[PATCH V3 1/1] X86: Probe for PIC and set legacy_pic appropriately

["K. Y. Srinivasan" <kys@microsoft.com>]

This patch implements the proposal put forth by H. Peter Anvin <hpa@linux.intel.com>.

In  version V1 of the patch, I had cleaned up the code based on comments from Peter.
In  version V2 of the patch, I have addressed additional comments from Peter.
In this version of the patch, I have addressed Jan's comments (JBeulich@suse.com).

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
---
 arch/x86/kernel/cpu/mshyperv.c |    2 --
 arch/x86/kernel/i8259.c        |   20 +++++++++++++++++++-
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 47359f7..9ca9587 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -133,8 +133,6 @@ static void __init ms_hyperv_init_platform(void)
 		printk(KERN_INFO "HyperV: LAPIC Timer Frequency: %#x\n",
 				lapic_timer_frequency);
 
-		printk(KERN_INFO "HyperV: Using null_legacy_pic\n");
-		legacy_pic = &null_legacy_pic;
 	}
 #endif
 
diff --git a/arch/x86/kernel/i8259.c b/arch/x86/kernel/i8259.c
index 2e977b5..d5447f1 100644
--- a/arch/x86/kernel/i8259.c
+++ b/arch/x86/kernel/i8259.c
@@ -299,13 +299,31 @@ static void unmask_8259A(void)
 static void init_8259A(int auto_eoi)
 {
 	unsigned long flags;
+	unsigned char probe_val = ~(1 << PIC_CASCADE_IR);
+	unsigned char new_val;
 
 	i8259A_auto_eoi = auto_eoi;
 
 	raw_spin_lock_irqsave(&i8259A_lock, flags);
 
-	outb(0xff, PIC_MASTER_IMR);	/* mask all of 8259A-1 */
+	/*
+	 * Check to see if we have a PIC.
+	 * Mask all except the cascade and read
+	 * back the value we just wrote. If we don't
+	 * have a PIC, we will read 0xff as opposed to the
+	 * value we wrote.
+	 */
 	outb(0xff, PIC_SLAVE_IMR);	/* mask all of 8259A-2 */
+	outb(probe_val, PIC_MASTER_IMR);
+	new_val = inb(PIC_MASTER_IMR);
+	if (probe_val != new_val) {
+		printk(KERN_INFO "Using NULL legacy PIC\n");
+		legacy_pic = &null_legacy_pic;
+		raw_spin_unlock_irqrestore(&i8259A_lock, flags);
+		return;
+	}
+
+	outb(0xff, PIC_MASTER_IMR);	/* mask all of 8259A-1 */
 
 	/*
 	 * outb_pic - this has to work on a wide range of PC hardware.
-- 
1.7.4.1