Messages in this thread Patch in this message | | | Subject | Re: New objtool warning.. | From | Joe Perches <> | Date | Thu, 17 Dec 2020 10:25:29 -0800 |
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On Thu, 2020-12-17 at 09:27 -0800, Linus Torvalds wrote: > 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.
Using default somewhere other than the bottom of a switch/case block isn't common/pretty. It's easy to visually skip/glaze over.
Perhaps reorder the block. Maybe add static to the const arrays too. --- drivers/gpu/drm/drm_edid.c | 18 ++++++++---------- 1 file changed, 8 insertions(+), 10 deletions(-)
diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c index 74f5a3197214..53b7bb281edb 100644 --- a/drivers/gpu/drm/drm_edid.c +++ b/drivers/gpu/drm/drm_edid.c @@ -3089,8 +3089,8 @@ static int drm_cvt_modes(struct drm_connector *connector, struct drm_display_mode *newmode; struct drm_device *dev = connector->dev; struct cvt_timing *cvt; - const int rates[] = { 60, 85, 75, 60, 50 }; - const u8 empty[3] = { 0, 0, 0 }; + static const int rates[] = { 60, 85, 75, 60, 50 }; + static const u8 empty[3] = { 0, 0, 0 }; for (i = 0; i < 4; i++) { int width, height; @@ -3102,20 +3102,18 @@ 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) { - case 0x00: - width = height * 4 / 3; - break; - case 0x04: - width = height * 16 / 9; + case 0x0c: + width = height * 15 / 9; break; case 0x08: width = height * 16 / 10; break; - case 0x0c: - width = height * 15 / 9; + case 0x04: + width = height * 16 / 9; break; default: - unreachable(); + width = height * 4 / 3; + break; } for (j = 1; j < 5; j++) {
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