[PATCH v2] powerpc/mpc85xx: Pass correct cpu compiler flags

[Pali Rohár]

When gcc's default cpu (selected by --with-cpu= during gcc's configure
phase) does not match target U-Boot board cpu then U-Boot binary does not
have to be compiled correctly. Lot of distributions sets gcc's default cpu
to generic powerpc variant which works fine.

U-Boot already pass -Wa,-me500 flag to gcc which instructs GNU AS to accept
e500 specific instructions when processing assembler source files (.S).

This affects also assembly files generated by gcc from C source files. And
because gcc for generic powerpc cpu puts '.machine ppc' at the beginning of
the generated assembly file, it basically overwrites -me500 flag by which
was GNU AS invoked (from U-boot build system).

It started to be an issue since binutils 2.38 which does not keep enabled
extra functional units selected by previous cpu. Hence issuing directive
'.machine ppc' (generated by gcc for generic powerpc) after '.machine e500'
(specifying at command line) disables usage of e500 specific instructions.

And compiling arch/powerpc/cpu/mpc85xx/tlb.c code throws following
assembler errors:

    {standard input}: Assembler messages:
    {standard input}:127: Error: unrecognized opcode: `tlbre'
    {standard input}:418: Error: unrecognized opcode: `tlbre'
    {standard input}:821: Error: unrecognized opcode: `msync'
    {standard input}:821: Error: unrecognized opcode: `tlbwe'
    {standard input}:884: Error: unrecognized opcode: `tlbsx'

This issue was already hit by Debian people and is reported in bug tracker:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1003490

Calling gcc with -mcpu=8540 flag fixes this issue because -mcpu=8540 tells
gcc to compile code for e500 core/cpu (overwriting gcc's default cpu) and
does not put '.machine ppc' directive into assembly anymore.

Also if gcc is invoked with -mcpu=8540 then it pass -me500 flag to GNU AS.
So it is unnecessary to manually specify -Wa,-me500 flag because it is
implicitly added.

Fix this issue properly by specifying correct -mcpu compiler flag for all
supported powerpc cores in U-Boot mpc85xx platform, which are: e500v1,
e500v2, e500mc, e5500 and e6500. For specifying e500v1 and e500v2 cores,
gcc has unintuitive -mcpu=8540 and -mcpu=8548 flag names, for other cores
-mcpu matches core name.

The only difference between gcc's -mcpu=8540 and -mcpu=8548 flags is
support for double precision floating point SPE instructions. As U-Boot
does not use floating point, it is fine to use -mcpu=8540 for both e500v1
and e500v2 cores. Moreover gcc 9 completely removed e500 floating point
support, so since gcc 9 -mcpu=8548 is just alias to -mcpu=8540.

Note that U-Boot's CONFIG_E500 option is set also for other cpus, not only
for e500v1 and e500v2. So do not check for CONFIG_E500 and rather set e500
as last fallback value when no other mpc85xx cpu matches.

Signed-off-by: Pali Rohár <pali at kernel.org>

---
Changes in v2:
* Fix commit message and fact that -mcpu=8548 flag works also with last gcc
---
 arch/powerpc/cpu/mpc85xx/config.mk | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/cpu/mpc85xx/config.mk b/arch/powerpc/cpu/mpc85xx/config.mk
index 7a1d81cf2d76..b6b5d2053aea 100644
--- a/arch/powerpc/cpu/mpc85xx/config.mk
+++ b/arch/powerpc/cpu/mpc85xx/config.mk
@@ -3,7 +3,7 @@
 # (C) Copyright 2002,2003 Motorola Inc.
 # Xianghua Xiao, X.Xiao at motorola.com
 
-PLATFORM_CPPFLAGS += -Wa,-me500 -msoft-float -mno-string
+PLATFORM_CPPFLAGS += -msoft-float -mno-string
 PLATFORM_RELFLAGS += -msingle-pic-base -fno-jump-tables
 
 # -mspe=yes is needed to have -mno-spe accepted by a buggy GCC;
@@ -11,3 +11,13 @@ PLATFORM_RELFLAGS += -msingle-pic-base -fno-jump-tables
 # http://gcc.gnu.org/ml/gcc-patches/2008-04/msg00311.html
 PLATFORM_CPPFLAGS += $(call cc-option,-mspe=yes) \
 		   $(call cc-option,-mno-spe)
+
+ifdef CONFIG_E6500
+PLATFORM_CPPFLAGS += -mcpu=e6500
+else ifdef CONFIG_E5500
+PLATFORM_CPPFLAGS += -mcpu=e5500
+else ifdef CONFIG_E500MC
+PLATFORM_CPPFLAGS += -mcpu=e500mc
+else
+PLATFORM_CPPFLAGS += -mcpu=8540
+endif
-- 
2.20.1