nds32: support denormalized result through FP emulator

Currently, the nds32 FPU dose not support the arithmetic of denormalized
number. When the nds32 FPU finds the result of the instruction is a
denormlized number, the nds32 FPU considers it to be an underflow condition
and rounds the result to an appropriate number. It may causes some loss
of precision. This commit proposes a solution to re-execute the
instruction by the FPU emulator to enhance the precision. To transfer
calculations from user space to kernel space, this feature will enable
the underflow exception trap by default. Enabling this feature may cause
some side effects:
  1. Performance loss due to extra FPU exception
  2. Need another scheme to control real underflow trap
       A new parameter, UDF_trap, which is belong to FPU context is used
     to control underflow trap.

User can configure this feature via CONFIG_SUPPORT_DENORMAL_ARITHMETIC

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

2 files changed, 53 insertions, 4 deletions

diff --git a/arch/nds32/kernel/fpu.c b/arch/nds32/kernel/fpu.c
index 2942df6f93e6..fddd40c7a16f 100644
--- a/arch/nds32/kernel/fpu.c
+++ b/arch/nds32/kernel/fpu.c
@@ -12,7 +12,10 @@
 
 const struct fpu_struct init_fpuregs = {
 	.fd_regs = {[0 ... 31] = sNAN64},
-	.fpcsr = FPCSR_INIT
+	.fpcsr = FPCSR_INIT,
+#if IS_ENABLED(CONFIG_SUPPORT_DENORMAL_ARITHMETIC)
+	.UDF_trap = 0
+#endif
 };
 
 void save_fpu(struct task_struct *tsk)
@@ -174,6 +177,9 @@ inline void do_fpu_context_switch(struct pt_regs *regs)
 	} else {
 		/* First time FPU user.  */
 		load_fpu(&init_fpuregs);
+#if IS_ENABLED(CONFIG_SUPPORT_DENORMAL_ARITHMETIC)
+		current->thread.fpu.UDF_trap = init_fpuregs.UDF_trap;
+#endif
 		set_used_math();
 	}
 
@@ -183,10 +189,12 @@ inline void fill_sigfpe_signo(unsigned int fpcsr, int *signo)
 {
 	if (fpcsr & FPCSR_mskOVFT)
 		*signo = FPE_FLTOVF;
-	else if (fpcsr & FPCSR_mskIVOT)
-		*signo = FPE_FLTINV;
+#ifndef CONFIG_SUPPORT_DENORMAL_ARITHMETIC
 	else if (fpcsr & FPCSR_mskUDFT)
 		*signo = FPE_FLTUND;
+#endif
+	else if (fpcsr & FPCSR_mskIVOT)
+		*signo = FPE_FLTINV;
 	else if (fpcsr & FPCSR_mskDBZT)
 		*signo = FPE_FLTDIV;
 	else if (fpcsr & FPCSR_mskIEXT)
@@ -197,11 +205,20 @@ inline void handle_fpu_exception(struct pt_regs *regs)
 {
 	unsigned int fpcsr;
 	int si_code = 0, si_signo = SIGFPE;
+#if IS_ENABLED(CONFIG_SUPPORT_DENORMAL_ARITHMETIC)
+	unsigned long redo_except = FPCSR_mskDNIT|FPCSR_mskUDFT;
+#else
+	unsigned long redo_except = FPCSR_mskDNIT;
+#endif
 
 	lose_fpu();
 	fpcsr = current->thread.fpu.fpcsr;
 
-	if (fpcsr & FPCSR_mskDNIT) {
+	if (fpcsr & redo_except) {
+#if IS_ENABLED(CONFIG_SUPPORT_DENORMAL_ARITHMETIC)
+		if (fpcsr & FPCSR_mskUDFT)
+			current->thread.fpu.fpcsr &= ~FPCSR_mskIEX;
+#endif
 		si_signo = do_fpuemu(regs, &current->thread.fpu);
 		fpcsr = current->thread.fpu.fpcsr;
 		if (!si_signo)
diff --git a/arch/nds32/kernel/sys_nds32.c b/arch/nds32/kernel/sys_nds32.c
index 9de93ab4c52b..0835277636ce 100644
--- a/arch/nds32/kernel/sys_nds32.c
+++ b/arch/nds32/kernel/sys_nds32.c
@@ -6,6 +6,8 @@
 
 #include <asm/cachectl.h>
 #include <asm/proc-fns.h>
+#include <asm/udftrap.h>
+#include <asm/fpu.h>
 
 SYSCALL_DEFINE6(mmap2, unsigned long, addr, unsigned long, len,
 	       unsigned long, prot, unsigned long, flags,
@@ -48,3 +50,33 @@ SYSCALL_DEFINE3(cacheflush, unsigned int, start, unsigned int, end, int, cache)
 
 	return 0;
 }
+
+SYSCALL_DEFINE1(udftrap, int, option)
+{
+#if IS_ENABLED(CONFIG_SUPPORT_DENORMAL_ARITHMETIC)
+	int old_udftrap;
+
+	if (!used_math()) {
+		load_fpu(&init_fpuregs);
+		current->thread.fpu.UDF_trap = init_fpuregs.UDF_trap;
+		set_used_math();
+	}
+
+	old_udftrap = current->thread.fpu.UDF_trap;
+	switch (option) {
+	case DISABLE_UDFTRAP:
+		current->thread.fpu.UDF_trap = 0;
+		break;
+	case ENABLE_UDFTRAP:
+		current->thread.fpu.UDF_trap = FPCSR_mskUDFE;
+		break;
+	case GET_UDFTRAP:
+		break;
+	default:
+		return -EINVAL;
+	}
+	return old_udftrap;
+#else
+	return -ENOTSUPP;
+#endif
+}