[x86] only print out DR registers if they are not power-on defaults.

[Dave Jones <davej@redhat.com>]

The DR registers are rarely useful when decoding oopses.
With screen real estate during oopses at a premium, we can save two lines
by only printing out these registers when they are set to something other
than they power-on state.

Signed-off-by: Dave Jones <davej@redhat.com>

diff -durpN '--exclude-from=/home/davej/.exclude' /home/davej/src/kernel/git-trees/linux/arch/x86/kernel/process_64.c linux-dj/arch/x86/kernel/process_64.c
--- /home/davej/src/kernel/git-trees/linux/arch/x86/kernel/process_64.c	2013-05-01 10:02:52.064151923 -0400
+++ linux-dj/arch/x86/kernel/process_64.c	2013-05-06 20:35:09.219868881 -0400
@@ -105,11 +105,18 @@ void __show_regs(struct pt_regs *regs, i
 	get_debugreg(d0, 0);
 	get_debugreg(d1, 1);
 	get_debugreg(d2, 2);
-	printk(KERN_DEFAULT "DR0: %016lx DR1: %016lx DR2: %016lx\n", d0, d1, d2);
 	get_debugreg(d3, 3);
 	get_debugreg(d6, 6);
 	get_debugreg(d7, 7);
+
+	/* Only print out debug registers if they are in their non-default state. */
+	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
+	    (d6 & ~DR6_RESERVED) && (d7 == 0x400))
+		return;
+
+	printk(KERN_DEFAULT "DR0: %016lx DR1: %016lx DR2: %016lx\n", d0, d1, d2);
 	printk(KERN_DEFAULT "DR3: %016lx DR6: %016lx DR7: %016lx\n", d3, d6, d7);
+
 }
 
 void release_thread(struct task_struct *dead_task)

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 7305f7d..5905dc5 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -110,11 +110,16 @@ void __show_regs(struct pt_regs *regs, int all)
 	get_debugreg(d1, 1);
 	get_debugreg(d2, 2);
 	get_debugreg(d3, 3);
-	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
-			d0, d1, d2, d3);
-
 	get_debugreg(d6, 6);
 	get_debugreg(d7, 7);
+
+	/* Only print out debug registers if they are in their non-default state. */
+	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
+	    (d6 & ~DR6_RESERVED) && (d7 == 0x400))
+		return;
+
+	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
+			d0, d1, d2, d3);
 	printk(KERN_DEFAULT "DR6: %08lx DR7: %08lx\n",
 			d6, d7);
 }