[IA64] hooks to wait for mmio writes to drain when migrating processes

On SN2, MMIO writes which are issued from separate processors are not
guaranteed to arrive in any particular order at the IO hardware.  When
performing such writes from the kernel this is not a problem, as a
kernel thread will not migrate to another CPU during execution, and
mmiowb() calls can guarantee write ordering when control of the IO
resource is allowed to move between threads.

However, when MMIO writes can be performed from user space (e.g. DRM)
there are no such guarantees and mechanisms, as the process may
context-switch at any time, and may migrate to a different CPU as part
of the switch.  For such programs/hardware to operate correctly, it is
required that the MMIO writes from the old CPU be accepted by the IO
hardware before subsequent writes from the new CPU can be issued.

The following patch implements this behavior on SN2 by waiting for a
Shub register to indicate that these writes have been accepted.  This
is placed in the context switch-in path, and only performs the wait
when the newly scheduled task changes CPUs.

Signed-off-by: Prarit Bhargava <prarit@sgi.com>
Signed-off-by: Brent Casavant <bcasavan@sgi.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/arch/ia64/sn/kernel/sn2/sn2_smp.c b/arch/ia64/sn/kernel/sn2/sn2_smp.c
index 471bbaa65d1b..1b33fd5e4e3a 100644
--- a/arch/ia64/sn/kernel/sn2/sn2_smp.c
+++ b/arch/ia64/sn/kernel/sn2/sn2_smp.c
@@ -5,7 +5,7 @@
  * License.  See the file "COPYING" in the main directory of this archive
  * for more details.
  *
- * Copyright (C) 2000-2005 Silicon Graphics, Inc. All rights reserved.
+ * Copyright (C) 2000-2006 Silicon Graphics, Inc. All rights reserved.
  */
 
 #include <linux/init.h>
@@ -169,6 +169,27 @@ static inline unsigned long wait_piowc(void)
         return ws;
 }
 
+/**
+ * sn_migrate - SN-specific task migration actions
+ * @task: Task being migrated to new CPU
+ *
+ * SN2 PIO writes from separate CPUs are not guaranteed to arrive in order.
+ * Context switching user threads which have memory-mapped MMIO may cause
+ * PIOs to issue from seperate CPUs, thus the PIO writes must be drained
+ * from the previous CPU's Shub before execution resumes on the new CPU.
+ */
+void sn_migrate(struct task_struct *task)
+{
+        pda_t *last_pda = pdacpu(task_thread_info(task)->last_cpu);
+        volatile unsigned long *adr = last_pda->pio_write_status_addr;
+        unsigned long val = last_pda->pio_write_status_val;
+
+        /* Drain PIO writes from old CPU's Shub */
+        while (unlikely((*adr & SH_PIO_WRITE_STATUS_PENDING_WRITE_COUNT_MASK)
+                        != val))
+                cpu_relax();
+}
+
 void sn_tlb_migrate_finish(struct mm_struct *mm)
 {
         if (mm == current->mm)