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

[Rafael J. Wysocki]

On Thu, Jun 25, 2020 at 6:49 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> Dropping Feng Kan <fkan@xxxxxxx> and Toan Le <toanle@xxxxxxx> because
> their mails are bouncing.
>
> On Thu, Jun 25, 2020 at 8:19 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> >
> > 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.
>
> Yes, lots of unnecessary reordering, but doing it only for deferred
> probes IS the problem. In the example I gave, the consumer is never
> deferred probe because the supplier happens to finish probing before
> the consumer probe is even attempted.

But why would the supplier be moved to the end of dpm_list without
moving the consumer along with it?