virtio: console: wait for first console port for early console output

On s390 I have seen some random

"Warning: unable to open an initial console"

boot failure. Turns out that tty_open fails, because the
hvc_alloc was not yet done. In former times this could not happen,
since the probe function automatically called hvc_alloc. With newer
versions (multiport) some host<->guest interaction is required
before hvc_alloc is called. This might be too late, especially if
an initramfs is involved. Lets use a completion if we have
multiport and an early console.

[Amit:
  * Use NULL instead of 0 for pointer comparison
  * Rename 'port_added' to 'early_console_added'
  * Re-format, re-word commit message
  * Rebase patch on top of current queue]

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Acked-by: Chrstian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 387fcdf..4ca181f 100644 (file)
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -19,6 +19,7 @@
  */
 #include <linux/cdev.h>
 #include <linux/debugfs.h>
+#include <linux/completion.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/freezer.h>
@@ -74,6 +75,7 @@ struct ports_driver_data {
 static struct ports_driver_data pdrvdata;
 
 DEFINE_SPINLOCK(pdrvdata_lock);
+DECLARE_COMPLETION(early_console_added);
 
 /* This struct holds information that's relevant only for console ports */
 struct console {
@@ -1366,6 +1368,7 @@ static void handle_control_message(struct ports_device *portdev,
                        break;
 
                init_port_console(port);
+               complete(&early_console_added);
                /*
                 * Could remove the port here in case init fails - but
                 * have to notify the host first.
@@ -1668,6 +1671,10 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
        struct ports_device *portdev;
        int err;
        bool multiport;
+       bool early = early_put_chars != NULL;
+
+       /* Ensure to read early_put_chars now */
+       barrier();
 
        portdev = kmalloc(sizeof(*portdev), GFP_KERNEL);
        if (!portdev) {
@@ -1737,6 +1744,19 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
 
        __send_control_msg(portdev, VIRTIO_CONSOLE_BAD_ID,
                           VIRTIO_CONSOLE_DEVICE_READY, 1);
+
+       /*
+        * If there was an early virtio console, assume that there are no
+        * other consoles. We need to wait until the hvc_alloc matches the
+        * hvc_instantiate, otherwise tty_open will complain, resulting in
+        * a "Warning: unable to open an initial console" boot failure.
+        * Without multiport this is done in add_port above. With multiport
+        * this might take some host<->guest communication - thus we have to
+        * wait.
+        */
+       if (multiport && early)
+               wait_for_completion(&early_console_added);
+
        return 0;
 
 free_vqs: