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    SubjectRe: [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@kernel.org> wrote:
    >
    > On Mon, Jul 9, 2018 at 8:48 AM, Pingfan Liu <kernelfans@gmail.com> wrote:
    > > On Sun, Jul 8, 2018 at 4:25 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
    > >>
    > >> On Sat, Jul 7, 2018 at 6:24 AM, Pingfan Liu <kernelfans@gmail.com> wrote:
    > >> > On Fri, Jul 6, 2018 at 9:55 PM Pingfan Liu <kernelfans@gmail.com> wrote:
    > >> >>
    > >> >> 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.
    > >> >>
    > >> > 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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