Delay creation of khcvd thread

This changes hvc_init() to be called only when someone actually uses the
hvc_console driver.  Dave Jones complained when profiling bootup.

hvc_console used to only be for Power aka pSeries: now lguest and Xen both
want it built-in in case the kernel is a guest under one of those, even
though usually it will be a native boot.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 42 insertions, 18 deletions

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 1232ace097e1..57b801716fbe 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -585,8 +585,8 @@ config TIPAR
 config HVC_DRIVER
         bool
         help
-          Users of pSeries machines that want to utilize the hvc console front-end
-          module for their backend console driver should select this option.
+          Generic "hypervisor virtual console" infrastructure for various
+          hypervisors (pSeries, Xen, lguest).
           It will automatically be selected if one of the back-end console drivers
           is selected.
 
diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index 83c1151ec7a2..8252f8668538 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -69,6 +69,8 @@ static struct task_struct *hvc_task;
 /* Picks up late kicks after list walk but before schedule() */
 static int hvc_kicked;
 
+static int hvc_init(void);
+
 #ifdef CONFIG_MAGIC_SYSRQ
 static int sysrq_pressed;
 #endif
@@ -754,6 +756,13 @@ struct hvc_struct __devinit *hvc_alloc(uint32_t vtermno, int irq,
         struct hvc_struct *hp;
         int i;
 
+        /* We wait until a driver actually comes along */
+        if (!hvc_driver) {
+                int err = hvc_init();
+                if (err)
+                        return ERR_PTR(err);
+        }
+
         hp = kmalloc(ALIGN(sizeof(*hp), sizeof(long)) + outbuf_size,
                         GFP_KERNEL);
         if (!hp)
@@ -829,16 +838,18 @@ int __devexit hvc_remove(struct hvc_struct *hp)
         return 0;
 }
 
-/* Driver initialization.  Follow console initialization.  This is where the TTY
- * interfaces start to become available. */
-static int __init hvc_init(void)
+/* Driver initialization: called as soon as someone uses hvc_alloc(). */
+static int hvc_init(void)
 {
         struct tty_driver *drv;
+        int err;
 
         /* We need more than hvc_count adapters due to hotplug additions. */
         drv = alloc_tty_driver(HVC_ALLOC_TTY_ADAPTERS);
-        if (!drv)
-                return -ENOMEM;
+        if (!drv) {
+                err = -ENOMEM;
+                goto out;
+        }
 
         drv->owner = THIS_MODULE;
         drv->driver_name = "hvc";
@@ -854,30 +865,43 @@ static int __init hvc_init(void)
          * added later. */
         hvc_task = kthread_run(khvcd, NULL, "khvcd");
         if (IS_ERR(hvc_task)) {
-                panic("Couldn't create kthread for console.\n");
-                put_tty_driver(drv);
-                return -EIO;
+                printk(KERN_ERR "Couldn't create kthread for console.\n");
+                err = PTR_ERR(hvc_task);
+                goto put_tty;
         }
 
-        if (tty_register_driver(drv))
-                panic("Couldn't register hvc console driver\n");
+        err = tty_register_driver(drv);
+        if (err) {
+                printk(KERN_ERR "Couldn't register hvc console driver\n");
+                goto stop_thread;
+        }
 
+        /* FIXME: This mb() seems completely random.  Remove it. */
         mb();
         hvc_driver = drv;
         return 0;
+
+put_tty:
+        put_tty_driver(hvc_driver);
+stop_thread:
+        kthread_stop(hvc_task);
+        hvc_task = NULL;
+out:
+        return err;
 }
-module_init(hvc_init);
 
 /* This isn't particularly necessary due to this being a console driver
  * but it is nice to be thorough.
  */
 static void __exit hvc_exit(void)
 {
-        kthread_stop(hvc_task);
+        if (hvc_driver) {
+                kthread_stop(hvc_task);
 
-        tty_unregister_driver(hvc_driver);
-        /* return tty_struct instances allocated in hvc_init(). */
-        put_tty_driver(hvc_driver);
-        unregister_console(&hvc_con_driver);
+                tty_unregister_driver(hvc_driver);
+                /* return tty_struct instances allocated in hvc_init(). */
+                put_tty_driver(hvc_driver);
+                unregister_console(&hvc_con_driver);
+        }
 }
 module_exit(hvc_exit);