On Thu, Aug 04, 2022 at 09:24:41PM +0200, Arnd Bergmann wrote: > On Thu, Aug 4, 2022 at 8:52 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, Aug 4, 2022 at 11:37 AM Sudip Mukherjee (Codethink) > > <sudipm.mukherjee@xxxxxxxxx> wrote:cov_trace_cmp > > > > > > git bisect points to 3876a8b5e241 ("drm/amd/display: Enable building new display engine with KCOV enabled"). > > > > Ahh. So that was presumably why it was disabled before - because it > > presumably does disgusting things that make KCOV generate even bigger > > stack frames than it already has. > > > > Those functions do seem to have fairly big stack footprints already (I > > didn't try to look into why, I assume it's partly due to aggressive > > inlining, and probably some automatic structures on stack). But gcc > > doesn't seem to make it all that much worse with KCOV (and my clang > > build doesn't enable KCOV). > > > > So it's presumably some KCOV-vs-clang thing. Nathan? Looks like Arnd beat me to it :) > The dependency was originally added to avoid a link failure in 9d1d02ff3678 > ("drm/amd/display: Don't build DCN1 when kcov is enabled") after I reported the > problem in https://lists.freedesktop.org/archives/dri-devel/2018-August/186131.html > > The commit from the bisection just turns off KCOV for the entire directory > to avoid the link failure, so it's not actually a problem with KCOV vs clang, > but I think a problem with clang vs badly written code that was obscured > in allmodconfig builds prior to this. Right, I do think the sanitizers make things worse here too, as those get enabled with allmodconfig. I ran some really quick tests with allmodconfig and a few instrumentation options flipped on/off: allmodconfig (CONFIG_KASAN=y, CONFIG_KCSAN=n, CONFIG_KCOV=y, and CONFIG_UBSAN=y): warning: stack frame size (2216) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2184) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2176) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] allmodconfig + CONFIG_KASAN=n: warning: stack frame size (2112) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] allmodconfig + CONFIG_KCOV=n: warning: stack frame size (2216) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2184) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2176) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] allmodconfig + CONFIG_UBSAN=n: warning: stack frame size (2584) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2680) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2352) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] allmodconfig + CONFIG_KASAN=n + CONFIG_KCSAN=y + CONFIG_UBSAN=n: warning: stack frame size (2504) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2600) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] warning: stack frame size (2264) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] allmodconfig + CONFIG_KASAN=n + CONFIG_KCSAN=n + CONFIG_UBSAN=n: warning: stack frame size (2072) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than] There might be other debugging configurations that make this worse too, as I don't see those warnings on my distribution configuration. > The dml30_ModeSupportAndSystemConfigurationFull() function exercises > a few paths in the compiler that are otherwise rare. On thing it does is to > pass up to 60 arguments to other functions, and it heavily uses float and > double variables. Both of these make it rather fragile when it comes to > unusual compiler options, so the files keep coming up whenever a new > instrumentation feature gets added. There is probably some other flag > in allmodconfig that we can disable to improve this again, but I have not > checked this time. I do notice that these files build with a non-configurable -Wframe-large-than value: $ rg frame_warn_flag drivers/gpu/drm/amd/display/dc/dml/Makefile 54:frame_warn_flag := -Wframe-larger-than=2048 70:CFLAGS_$(AMDDALPATH)/dc/dml/dcn30/display_mode_vba_30.o := $(dml_ccflags) $(frame_warn_flag) 72:CFLAGS_$(AMDDALPATH)/dc/dml/dcn31/display_mode_vba_31.o := $(dml_ccflags) $(frame_warn_flag) 76:CFLAGS_$(AMDDALPATH)/dc/dml/dcn32/display_mode_vba_32.o := $(dml_ccflags) $(frame_warn_flag) I suppose that could just be bumped as a quick workaround? Two of those files have a comment that implies modifying them in non-trivial ways is not recommended. /* * NOTE: * This file is gcc-parsable HW gospel, coming straight from HW engineers. * * It doesn't adhere to Linux kernel style and sometimes will do things in odd * ways. Unless there is something clearly wrong with it the code should * remain as-is as it provides us with a guarantee from HW that it is correct. */ I do note that commit 1b54a0121dba ("drm/amd/display: Reduce stack size in the mode support function") did have a workaround for GCC. It appears clang will still inline mode_support_configuration(). If I mark it as 'noinline', the warning disappears in that file. Cheers, Nathan