Message-ID: <CAFgQCTvGJVBdGLz0R74jO3f1nY3aWprGoADUXzw4E6jxOxWHBg@mail.gmail.com>
Date:   Mon, 9 Jul 2018 16:40:52 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Rafael Wysocki <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 Mon, Jul 9, 2018 at 3:48 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Mon, Jul 9, 2018 at 8:48 AM, Pingfan Liu <kernelfans@...il.com> wrote:
> > On Sun, Jul 8, 2018 at 4:25 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >>
> >> 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?
> >>
> > The drivers use device_link_add() to build "supplier<-consumer" order
> > without knowing each other. Hence there is the following potential
> > odds: (consumerX, child_a, ...) (consumer_a,..) (supplierX), where
> > consumer_a consumes child_a.
>
> Well, what's the initial state of the list?
>
> > When device_link_add()->device_reorder_to_tail() moves all descendant of
> > consumerX to the tail, it breaks the "supplier<-consumer" order by
> > "consumer_a <- child_a".
>
> That depends on what the initial ordering of the list is and please
> note that circular dependencies are explicitly assumed to be not
> present.
>
> The assumption is that the initial ordering of the list reflects the
> correct suspend (or shutdown) order without the new link.  Therefore
> initially all children are located after their parents and all known
> consumers are located after their suppliers.
>
> If a new link is added, the new consumer goes to the end of the list
> and all of its children and all of its consumers go after it.
> device_reorder_to_tail() is recursive, so for each of the devices that
> went to the end of the list, all of its children and all of its
> consumers go after it and so on.
>
> Now, that operation doesn't change the order of any of the
> parent<-child or supplier<-consumer pairs that get moved and since all
> of the devices that depend on any device that get moved go to the end
> of list after it, the only devices that don't go to the end of list
> are guaranteed to not depend on any of them (they may be parents or
> suppliers of the devices that go to the end of the list, but not their
> children or suppliers).
>
Thanks for the detailed explain. It is clear now, and you are right.

> > And we need recrusion to resolve the item in
> > (consumer_a,..), each time when moving a consumer behind its supplier,
> > we may break "parent<-child".
>
> I don't see this as per the above.
>
> Say, device_reorder_to_tail() moves a parent after its child.  This
> means that device_reorder_to_tail() was not called for the child after
> it had been called for the parent, but that is not true, because it is
> called for all of the children of each device that gets moved *after*
> moving that device.
>
Yes, you are right.

> >> > 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.
> >>
> > If we consider device_move()-> device_pm_move_after/_before() more
> > carefully like the above description, then we can hide the detail from
> > caller. And keep the info of the pm order inside the core.
>
> Yes, we can.
>
> My point is that we have not been doing that so far and the current
> callers of those routines are expected to know that.
>
> We can do that to make the life of *future* callers easier (and maybe
> to simplify the current ones), but currently the caller is expected to
> do the right thing.
>
OK, I get your point.

> >> > It is hard to extract high dimension info and pack them into one dimension
> >> > linked-list.
> >>
> >> Well, yes and no.
> >>
> > For "hard", I means that we need two interleaved recursion to make the
> > order correct. Otherwise, I think it is a bug or limitation.
>
> So the limitation is that circular dependencies may not exist, because
> if they did, there would be no suitable suspend/shutdown ordering
> between devices.
>
Yes.

> >> 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.
> >>
> > Yeah, I agree that iterating over device tree may hurt performance. I
> > guess the iterating will not cost the majority of the suspend time,
> > comparing to the device_suspend(), which causes hardware's sync. But
> > data is more persuasive. Besides the performance, do you have other
> > concern till now?
>
> I simply think that there should be one way to iterate over devices
> for both system-wide PM and shutdown.
>
> The reason why it is not like that today is because of the development
> history, but if it doesn't work and we want to fix it, let's just
> consolidate all of that.
>
> Now, system-wide suspend resume sometimes iterates the list in the
> reverse order which would be hard without having a list, wouldn't it?
>
Yes, it would be hard without having a list. I just thought to use
device tree info to build up a shadowed list, and rebuild the list
until there is new device_link_add() operation. For
device_add/_remove(), it can modify the shadowed list directly.

Thanks,
Pingfan

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