Am 09.04.20 um 22:01 schrieb Peter Zijlstra:
On Thu, Apr 09, 2020 at 08:15:57PM +0200, Christian König wrote:
Am 09.04.20 um 19:09 schrieb Peter Zijlstra:
On Thu, Apr 09, 2020 at 05:59:56PM +0200, Peter Zijlstra wrote:
[SNIP]
I'll need another approach, let me consider.
Christian; it says these files are generated, does that generator know
which functions are wholly in FPU context and which are not?
Well that "generator" is still a human being :)
It's just that the formulae for the calculation come from the hardware team
and we are not able to easily transcript them to fixed point calculations.
Well, if it's a human, can this human respect the kernel coding style a
bit more :-) Some of that stuff is atrocious.
Yes, I know. That's unfortunately something we still need to work on as
well.
We are currently in the process of moving all the stuff which requires
floating point into a single C file(s) and then make sure that we only call
those within kernel_fpu_begin()/end() blocks.
Can you make the build system stick all those .o files in a single
archive? That's the only way I can do call validation; external
relocatoin records do not contain the section.
Need to double check that with the display team responsible for the
code, but I think that shouldn't be much of a problem.
Annotating those function with __fpu or even saying to gcc that all code of
those files should go into a special text.fpu segment shouldn't be much of a
problem.
Guess what the __fpu attribute does ;-)
Good to know that my suspicion how this is implemented was correct :)
With the below patch (which is on to of newer versions of the objtool
patches send earlier, let me know if you want a full set
Getting a branch somewhere would be perfect.
) that only
converts a few files, but fully converts:
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c
But building it (and this is an absolute pain; when you're reworking
this, can you pretty please also fix the Makefiles?), we get:
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o: warning: objtool: dcn_validate_bandwidth()+0x34fa: FPU instruction outside of kernel_fpu_{begin,end}()
$ ./scripts/faddr2line defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o dcn_validate_bandwidth+0x34fa
dcn_validate_bandwidth+0x34fa/0x57ce:
dcn_validate_bandwidth at /usr/src/linux-2.6/defconfig-build/../drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:1293 (discriminator 5)
# ./objdump-func.sh defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o dcn_validate_bandwidth | grep 34fa
34fa 50fa: f2 0f 10 b5 60 ff ff movsd -0xa0(%rbp),%xmm6
Which seems to indicate there's still problms with the current code.
Making an educated guess I would say the compiler has no idea that it
shouldn't use instructions which touch fp registers outside of
kernel_fpu_{begin,end}().
Going to talk with the display team about this whole topic internally
once more. Since this discussion already raised attention in our
technical management it shouldn't be to much of a problem to get
manpower to get this fixed properly.
Can we put this new automated check will be behind a configuration flag
initially? Or at least make it a warning and not a hard error.
Thanks,
Christian.
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