Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer ordering in device_kset

[Pingfan Liu <kernelfans@gmail.com>]

On Fri, Jul 6, 2018 at 4:47 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Fri, Jul 6, 2018 at 10:36 AM, Lukas Wunner <lukas@wunner.de> wrote:
> > [cc += Kishon Vijay Abraham]
> >
> > On Thu, Jul 05, 2018 at 11:18:28AM +0200, Rafael J. Wysocki wrote:
> >> OK, so calling devices_kset_move_last() from really_probe() clearly is
> >> a mistake.
> >>
> >> I'm not really sure what the intention of it was as the changelog of
> >> commit 52cdbdd49853d doesn't really explain that (why would it be
> >> insufficient without that change?)
> >
> > It seems 52cdbdd49853d fixed an issue with boards which have an MMC
> > whose reset pin needs to be driven high on shutdown, lest the MMC
> > won't be found on the next boot.
> >
> > The boards' devicetrees use a kludge wherein the reset pin is modelled
> > as a regulator.  The regulator is enabled when the MMC probes and
> > disabled on driver unbind and shutdown.  As a result, the pin is driven
> > low on shutdown and the MMC is not found on the next boot.
> >
> > To fix this, another kludge was invented wherein the GPIO expander
> > driving the reset pin unconditionally drives all its pins high on
> > shutdown, see pcf857x_shutdown() in drivers/gpio/gpio-pcf857x.c
> > (commit adc284755055, "gpio: pcf857x: restore the initial line state
> > of all pcf lines").
> >
> > For this kludge to work, the GPIO expander's ->shutdown hook needs to
> > be executed after the MMC expander's ->shutdown hook.
> >
> > Commit 52cdbdd49853d achieved that by reordering devices_kset according
> > to the probe order.  Apparently the MMC probes after the GPIO expander,
> > possibly because it returns -EPROBE_DEFER if the vmmc regulator isn't
> > available yet, see mmc_regulator_get_supply().
> >
> > Note, I'm just piecing the information together from git history,
> > I'm not responsible for these kludges.  (I'm innocent!)
>
> Sure enough. :-)
>
> In any case, calling devices_kset_move_last() in really_probe() is
> plain broken and if its only purpose was to address a single, arguably
> kludgy, use case, let's just get rid of it in the first place IMO.
>
Yes, if it is only used for a single use case.

> > @Pingfan Liu, if you just remove the call to devices_kset_move_last()
> > from really_probe(), does the issue go away?
>
> I would think so from the description of the problem (elsewhere in this thread).
>
Yes.

Thanks,
Pingfan