MIPS: Fix a preemption issue with thread's FPU defaults

Fix "BUG: using smp_processor_id() in preemptible" reported in accesses
to thread's FPU defaults: the value to initialise FSCR to at program
startup, the FCSR r/w mask and the contents of FIR in full FPU
emulation, removing a regression introduced with 9b26616c [MIPS: Respect
the ISA level in FCSR handling] and f6843626 [MIPS: math-emu: Set FIR
feature flags for full emulation].

Use `boot_cpu_data' to obtain the data from, following the approach that
`cpu_has_*' macros take and avoiding the call to `smp_processor_id' made
in the reference to `current_cpu_data'.  The contents of FSCR have to be
consistent across processors in an SMP system, the settings there must
not change as a thread is migrated across processors.  And the contents
of FIR are guaranteed to be consistent in FPU emulation, by definition.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Tested-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Tested-by: Paul Martin <paul.martin@codethink.co.uk>
Cc: Markos Chandras <Markos.Chandras@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10030/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/mips/include/asm/elf.h b/arch/mips/include/asm/elf.h
index a594d8ed9698..f19e890b99d2 100644
--- a/arch/mips/include/asm/elf.h
+++ b/arch/mips/include/asm/elf.h
@@ -304,7 +304,7 @@ do {									\
                                                                         \
         current->thread.abi = &mips_abi;                                \
                                                                         \
-        current->thread.fpu.fcr31 = current_cpu_data.fpu_csr31;         \
+        current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31;            \
 } while (0)
 
 #endif /* CONFIG_32BIT */
@@ -366,7 +366,7 @@ do {									\
         else                                                            \
                 current->thread.abi = &mips_abi;                        \
                                                                         \
-        current->thread.fpu.fcr31 = current_cpu_data.fpu_csr31;         \
+        current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31;            \
                                                                         \
         p = personality(current->personality);                          \
         if (p != PER_LINUX32 && p != PER_LINUX)                         \