x86/platform/uv: Disable UV BAU by default

For several years, the common practice has been to boot UVs with the
"nobau" parameter on the command line, to disable the BAU.  We've
decided that it makes more sense to just disable the BAU by default in
the kernel, and provide the option to turn it on, if desired.

For now, having the on/off switch doesn't buy us any more than just
reversing the logic would, but we're working towards having the BAU
enabled by default on UV4.  When those changes are in place, having the
on/off switch will make more sense than an enable flag, since the
default behavior will be different depending on the system version.

I've also added a bit of documentation for the new parameter to
Documentation/kernel-parameters.txt.

Signed-off-by: Alex Thorlton <athorlton@sgi.com>
Reviewed-by: Hedi Berriche <hedi@sgi.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1459451909-121845-1-git-send-email-athorlton@sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/kernel-parameters.txt

@@ -131,6 +131,7 @@ parameter is applicable:
 			More X86-64 boot options can be found in
 			Documentation/x86/x86_64/boot-options.txt .
 	X86	Either 32-bit or 64-bit x86 (same as X86-32+X86-64)
+	X86_UV	SGI UV support is enabled.
 	XEN	Xen support is enabled
 
 In addition, the following text indicates that the option:
@@ -542,6 +543,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Format: <int> (must be >=0)
 			Default: 64
 
+	bau=		[X86_UV] Enable the BAU on SGI UV.  The default
+			behavior is to disable the BAU (i.e. bau=0).
+			Format: { "0" | "1" }
+			0 - Disable the BAU.
+			1 - Enable the BAU.
+			unset - Disable the BAU.
+
 	baycom_epp=	[HW,AX25]
 			Format: <io>,<mode>
 

arch/x86/include/asm/uv/uv_bau.h

@@ -598,7 +598,7 @@ struct bau_control {
 	int			timeout_tries;
 	int			ipi_attempts;
 	int			conseccompletes;
-	short			nobau;
+	bool			nobau;
 	short			baudisabled;
 	short			cpu;
 	short			osnode;

arch/x86/platform/uv/tlb_uv.c

@@ -37,7 +37,7 @@ static int timeout_base_ns[] = {
 };
 
 static int timeout_us;
-static int nobau;
+static bool nobau = true;
 static int nobau_perm;
 static cycles_t congested_cycles;
 
@@ -106,13 +106,28 @@ static char *stat_description[] = {
 	"enable:   number times use of the BAU was re-enabled"
 };
 
-static int __init
+static int __init setup_bau(char *arg)
-setup_nobau(char *arg)
 {
-	nobau = 1;
+	int result;
+
+	if (!arg)
+		return -EINVAL;
+
+	result = strtobool(arg, &nobau);
+	if (result)
+		return result;
+
+	/* we need to flip the logic here, so that bau=y sets nobau to false */
+	nobau = !nobau;
+
+	if (!nobau)
+		pr_info("UV BAU Enabled\n");
+	else
+		pr_info("UV BAU Disabled\n");
+
 	return 0;
 }
-early_param("nobau", setup_nobau);
+early_param("bau", setup_bau);
 
 /* base pnode in this partition */
 static int uv_base_pnode __read_mostly;
@@ -131,10 +146,10 @@ set_bau_on(void)
 		pr_info("BAU not initialized; cannot be turned on\n");
 		return;
 	}
-	nobau = 0;
+	nobau = false;
 	for_each_present_cpu(cpu) {
 		bcp = &per_cpu(bau_control, cpu);
-		bcp->nobau = 0;
+		bcp->nobau = false;
 	}
 	pr_info("BAU turned on\n");
 	return;
@@ -146,10 +161,10 @@ set_bau_off(void)
 	int cpu;
 	struct bau_control *bcp;
 
-	nobau = 1;
+	nobau = true;
 	for_each_present_cpu(cpu) {
 		bcp = &per_cpu(bau_control, cpu);
-		bcp->nobau = 1;
+		bcp->nobau = true;
 	}
 	pr_info("BAU turned off\n");
 	return;
@@ -1886,7 +1901,7 @@ static void __init init_per_cpu_tunables(void)
 		bcp = &per_cpu(bau_control, cpu);
 		bcp->baudisabled		= 0;
 		if (nobau)
-			bcp->nobau		= 1;
+			bcp->nobau		= true;
 		bcp->statp			= &per_cpu(ptcstats, cpu);
 		/* time interval to catch a hardware stay-busy bug */
 		bcp->timeout_interval		= usec_2_cycles(2*timeout_us);