Re: New objtool warning..

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Thu, Dec 17, 2020 at 8:25 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> Oh yeah, I forgot about that.  That would be another option if my patch
> doesn't work out.

Well, one option is to just say "ok, we know gcc generates horrible
code that falls through to another function in a situation that we
claim is unreachable, so let's not claim it is unreachable".

IOW, the problem here is that the compiler fundamentally isn't smart
enough to see that something is unreachable, and the "unreachable()"
annotation we did didn't actually really cause any code that makes it
so. So we basically have code that _if_ we ever change it, it will
simply be wrong, and we'll never see any warnings about it but it will
fall through to nonsensical code.

So maybe the option here is simply "objtool was right before, the
unreachable() is fragile and wrong".

We can easily write that case statement in a way that actually makes
the compiler generate better code and avoids the issue by just making
case 0x00 also be the default case.

So I think I'll just apply this patch instead.

            Linus

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 drivers/gpu/drm/drm_edid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 74f5a3197214..e95cce8e736d 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -3102,6 +3102,8 @@ static int drm_cvt_modes(struct drm_connector *connector,
 
 		height = (cvt->code[0] + ((cvt->code[1] & 0xf0) << 4) + 1) * 2;
 		switch (cvt->code[1] & 0x0c) {
+		/* default - because compiler doesn't see that we've enumerated all cases */
+		default:
 		case 0x00:
 			width = height * 4 / 3;
 			break;
@@ -3114,8 +3116,6 @@ static int drm_cvt_modes(struct drm_connector *connector,
 		case 0x0c:
 			width = height * 15 / 9;
 			break;
-		default:
-			unreachable();
 		}
 
 		for (j = 1; j < 5; j++) {