[PATCH v2] drm/xe/guc: Guard page-fault ack with runtime PM check

[Varun Gupta <varun.gupta@intel.com>]

During VM teardown, the VM's runtime PM reference is dropped
asynchronously, allowing the device to autosuspend while stale page
faults belonging to the now-dead VM are still queued. When the
page-fault worker later tries to ack one of these, it calls into
guc_ct_send_locked() on an already-suspended device, tripping:

  Assertion `!xe_pm_runtime_suspended(xe)` failed!
  WARNING at xe_device.c:1267 xe_device_assert_mem_access+0x11c/0x140 [xe]

A live VM/exec queue always holds a PM reference while it has
outstanding work, so if the device is suspended at ack time, the
owning context is already gone and the fault is stale.

Take a runtime PM reference across the entire page-fault ack batch
preventing mid-batch suspends.

v2:
 - Hold PM ref across the entire batch (begin/end) instead of per-ack.
   This prevents the device from autosuspending mid-batch, which would
   leave write_only acks written but the end flush skipped, and skip
   counter++, desyncing the cadence check.(Himal)
 - Add a comment explaining stale faults.(Himal)

Fixes: f289f7807119 ("drm/xe: Add xe_guc_pagefault layer")
Reported-by: Szymon Markiewicz <szymon.markiewicz@intel.com>
Signed-off-by: Varun Gupta <varun.gupta@intel.com>
---
 drivers/gpu/drm/xe/xe_guc_pagefault.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_guc_pagefault.c b/drivers/gpu/drm/xe/xe_guc_pagefault.c
index 8f8210a732e9..9c9cd6e056fb 100644
--- a/drivers/gpu/drm/xe/xe_guc_pagefault.c
+++ b/drivers/gpu/drm/xe/xe_guc_pagefault.c
@@ -9,12 +9,21 @@
 #include "xe_guc_pagefault.h"
 #include "xe_pagefault.h"
 #include "xe_pagefault_types.h"
+#include "xe_pm.h"
 
 #define XE_GUC_PAGEFAULT_FLUSH_PERIOD	BIT(4)	/* Sixteen */
 
 static void guc_ack_fault_begin(void *private)
 {
 	struct xe_guc *guc = private;
+	struct xe_device *xe = guc_to_xe(guc);
+
+	/*
+	 * Live VMs hold a PM ref, so faults during suspend are stale.
+	 * Hold a PM ref across the entire batch to safely drain them
+	 * and prevent mid-batch autosuspend from desyncing CT flushes.
+	 */
+	xe_pm_runtime_get(xe);
 
 	xe_guc_ct_lock(&guc->ct);
 
@@ -62,10 +71,13 @@ static void guc_ack_fault(struct xe_pagefault *pf, int err)
 static void guc_ack_fault_end(void *private)
 {
 	struct xe_guc *guc = private;
+	struct xe_device *xe = guc_to_xe(guc);
 
 	if ((guc->pagefault_ack_counter & (XE_GUC_PAGEFAULT_FLUSH_PERIOD - 1)) != 1)
 		xe_guc_ct_send_flush(&guc->ct);
 	xe_guc_ct_unlock(&guc->ct);
+
+	xe_pm_runtime_put(xe);
 }
 
 static const struct xe_pagefault_ops guc_pagefault_ops = {
-- 
2.43.0