[PATCH v1] driver core: Fix suspend/resume order issue with deferred probe

[Saravana Kannan <saravanak@google.com>]

Under the following conditions:
- driver A is built in and can probe device-A
- driver B is a module and can probe device-B
- device-A is supplier of device-B

Without this patch:
1. device-A is added.
2. device-B is added.
3. dpm_list is now [device-A, device-B].
4. driver-A defers probe of device-A.
5. deferred probe of device-A is reattempted
6. device-A is moved to end of dpm_list.
6. dpm_list is now [device-B, device-A].
7. driver-B is loaded and probes device-B.
8. dpm_list stays as [device-B, device-A].

Suspend (which goes in the reverse order of dpm_list) fails because
device-A (supplier) is suspended before device-B (consumer).

With this patch:
1. device-A is added.
2. device-B is added.
3. dpm_list is now [device-A, device-B].
4. driver-A defers probe of device-A.
5. deferred probe of device-A is reattempted later.
6. dpm_list is now [device-B, device-A].
7. driver-B is loaded and probes device-B.
8. dpm_list is now [device-A, device-B].

Suspend works because device-B (consumer) is suspended before device-A
(supplier).

Fixes: 494fd7b7ad10 ("PM / core: fix deferred probe breaking suspend resume order")
Fixes: 716a7a259690 ("driver core: fw_devlink: Add support for batching fwnode parsing")
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Saravana Kannan <saravanak@google.com>
---
 drivers/base/dd.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 9a1d940342ac..52b2148c7983 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -109,6 +109,8 @@ static void deferred_probe_work_func(struct work_struct *work)
 		 * probe makes that very unsafe.
 		 */
 		device_pm_move_to_tail(dev);
+		/* Greg/Rafael: SHOULD I DELETE THIS? ^^ I think I should, but
+		 * I'm worried if it'll have some unintended consequeneces. */
 
 		dev_dbg(dev, "Retrying from deferred list\n");
 		bus_probe_device(dev);
@@ -557,6 +559,20 @@ static int really_probe(struct device *dev, struct device_driver *drv)
 		goto re_probe;
 	}
 
+	/*
+	 * The devices are added to the dpm_list (resume/suspend (reverse
+	 * order) list) as they are registered with the driver core. But the
+	 * order the devices are added doesn't necessarily match the real
+	 * dependency order.
+	 *
+	 * The successful probe order is a much better signal. If a device just
+	 * probed successfully, then we know for sure that all the devices that
+	 * probed before it don't depend on the device. So, we can safely move
+	 * the device to the end of the dpm_list. As more devices probe,
+	 * they'll automatically get ordered correctly.
+	 */
+	device_pm_move_to_tail(dev);
+
 	pinctrl_init_done(dev);
 
 	if (dev->pm_domain && dev->pm_domain->sync)
-- 
2.27.0.111.gc72c7da667-goog