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Subject[PATCH v4 5/5] Auto-detect whether a FPU exists
Date
We expect that a kernel with CONFIG_FPU=y can still support no-FPU machines.
To do so, the kernel should first examine the existence of a FPU, then
do nothing if a FPU does exist; otherwise, it should disable/bypass all
FPU-related functions.

In this patch, a new global variable, no_fpu, is created and determined
when parsing the hardware capability from device tree during booting.
This variable is used in those FPU-related functions.

Signed-off-by: Alan Kao <alankao@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Cc: Vincent Chen <vincentc@andestech.com>
Cc: Zong Li <zong@andestech.com>
Cc: Nick Hu <nickhu@andestech.com>
---
arch/riscv/include/asm/hwcap.h | 3 +++
arch/riscv/include/asm/switch_to.h | 13 ++++++++++++-
arch/riscv/kernel/cpufeature.c | 11 +++++++++++
arch/riscv/kernel/signal.c | 6 ++++++
4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index 8a4ed7bbcbea..1b870086a869 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -33,5 +33,8 @@ enum {
};

extern unsigned long elf_hwcap;
+#ifdef CONFIG_FPU
+extern bool no_fpu;
+#endif
#endif
#endif
diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 093050b03543..7278e3eb7a70 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -17,6 +17,7 @@
#include <asm/processor.h>
#include <asm/ptrace.h>
#include <asm/csr.h>
+#include <asm/hwcap.h>

#ifdef CONFIG_FPU
extern void __fstate_save(struct task_struct *save_to);
@@ -30,6 +31,9 @@ static inline void __fstate_clean(struct pt_regs *regs)
static inline void fstate_save(struct task_struct *task,
struct pt_regs *regs)
{
+ if (unlikely(no_fpu))
+ return;
+
if ((regs->sstatus & SR_FS) == SR_FS_DIRTY) {
__fstate_save(task);
__fstate_clean(regs);
@@ -39,6 +43,9 @@ static inline void fstate_save(struct task_struct *task,
static inline void fstate_restore(struct task_struct *task,
struct pt_regs *regs)
{
+ if (unlikely(no_fpu))
+ return;
+
if ((regs->sstatus & SR_FS) != SR_FS_OFF) {
__fstate_restore(task);
__fstate_clean(regs);
@@ -50,13 +57,17 @@ static inline void __switch_to_aux(struct task_struct *prev,
{
struct pt_regs *regs;

+ if (unlikely(no_fpu))
+ return;
+
regs = task_pt_regs(prev);
if (unlikely(regs->sstatus & SR_SD))
fstate_save(prev, regs);
fstate_restore(next, task_pt_regs(next));
}

-#define DEFAULT_SSTATUS (SR_SPIE | SR_FS_INITIAL)
+#define DEFAULT_SSTATUS \
+ ((unlikely(no_fpu)) ? (SR_SPIE | SR_FS_OFF) : (SR_SPIE | SR_FS_INITIAL))

#else
#define fstate_save(task, regs) do { } while (0)
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 17011a870044..bc269c1e0b1a 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -22,6 +22,9 @@
#include <asm/hwcap.h>

unsigned long elf_hwcap __read_mostly;
+#ifdef CONFIG_FPU
+bool no_fpu __read_mostly;
+#endif

void riscv_fill_hwcap(void)
{
@@ -58,4 +61,12 @@ void riscv_fill_hwcap(void)
elf_hwcap |= isa2hwcap[(unsigned char)(isa[i])];

pr_info("elf_hwcap is 0x%lx", elf_hwcap);
+
+#ifdef CONFIG_FPU
+ no_fpu = 0;
+ if (!(elf_hwcap & (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D))) {
+ pr_info("Bypass FPU code.");
+ no_fpu = 1;
+ }
+#endif
}
diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
index 2450b824d799..9714e4fccb69 100644
--- a/arch/riscv/kernel/signal.c
+++ b/arch/riscv/kernel/signal.c
@@ -45,6 +45,9 @@ static long restore_fp_state(struct pt_regs *regs,
struct __riscv_d_ext_state __user *state = &sc_fpregs->d;
size_t i;

+ if (unlikely(no_fpu))
+ return 0;
+
err = __copy_from_user(&current->thread.fstate, state, sizeof(*state));
if (unlikely(err))
return err;
@@ -72,6 +75,9 @@ static long save_fp_state(struct pt_regs *regs,
struct __riscv_d_ext_state __user *state = &sc_fpregs->d;
size_t i;

+ if (unlikely(no_fpu))
+ return 0;
+
fstate_save(current, regs);
err = __copy_to_user(state, &current->thread.fstate, sizeof(*state));
if (unlikely(err))
--
2.18.0
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