reachable (ie an UD instruction or similar).
#     
#     Solve the "compiler isn't smart enough" problem by just marking one of
#     the cases as "default", so that even when the compiler doesn't otherwise
#     see that we've enumerated all cases, the compiler will feel happy and
#     safe about there always being a valid case that initializes the 'width'
#     variable.
#     
#     This also generates better code, since now the compiler doesn't generate
#     comparisons for five different possibilities (the four real ones and the
#     one that can't happen), but just for the three real ones and "the rest"
#     (which is that last one).
#     
#     A smart enough compiler that sees that we cover all the cases won't care.
#     
#     Cc: Lyude Paul <lyude@redhat.com>
#     Cc: Ilia Mirkin <imirkin@alum.mit.edu>
#     Cc: Josh Poimboeuf <jpoimboe@redhat.com>
#     Cc: Peter Zijlstra <peterz@infradead.org>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-gcc --version
# < /opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-ld --version
# < git log --format=%s --max-count=1 d652d5f1eeeb06046009f4fcb9b4542249526916
# < make -s -j 48 ARCH=arm O=/kisskb/build/linus_magician_defconfig_arm-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-  magician_defconfig
# < make -s -j 48 ARCH=arm O=/kisskb/build/linus_magician_defconfig_arm-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-  help
# make -s -j 48 ARCH=arm O=/kisskb/build/linus_magician_defconfig_arm-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-  olddefconfig
# make -s -j 48 ARCH=arm O=/kisskb/build/linus_magician_defconfig_arm-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-  
Completed OK
# rm -rf /kisskb/build/linus_magician_defconfig_arm-gcc4.9
# Build took: 0:00:51.090418