ata_timing: ensure t->cycle is always correct

Russell King hit a case where quantisation errors accumulated such that
the cycle time was shorter than rather than equal to the active/recovery
time. The code already knows how to stretch times to fit the cycle time
but does not know about the reverse.
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -2639,6 +2639,12 @@ int ata_timing_compute(struct ata_device *adev, unsigned short speed,
 		t->active += (t->cycle - (t->active + t->recover)) / 2;
 		t->recover = t->cycle - t->active;
 	}
+	
+	/* In a few cases quantisation may produce enough errors to
+	   leave t->cycle too low for the sum of active and recovery
+	   if so we must correct this */
+	if (t->active + t->recover > t->cycle)
+		t->cycle = t->active + t->recover;
 
 	return 0;
 }