drm/i915: Skip the ERR_PTR error state

Although commit fb6f0b64 ("drm/i915: Prevent machine hang from
Broxton's vtd w/a and error capture") applied cleanly after a 24 month
hiatus, the code had moved on with new methods for peeking and fetching
the captured gpu info. Make sure we catch all uses of the stashed error
state and avoid dereferencing the error pointer.

v2: Move error pointer determination into i915_gpu_capture_state
v3: Restore early check to avoid capturing and then throwing away
subsequent GPU error states.

Fixes: fb6f0b64 ("drm/i915: Prevent machine hang from Broxton's vtd w/a and error capture")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181207110554.19897-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -984,8 +984,8 @@ static int i915_gpu_info_open(struct inode *inode, struct file *file)
 	intel_runtime_pm_get(i915);
 	gpu = i915_capture_gpu_state(i915);
 	intel_runtime_pm_put(i915);
-	if (!gpu)
-		return -ENOMEM;
+	if (IS_ERR(gpu))
+		return PTR_ERR(gpu);
 
 	file->private_data = gpu;
 	return 0;
@@ -1018,7 +1018,13 @@ i915_error_state_write(struct file *filp,
 
 static int i915_error_state_open(struct inode *inode, struct file *file)
 {
-	file->private_data = i915_first_error_state(inode->i_private);
+	struct i915_gpu_state *error;
+
+	error = i915_first_error_state(inode->i_private);
+	if (IS_ERR(error))
+		return PTR_ERR(error);
+
+	file->private_data  = error;
 	return 0;
 }
 

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1907,9 +1907,16 @@ i915_capture_gpu_state(struct drm_i915_private *i915)
 {
 	struct i915_gpu_state *error;
 
+	/* Check if GPU capture has been disabled */
+	error = READ_ONCE(i915->gpu_error.first_error);
+	if (IS_ERR(error))
+		return error;
+
 	error = kzalloc(sizeof(*error), GFP_ATOMIC);
-	if (!error)
-		return NULL;
+	if (!error) {
+		i915_disable_error_state(i915, -ENOMEM);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	kref_init(&error->ref);
 	error->i915 = i915;
@@ -1945,11 +1952,8 @@ void i915_capture_error_state(struct drm_i915_private *i915,
 		return;
 
 	error = i915_capture_gpu_state(i915);
-	if (!error) {
-		DRM_DEBUG_DRIVER("out of memory, not capturing error state\n");
-		i915_disable_error_state(i915, -ENOMEM);
+	if (IS_ERR(error))
 		return;
-	}
 
 	i915_error_capture_msg(i915, error, engine_mask, error_msg);
 	DRM_INFO("%s\n", error->error_msg);
@@ -1987,7 +1991,7 @@ i915_first_error_state(struct drm_i915_private *i915)
 
 	spin_lock_irq(&i915->gpu_error.lock);
 	error = i915->gpu_error.first_error;
-	if (error)
+	if (!IS_ERR_OR_NULL(error))
 		i915_gpu_state_get(error);
 	spin_unlock_irq(&i915->gpu_error.lock);
 
@@ -2000,10 +2004,11 @@ void i915_reset_error_state(struct drm_i915_private *i915)
 
 	spin_lock_irq(&i915->gpu_error.lock);
 	error = i915->gpu_error.first_error;
+	if (error != ERR_PTR(-ENODEV)) /* if disabled, always disabled */
 		i915->gpu_error.first_error = NULL;
 	spin_unlock_irq(&i915->gpu_error.lock);
 
-	if (!IS_ERR(error))
+	if (!IS_ERR_OR_NULL(error))
 		i915_gpu_state_put(error);
 }
 

drivers/gpu/drm/i915/i915_sysfs.c

@@ -521,7 +521,9 @@ static ssize_t error_state_read(struct file *filp, struct kobject *kobj,
 	ssize_t ret;
 
 	gpu = i915_first_error_state(i915);
-	if (gpu) {
+	if (IS_ERR(gpu)) {
+		ret = PTR_ERR(gpu);
+	} else if (gpu) {
 		ret = i915_gpu_state_copy_to_buffer(gpu, buf, off, count);
 		i915_gpu_state_put(gpu);
 	} else {