diff --git a/arch/mips/txx9/rbtx4927/irq.c b/arch/mips/txx9/rbtx4927/irq.c
index 22076e3f03a8f10b1bfd82c54ac1cf9ea10540ef..9c14ebb26cb4caf63f7448b02b83ad4e435f2332 100644
--- a/arch/mips/txx9/rbtx4927/irq.c
+++ b/arch/mips/txx9/rbtx4927/irq.c
@@ -133,9 +133,9 @@ static int toshiba_rbtx4927_irq_nested(int sw_irq)
 	u8 level3;
 
 	level3 = readb(rbtx4927_imstat_addr) & 0x1f;
-	if (level3)
-		sw_irq = RBTX4927_IRQ_IOC + fls(level3) - 1;
-	return sw_irq;
+	if (unlikely(!level3))
+		return -1;
+	return RBTX4927_IRQ_IOC + __fls8(level3);
 }
 
 static void __init toshiba_rbtx4927_irq_ioc_init(void)
diff --git a/arch/mips/txx9/rbtx4938/irq.c b/arch/mips/txx9/rbtx4938/irq.c
index ca2f8306ce93b1e95fde8594793507c4bdc356df..7d21befb8932c5ba36df028cb74b1c66e61c652d 100644
--- a/arch/mips/txx9/rbtx4938/irq.c
+++ b/arch/mips/txx9/rbtx4938/irq.c
@@ -85,10 +85,10 @@ static int toshiba_rbtx4938_irq_nested(int sw_irq)
 	u8 level3;
 
 	level3 = readb(rbtx4938_imstat_addr);
-	if (level3)
-		/* must use fls so onboard ATA has priority */
-		sw_irq = RBTX4938_IRQ_IOC + fls(level3) - 1;
-	return sw_irq;
+	if (unlikely(!level3))
+		return -1;
+	/* must use fls so onboard ATA has priority */
+	return RBTX4938_IRQ_IOC + __fls8(level3);
 }
 
 static void __init
diff --git a/include/asm-mips/txx9/generic.h b/include/asm-mips/txx9/generic.h
index 1e1a9f2d2379653254484a96d496ebb9e4c31ea3..dc85515eac13659e2b909258ddb22fdc5d7efb3e 100644
--- a/include/asm-mips/txx9/generic.h
+++ b/include/asm-mips/txx9/generic.h
@@ -64,4 +64,22 @@ struct physmap_flash_data;
 void txx9_physmap_flash_init(int no, unsigned long addr, unsigned long size,
 			     const struct physmap_flash_data *pdata);
 
+/* 8 bit version of __fls(): find first bit set (returns 0..7) */
+static inline unsigned int __fls8(unsigned char x)
+{
+	int r = 7;
+
+	if (!(x & 0xf0)) {
+		r -= 4;
+		x <<= 4;
+	}
+	if (!(x & 0xc0)) {
+		r -= 2;
+		x <<= 2;
+	}
+	if (!(x & 0x80))
+		r -= 1;
+	return r;
+}
+
 #endif /* __ASM_TXX9_GENERIC_H */