platform/surface: aggregator: Allow devices to be marked as hot-removed

Some SSAM devices, notably the keyboard cover (keyboard and touchpad) on
the Surface Pro 8, can be hot-removed. When this occurs, communication
with the device may fail and time out. This timeout can unnecessarily
block and slow down device removal and even cause issues when the
devices are detached and re-attached quickly. Thus, communication should
generally be avoided once hot-removal is detected.

While we already remove a device as soon as we detect its (hot-)removal,
the corresponding device driver may still attempt to communicate with
the device during teardown. This is especially critical as communication
failure may also extend to disabling of events, which is typically done
at that stage.

Add a flag to allow marking devices as hot-removed. This can then be
used during client driver teardown to check if any communication
attempts should be avoided.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20220527023447.2460025-3-luzmaximilian@gmail.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

include/linux/surface_aggregator/device.h

@@ -148,17 +148,30 @@ struct ssam_device_uid {
 #define SSAM_SDEV(cat, tid, iid, fun) \
 	SSAM_DEVICE(SSAM_DOMAIN_SERIALHUB, SSAM_SSH_TC_##cat, tid, iid, fun)
 
+/*
+ * enum ssam_device_flags - Flags for SSAM client devices.
+ * @SSAM_DEVICE_HOT_REMOVED_BIT:
+ *	The device has been hot-removed. Further communication with it may time
+ *	out and should be avoided.
+ */
+enum ssam_device_flags {
+	SSAM_DEVICE_HOT_REMOVED_BIT = 0,
+};
+
 /**
  * struct ssam_device - SSAM client device.
  * @dev:   Driver model representation of the device.
  * @ctrl:  SSAM controller managing this device.
  * @uid:   UID identifying the device.
+ * @flags: Device state flags, see &enum ssam_device_flags.
  */
 struct ssam_device {
 	struct device dev;
 	struct ssam_controller *ctrl;
 
 	struct ssam_device_uid uid;
+
+	unsigned long flags;
 };
 
 /**
@@ -251,6 +264,35 @@ struct ssam_device *ssam_device_alloc(struct ssam_controller *ctrl,
 int ssam_device_add(struct ssam_device *sdev);
 void ssam_device_remove(struct ssam_device *sdev);
 
+/**
+ * ssam_device_mark_hot_removed() - Mark the given device as hot-removed.
+ * @sdev: The device to mark as hot-removed.
+ *
+ * Mark the device as having been hot-removed. This signals drivers using the
+ * device that communication with the device should be avoided and may lead to
+ * timeouts.
+ */
+static inline void ssam_device_mark_hot_removed(struct ssam_device *sdev)
+{
+	dev_dbg(&sdev->dev, "marking device as hot-removed\n");
+	set_bit(SSAM_DEVICE_HOT_REMOVED_BIT, &sdev->flags);
+}
+
+/**
+ * ssam_device_is_hot_removed() - Check if the given device has been
+ * hot-removed.
+ * @sdev: The device to check.
+ *
+ * Checks if the given device has been marked as hot-removed. See
+ * ssam_device_mark_hot_removed() for more details.
+ *
+ * Return: Returns ``true`` if the device has been marked as hot-removed.
+ */
+static inline bool ssam_device_is_hot_removed(struct ssam_device *sdev)
+{
+	return test_bit(SSAM_DEVICE_HOT_REMOVED_BIT, &sdev->flags);
+}
+
 /**
  * ssam_device_get() - Increment reference count of SSAM client device.
  * @sdev: The device to increment the reference count of.