On Thu, Apr 2, 2020 at 9:34 AM Christian König <christian.koenig@xxxxxxx> wrote: > Am 02.04.20 um 04:34 schrieb Jann Horn: > > [x86 folks in CC so that they can chime in on the precise rules for this stuff] > > I noticed that several makefiles under drivers/gpu/drm/amd/display/dc/ > > turn on floating-point instructions in the compiler flags > > (-mhard-float, -msse and -msse2) in order to make the "float" and > > "double" types usable from C code without requiring helper functions. > > > > However, as far as I know, code running in normal kernel context isn't > > allowed to use floating-point registers without special protection > > using helpers like kernel_fpu_begin() and kernel_fpu_end() (which also > > require that the protected code never blocks). If you violate that > > rule, that can lead to various issues - among other things, I think > > the kernel will clobber userspace FPU register state, and I think the > > kernel code can blow up if a context switch happens at the wrong time, > > since in-kernel task switches don't preserve FPU state. > > > > Is there some hidden trick I'm missing that makes it okay to use FPU > > registers here? > > > > I would try testing this, but unfortunately none of the AMD devices I > > have here have the appropriate graphics hardware... > > yes, using the floating point calculations in the display code has been > a source of numerous problems and confusion in the past. > > The calls to kernel_fpu_begin() and kernel_fpu_end() are hidden behind > the DC_FP_START() and DC_FP_END() macros which are supposed to hide the > architecture depend handling for x86 and PPC64. Hmm... but as far as I can tell, you're using those macros from inside functions that are already compiled with the FPU on: - drivers/gpu/drm/amd/display/dc/calcs/dcn_calcs.c uses the macros, but is already compiled with calcs_ccflags - drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c uses the macros, but is already compiled with "-mhard-float -msse -msse2" - drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c uses the macros, but is already compiled with "-mhard-float -msse -msse2" AFAIK as soon as you enter any function in any file compiled with FPU instructions, you may encounter SSE instructions, e.g. via things like compiler-generated memory-zeroing code - not just when you're actually using doubles or floats. _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
