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Subject[PATCH v6 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, has_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/switch_to.h | 8 ++++----
arch/riscv/kernel/cpufeature.c | 8 ++++++++
arch/riscv/kernel/process.c | 4 +++-
arch/riscv/kernel/signal.c | 6 ++++--
4 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 093050b03543..733559083f24 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -56,13 +56,12 @@ static inline void __switch_to_aux(struct task_struct *prev,
fstate_restore(next, task_pt_regs(next));
}

-#define DEFAULT_SSTATUS (SR_SPIE | SR_FS_INITIAL)
-
+extern bool has_fpu;
#else
+#define has_fpu false
#define fstate_save(task, regs) do { } while (0)
#define fstate_restore(task, regs) do { } while (0)
#define __switch_to_aux(__prev, __next) do { } while (0)
-#define DEFAULT_SSTATUS (SR_SPIE | SR_FS_OFF)
#endif

extern struct task_struct *__switch_to(struct task_struct *,
@@ -72,7 +71,8 @@ extern struct task_struct *__switch_to(struct task_struct *,
do { \
struct task_struct *__prev = (prev); \
struct task_struct *__next = (next); \
- __switch_to_aux(__prev, __next); \
+ if (has_fpu) \
+ __switch_to_aux(__prev, __next); \
((last) = __switch_to(__prev, __next)); \
} while (0)

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 17011a870044..46942e635266 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 has_fpu __read_mostly;
+#endif

void riscv_fill_hwcap(void)
{
@@ -58,4 +61,9 @@ void riscv_fill_hwcap(void)
elf_hwcap |= isa2hwcap[(unsigned char)(isa[i])];

pr_info("elf_hwcap is 0x%lx", elf_hwcap);
+
+#ifdef CONFIG_FPU
+ if (elf_hwcap & (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D))
+ has_fpu = true;
+#endif
}
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 3820d89e2db9..97155aee9e71 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -83,7 +83,9 @@ void show_regs(struct pt_regs *regs)
void start_thread(struct pt_regs *regs, unsigned long pc,
unsigned long sp)
{
- regs->sstatus = DEFAULT_SSTATUS;
+ regs->sstatus = SR_SPIE;
+ if (has_fpu)
+ regs->sstatus |= SR_FS_INITIAL;
regs->sepc = pc;
regs->sp = sp;
set_fs(USER_DS);
diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
index 2450b824d799..f9b5e7e352ef 100644
--- a/arch/riscv/kernel/signal.c
+++ b/arch/riscv/kernel/signal.c
@@ -98,7 +98,8 @@ static long restore_sigcontext(struct pt_regs *regs,
/* sc_regs is structured the same as the start of pt_regs */
err = __copy_from_user(regs, &sc->sc_regs, sizeof(sc->sc_regs));
/* Restore the floating-point state. */
- err |= restore_fp_state(regs, &sc->sc_fpregs);
+ if (has_fpu)
+ err |= restore_fp_state(regs, &sc->sc_fpregs);
return err;
}

@@ -150,7 +151,8 @@ static long setup_sigcontext(struct rt_sigframe __user *frame,
/* sc_regs is structured the same as the start of pt_regs */
err = __copy_to_user(&sc->sc_regs, regs, sizeof(sc->sc_regs));
/* Save the floating-point state. */
- err |= save_fp_state(regs, &sc->sc_fpregs);
+ if (has_fpu)
+ err |= save_fp_state(regs, &sc->sc_fpregs);
return err;
}

--
2.18.0
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