powerpc/hvc_udbg: Don't crash when udbg_putc is NULL

Also while at it, add some help text indicating why you shouldn't
enable that driver under normal circumstances

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

2 files changed, 11 insertions, 1 deletions

diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig
index 8ea7f078b233..48cb8d3d1758 100644
--- a/drivers/tty/hvc/Kconfig
+++ b/drivers/tty/hvc/Kconfig
@@ -71,6 +71,10 @@ config HVC_UDBG
        depends on PPC && EXPERIMENTAL
        select HVC_DRIVER
        default n
+       help
+         This is meant to be used during HW bring up or debugging when
+         no other console mechanism exist but udbg, to get you a quick
+         console for userspace. Do NOT enable in production kernels. 
 
 config HVC_DCC
        bool "ARM JTAG DCC console"
diff --git a/drivers/tty/hvc/hvc_udbg.c b/drivers/tty/hvc/hvc_udbg.c
index b0957e61a7be..2259c6e72f06 100644
--- a/drivers/tty/hvc/hvc_udbg.c
+++ b/drivers/tty/hvc/hvc_udbg.c
@@ -36,7 +36,7 @@ static int hvc_udbg_put(uint32_t vtermno, const char *buf, int count)
 {
         int i;
 
-        for (i = 0; i < count; i++)
+        for (i = 0; i < count && udbg_putc; i++)
                 udbg_putc(buf[i]);
 
         return i;
@@ -67,6 +67,9 @@ static int __init hvc_udbg_init(void)
 {
         struct hvc_struct *hp;
 
+        if (!udbg_putc)
+                return -ENODEV;
+
         BUG_ON(hvc_udbg_dev);
 
         hp = hvc_alloc(0, NO_IRQ, &hvc_udbg_ops, 16);
@@ -88,6 +91,9 @@ module_exit(hvc_udbg_exit);
 
 static int __init hvc_udbg_console_init(void)
 {
+        if (!udbg_putc)
+                return -ENODEV;
+
         hvc_instantiate(0, 0, &hvc_udbg_ops);
         add_preferred_console("hvc", 0, NULL);