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

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Date:   Sat, 7 Jul 2018 12:24:26 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     rafael@...nel.org
Cc:     lukas@...ner.de, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Dave Young <dyoung@...hat.com>, linux-pci@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-pm@...r.kernel.org,
        kishon@...com
Subject: Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer
 ordering in device_kset

On Fri, Jul 6, 2018 at 9:55 PM Pingfan Liu <kernelfans@...il.com> wrote:
>
> On Fri, Jul 6, 2018 at 4:47 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Fri, Jul 6, 2018 at 10:36 AM, Lukas Wunner <lukas@...ner.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.
>
Think it again, I saw other potential issue with the current code.
device_link_add->device_reorder_to_tail() can break the
"supplier<-consumer" order. During moving children after parent's
supplier, it ignores the order of child's consumer.  Beside this,
essentially both devices_kset_move_after/_before() and
device_pm_move_after/_before() expose  the shutdown order to the
indirect caller,  and we can not expect that the caller can not handle
it correctly. It should be a job of drivers core.  It is hard to
extract high dimension info and pack them into one dimension
linked-list. And in theory, it is warranted that the shutdown seq is
correct by using device tree info. More important, it is cheap with
the data structure in hand. So I think it is time to resolve the issue
once for all.

Thanks and regards,
Pingfan