Re: mainline build failure for x86_64 allmodconfig with clang

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Aug 05, 2022 at 06:16:45PM +0200, Arnd Bergmann wrote:
> On Fri, Aug 5, 2022 at 5:32 PM Harry Wentland <harry.wentland@amd.com> wrote:
> > > 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
> >
> > Tbh, I was looking at the history and I can't find a good reason this
> > was added. It should be safe to drop this. I would much rather use
> > the CONFIG_FRAME_WARN value than override it.
> >
> > AFAIK most builds use 2048 by default anyways.
> 
> I'm fairly sure this was done for 32-bit builds, which default to a lower
> warning limit of 1024 bytes and would otherwise run into this
> problem when 64-bit platforms don't. With the default warning limit,
> clang warns even more about an i386 build:
> 
> display/dc/dml/dcn20/display_rq_dlg_calc_20.c:1549:6: error: stack
> frame size (1324) exceeds limit (1024) in 'dml20_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn20/display_rq_dlg_calc_20v2.c:1550:6: error: stack
> frame size (1324) exceeds limit (1024) in 'dml20v2_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn30/display_rq_dlg_calc_30.c:1742:6: error: stack
> frame size (1484) exceeds limit (1024) in 'dml30_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn31/display_rq_dlg_calc_31.c:1571:6: error: stack
> frame size (1548) exceeds limit (1024) in 'dml31_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn21/display_rq_dlg_calc_21.c:1657:6: error: stack
> frame size (1388) exceeds limit (1024) in 'dml21_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn32/display_rq_dlg_calc_32.c:206:6: error: stack
> frame size (1276) exceeds limit (1024) in 'dml32_rq_dlg_get_dlg_reg'
> display/dc/dml/dcn31/display_mode_vba_31.c:2049:13: error: stack frame
> size (1468) exceeds limit (1024) in
> 'DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation'
> display/dc/dml/dcn20/display_mode_vba_20v2.c:1145:13: error: stack
> frame size (1228) exceeds limit (1024) in
> 'dml20v2_DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation'
> display/dc/dml/dcn20/display_mode_vba_20.c:1085:13: error: stack frame
> size (1340) exceeds limit (1024) in
> 'dml20_DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation'
> display/dc/dml/dcn31/display_mode_vba_31.c:3908:6: error: stack frame
> size (1996) exceeds limit (1024) in
> 'dml31_ModeSupportAndSystemConfigurationFull'
> display/dc/dml/dcn21/display_mode_vba_21.c:1466:13: error: stack frame
> size (1308) exceeds limit (1024) in
> 'DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation'
> display/dc/dml/dcn20/display_mode_vba_20v2.c:3393:6: error: stack
> frame size (1356) exceeds limit (1024) in
> 'dml20v2_ModeSupportAndSystemConfigurationFull'
> display/dc/dml/dcn20/display_mode_vba_20.c:3286:6: error: stack frame
> size (1468) exceeds limit (1024) in
> 'dml20_ModeSupportAndSystemConfigurationFull'
> display/dc/dml/dcn21/display_mode_vba_21.c:3518:6: error: stack frame
> size (1228) exceeds limit (1024) in
> 'dml21_ModeSupportAndSystemConfigurationFull'
> display/dc/dml/dcn30/display_mode_vba_30.c:1906:13: error: stack frame
> size (1436) exceeds limit (1024) in
> 'DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation'
> display/dc/dml/dcn30/display_mode_vba_30.c:3596:6: error: stack frame
> size (2092) exceeds limit (1024) in
> 'dml30_ModeSupportAndSystemConfigurationFull'
> > > 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.
> >
> > That'd be the best quick fix. I guess if we split out functions to fix
> > stack usage we should mark them as 'noinline' in the future to avoid
> > agressive compiler optimizations.
> 
> While splitting out sub-functions can help reduce the maximum stack
> usage, it seems that in this case it makes the actual problem worse:
> I see 2168 bytes for the combined
> dml32_ModeSupportAndSystemConfigurationFull(), but marking
> mode_support_configuration() as noinline gives me 1992 bytes
> for the outer function plus 384 bytes for the inner one. So it does
> avoid the warning (barely), but not the problem that the warning tries
> to point out.

I haven't had a chance to take a look at splitting things up yet, would
you recommend a different approach?

Cheers,
Nathan