[S390] ETR support.

This patch adds support for clock synchronization to an external time
reference (ETR). The external time reference sends an oscillator
signal and a synchronization signal every 2^20 microseconds to keep
the TOD clocks of all connected servers in sync. For availability
two ETR units can be connected to a machine. If the clock deviates
for more than the sync-check tolerance all cpus get a machine check
that indicates that the clock is out of sync. For the lovely details
how to get the clock back in sync see the code below.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/drivers/s390/block/dasd.c b/drivers/s390/block/dasd.c
index f208940c463c..555e18a6b781 100644
--- a/drivers/s390/block/dasd.c
+++ b/drivers/s390/block/dasd.c
@@ -1232,6 +1232,19 @@ __dasd_process_blk_queue(struct dasd_device * device)
                 if (IS_ERR(cqr)) {
                         if (PTR_ERR(cqr) == -ENOMEM)
                                 break;  /* terminate request queue loop */
+                        if (PTR_ERR(cqr) == -EAGAIN) {
+                                /*
+                                 * The current request cannot be build right
+                                 * now, we have to try later. If this request
+                                 * is the head-of-queue we stop the device
+                                 * for 1/2 second.
+                                 */
+                                if (!list_empty(&device->ccw_queue))
+                                        break;
+                                device->stopped |= DASD_STOPPED_PENDING;
+                                dasd_set_timer(device, HZ/2);
+                                break;
+                        }
                         DBF_DEV_EVENT(DBF_ERR, device,
                                       "CCW creation failed (rc=%ld) "
                                       "on request %p",