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

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Date:   Sun, 8 Jul 2018 10:25:14 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Lukas Wunner <lukas@...ner.de>,
        Linux Kernel Mailing List <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 <linux-pci@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Kishon Vijay Abraham I <kishon@...com>
Subject: Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer
 ordering in device_kset

On Sat, Jul 7, 2018 at 6:24 AM, Pingfan Liu <kernelfans@...il.com> wrote:
> 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.

What do you mean?

> 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.

Arguably so, but that's how those functions were designed and the
callers should be aware of the limitation.

If they aren't, there is a bug in the caller.

> It is hard to extract high dimension info and pack them into one dimension
> linked-list.

Well, yes and no.

We know it for a fact that there is a linear ordering that will work.
It is inefficient to figure it out every time during system suspend
and resume, for one and that's why we have dpm_list.

Now, if we have it for suspend and resume, it can also be used for shutdown.

> 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.

Not the way you want to do that, though.

Thanks,
Rafael