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

[Rafael J. Wysocki]

On Thu, Jun 25, 2020 at 5:24 AM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> ---
>  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. */

Yes, this needs to go away if you make the other change.

>
>                 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);

But it would be good to somehow limit this to the devices affected by
deferred probing or we'll end up reordering dpm_list unnecessarily for
many times in the actual majority of cases.

> +
>         pinctrl_init_done(dev);
>
>         if (dev->pm_domain && dev->pm_domain->sync)
> --
> 2.27.0.111.gc72c7da667-goog
>