From b0c34f6155e2d8bbe096a85a770d63ee6be6c726 Mon Sep 17 00:00:00 2001 From: Ralf Baechle Date: Wed, 17 Dec 2014 11:39:30 +0100 Subject: [PATCH] MIPS: Do not fiddle with FRE unless FRE is actually available. Commit 4227a2d4efc9c84f35826dc4d1e6dc183f6c1c05 (MIPS: Support for hybrid FPRs) changes the kernel to execute read_c0_config5() even on processors that don't have a Config5 register. According to the arch spec the behaviour of trying to read or write this register is UNDEFINED where this register doesn't exist, that is merely looking at this register is already cruel because that might kill a kitten. In case of Qemu older than v2.2 Qemu has elected to implement this UNDEFINED behaviour by taking a RI exception - which then fries the kernel: [...] Freeing YAMON memory: 956k freed Freeing unused kernel memory: 240K (80674000 - 806b0000) Reserved instruction in kernel code[#1]: CPU: 0 PID: 1 Comm: init Not tainted 3.18.0-rc6-00058-g4227a2d #26 task: 86047588 ti: 86048000 task.ti: 86048000 $ 0 : 00000000 77a638cc 00000000 00000000 [...] For qemu v2.2.0 commit f31b035a9f10dc9b57f01c426110af845d453ce2 (target-mips: correctly handle access to unimplemented CP0 register) changed the behaviour to returning zero on read and ignoring writes which more matches how typical hardware implementations actually behave. Signed-off-by: Ralf Baechle --- arch/mips/include/asm/fpu.h | 43 +++++++++++++++++++++++-------------- 1 file changed, 27 insertions(+), 16 deletions(-) diff --git a/arch/mips/include/asm/fpu.h b/arch/mips/include/asm/fpu.h index 994d21939676..5528f4e2af6a 100644 --- a/arch/mips/include/asm/fpu.h +++ b/arch/mips/include/asm/fpu.h @@ -74,8 +74,10 @@ static inline int __enable_fpu(enum fpu_mode mode) #endif /* fall through */ case FPU_32BIT: - /* clear FRE */ - write_c0_config5(read_c0_config5() & ~MIPS_CONF5_FRE); + if (cpu_has_fre) { + /* clear FRE */ + write_c0_config5(read_c0_config5() & ~MIPS_CONF5_FRE); + } fr_common: /* set CU1 & change FR appropriately */ fr = (int)mode & FPU_FR_MASK; @@ -182,25 +184,34 @@ static inline int init_fpu(void) int ret = 0; if (cpu_has_fpu) { + unsigned int config5; + ret = __own_fpu(); - if (!ret) { - unsigned int config5 = read_c0_config5(); - - /* - * Ensure FRE is clear whilst running _init_fpu, since - * single precision FP instructions are used. If FRE - * was set then we'll just end up initialising all 32 - * 64b registers. - */ - write_c0_config5(config5 & ~MIPS_CONF5_FRE); - enable_fpu_hazard(); + if (ret) + return ret; + if (!cpu_has_fre) { _init_fpu(); - /* Restore FRE */ - write_c0_config5(config5); - enable_fpu_hazard(); + return 0; } + + config5 = read_c0_config5(); + + /* + * Ensure FRE is clear whilst running _init_fpu, since + * single precision FP instructions are used. If FRE + * was set then we'll just end up initialising all 32 + * 64b registers. + */ + write_c0_config5(config5 & ~MIPS_CONF5_FRE); + enable_fpu_hazard(); + + _init_fpu(); + + /* Restore FRE */ + write_c0_config5(config5); + enable_fpu_hazard(); } else fpu_emulator_init_fpu(); -- 2.20.1