ANDROID: tty: hvc_dcc: Add parameter to enable DCC

On cuttlefish device, DCC registers are unavailable and cause kernel to
crash if those registers are probed. Introduce a module parameter
("hvc_dcc.enable") to enable DCC at the kernel commandline.

Bug: 169129589
Change-Id: I0218d9e64443c881d163e484712edf18e42975fd
Signed-off-by: Elliot Berman <eberman@codeaurora.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -1550,6 +1550,10 @@
 				If specified, z/VM IUCV HVC accepts connections
 				from listed z/VM user IDs only.
 
+	hvc_dcc.enable=	[ARM,ARM64]	Enable DCC driver at runtime. For GKI,
+				disabled at runtime by default to prevent
+				crashes in devices which do not support DCC.
+
 	hv_nopvspin	[X86,HYPER_V] Disables the paravirt spinlock optimizations
 				      which allow the hypervisor to 'idle' the
 				      guest on lock contention.

drivers/tty/hvc/hvc_dcc.c

@@ -4,6 +4,7 @@
 #include <linux/console.h>
 #include <linux/init.h>
 #include <linux/kfifo.h>
+#include <linux/moduleparam.h>
 #include <linux/serial.h>
 #include <linux/serial_core.h>
 #include <linux/spinlock.h>
@@ -13,6 +14,13 @@
 
 #include "hvc_console.h"
 
+/*
+ * Disable DCC driver at runtime. Want driver enabled for GKI, but some devices
+ * do not support the registers and crash when driver pokes the registers
+ */
+static bool enable;
+module_param(enable, bool, 0444);
+
 /* DCC Status Bits */
 #define DCC_STATUS_RX		(1 << 30)
 #define DCC_STATUS_TX		(1 << 29)
@@ -244,7 +252,7 @@ static int __init hvc_dcc_console_init(void)
 {
 	int ret;
 
-	if (!hvc_dcc_check())
+	if (!enable || !hvc_dcc_check())
 		return -ENODEV;
 
 	/* Returns -1 if error */
@@ -258,7 +266,7 @@ static int __init hvc_dcc_init(void)
 {
 	struct hvc_struct *p;
 
-	if (!hvc_dcc_check())
+	if (!enable || !hvc_dcc_check())
 		return -ENODEV;
 
 	p = hvc_alloc(0, 0, &hvc_dcc_get_put_ops, 128);