Message-ID: <CAPDyKFotC5FBPkaTXHEqV_S=9RSsqBuM9U2YgUaSUk4vB0Kc2w@mail.gmail.com>
Date:   Tue, 7 Sep 2021 15:22:07 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Rob Herring <robh@...nel.org>, DTML <devicetree@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Stephen Boyd <sboyd@...nel.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] of: property: fw_devlink: Set 'optional_con_dev' for parse_power_domains

On Wed, 1 Sept 2021 at 23:49, Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Wed, Sep 1, 2021 at 1:13 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Tue, 31 Aug 2021 at 19:51, Saravana Kannan <saravanak@...gle.com> wrote:
> > >
> > > On Tue, Aug 31, 2021 at 3:21 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > >
> > > > The power-domain DT bindings [1] doesn't enforce a compatible string for a
> > > > provider node, even if this is common to use. In particular, when
> > > > describing a hierarchy with parent/child power-domains, as the psci DT
> > > > bindings [2] for example, it's sometimes not applicable to use a compatible
> > > > string.
> > >
> > > Ok, and fw_devlink handles that -- provider not having a compatible
> > > string is pretty common. In these cases, the parent node is the actual
> > > device that gets probed and registers the provider. So fw_devlink will
> > > create a link from the consumer to the parent device node.
> >
> > Yes, correct. That is working fine and isn't a problem.
> >
> > The first problem (I think) is that fw_devlink creates a fw_devlink
> > from a child provider node (consumer without compatible string) to a
> > parent node (supplier with a compatible string). I don't understand
> > the reason why this is needed, perhaps you can elaborate on why?
>
> I really should get around to doing a LWN article on this and also
> putting some of it into Documentation. Btw most of this info would be
> in one of my earlier commit texts or in the code comments. But it's
> still handy to have them all in one place I suppose.
>
> I'll try to answer all your questions with this example. Don't take
> the actual properties too literally, they are just used to point out
> dependencies.
>
> Device-A {
>         compatible="foo";
>
>         Device-B {
>                 compatible="flam";
>                 power-domains = <&Device-C>;
>         }
> }
>
> Device-C {
>         compatible="bar";
>
>         Device-D {
>                 compatible="baz";
>                 power-domains = <&Device-A>;
>         }
> }
>
> Legend:
> I'll use X -> Y to indicate links where X is the consumer and Y is the supplier.
> I'll call out the link types as fwnode or device links.
> If I don't explicitly state otherwise, when I say device links, I mean
> stateful/managed device link that is NOT sync-state-only device links.
>
> I think your first question is asking about fwnode link. So I'll answer that.
>
> fwnode links are created from the actual nodes that list the
> dependencies. So in this case from device-B -> device-C and device-D
> -> device-A. It needs to be done this way for a couple of reasons. But
> to answer your question on "why do this when Device-B doesn't have a
> compatible string?":
>
> 1. Not all devices have compatible strings (in an ideal world this
> won't be the case). So Device-A would create a struct device for
> Device-B, set the of_node/fwnode to point to Device-B DT node. Then
> device-B gets probed, etc. In those cases, we want the device links to
> be created between device-B -> device-C and NOT from device-A ->
> device-C. Because if we did follow that logic, we'll get device-A ->
> device-C and device-C -> device-A. This obviously can't work because
> it's a cyclic dependency and fw_devlink will have to give up on these.
>
> 2. When device-C is added (assuming device-A is added already), we
> need to create a sync-state-only device link from device-A to device-C
> as a proxy for the future device-B -> device-C device link that'll
> come up. This is to make sure device-C's sync_state() doesn't fire too
> early. So the way fw_devlink can tell apart device-A's real dependency
> (none in this case) vs device-B's dependency it's proxying for is by
> the fact the fwnode link is from device-B DT -> device-C DT.
>
> Hope that makes sense.

Yes, it does and I understand that it may become complicated in some
cases. If you get the time to put together an LWN article about
fw_devlinks, I would certainly read it. :-)

However, at least for power-domains, the DT example you describe above
is an invalid description of a HW. It doesn't make sense to try to
support if for fw_devlink, at least in my opinion. Let me elaborate.

So, I assume you have left out the #power-domain-cells property (for
simplicity) for Device-A and Device-C, as those seem to be the
power-domain providers.

*) If Device-B is a consumer of Device-C, it also means that Device-A
must be assigned as the child-power-domain to Device-C's power-domain.
**) If Device-D is the consumer of Device-A, it also means that
Device-C must be assigned as the child-power-domain to Device-A's
power-domain.

This simply can't be right from the HW point of view - and we don't
support this in the Linux kernel anyway. A power-domain can not be
both parent and child to another power-domain. In other words, cyclic
dependencies can't exist for power-domains, as it would be a wrong
description of the HW.

I wonder if the similar reasoning is applicable for other types of
resources, like clocks and regulators, for example.

>
> >
> > I come to the second and follow up problem from this behaviour, see below.
> >
> > >
> > > > Therefore, let's set the 'optional_con_dev' to true to avoid creating
> > > > incorrect fw_devlinks for power-domains.
> > >
> > > This part doesn't make sense or is incomplete. What is being done incorrectly?
> >
> > See below.
> >
> > >
> > > >
> > > > [1] Documentation/devicetree/bindings/power/power-domain.yaml
> > > > [2] Documentation/devicetree/bindings/arm/psci.yaml
> > > >
> > > > Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> > > > ---
> > > >
> > > > Some more details of what goes on here. I have added a debug print in
> > > > of_link_to_phandle() to see the fw_devlinks that gets created.
> > > >
> > > > This is what happens on Dragonboard 410c when 'optional_con_dev' isn't set:
> > > > ...
> > > > [    0.041274] device: 'psci': device_add
> > > > [    0.041366] OF: Linking power-domain-cpu0 (consumer) to psci (supplier)
> > > > [    0.041395] OF: Linking power-domain-cpu1 (consumer) to psci (supplier)
> > > > [    0.041423] OF: Linking power-domain-cpu2 (consumer) to psci (supplier)
> > > > [    0.041451] OF: Linking power-domain-cpu3 (consumer) to psci (supplier)
> > > > [    0.041494] device: 'platform:psci--platform:psci': device_add
> > > > [    0.041556] platform psci: Linked as a sync state only consumer to psci
> >
> > Because we created a fw_devlink for the child provider nodes
> > (consumer) that lacks compatible properties, we end up creating a sync
> > state only devlink. I don't think it serves a purpose, but I may be
> > wrong!?
>
> sync-state-only device links serve the purpose I explained in point 2 above.
>
> >
> > Additionally, the actual devlink that is created, has the same
> > supplier and consumer device, which indicates that this isn't the
> > right thing to do.
>
> THIS is the part that's kinda wrong. But it doesn't really break
> anything. It would also be very short lived as it would get deleted as
> soon as the consumer (in this case the same as supplier) ends up
> probing. But in your case the psci DT node doesn't use driver core for
> probing because it could be an early boot dependency. In which case
> you should really set the OF_POPULATED flag so a pointless struct
> device isn't created for the DT node or at least set
> FWNODE_FLAG_NOT_DEVICE so that fw_devlink knows not to wait on this
> supplier. This is good practice for fw_devlink (not just to work
> around the psci -> psci device link). Can you put up this patch
> please?
>
> Also sync-state-only links are explicitly present to allow cyclic
> dependencies (in the example above sync-state-only links will exist as
> device-A -> device-C and device-C -> device-A), but it kinda pointless
> to have a link where the supplier and the consumer are the same.
>
> With that said, if you want to avoid this short-lived pointless device
> link, I'd say the fix should be in device_link_add() in the following
> check:
>
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -705,6 +705,7 @@ struct device_link *device_link_add(struct device *consumer,
>          * because it only affects sync_state() callbacks.
>          */
>         if (!device_pm_initialized(supplier)
> +           || consumer == supplier
>             || (!(flags & DL_FLAG_SYNC_STATE_ONLY) &&
>                   device_is_dependent(consumer, supplier))) {
>                 link = NULL;

This change seems like the right thing to do, no matter what. I will
send a formal patch for it, thanks!

>
> >
> > > > ...
> > > >
> > > > This is what happens on Dragonboard 410c when 'optional_con_dev' is set:
> > > > ...
> > > > [    0.041179] device: 'psci': device_add
> > > > [    0.041265] OF: Not linking psci to psci - is descendant
> > > > [    0.041293] OF: Not linking psci to psci - is descendant
> > > > [    0.041319] OF: Not linking psci to psci - is descendant
> > > > [    0.041346] OF: Not linking psci to psci - is descendant
> > > > ...
> > >
> > > Can you please explain what exactly is going on that's wrong here? I
> > > notice that psci is not probed as a device at all. And when you aren't
> > > setting this flag the only difference I see is the creating of a sync
> > > state only link -- which shouldn't matter here because you don't even
> > > have a driver implemented.
> >
> > See above.
> >
> > >
> > > > The relevant dtsi file:
> > > > arch/arm64/boot/dts/qcom/msm8916.dtsi
> > > >
> > > > Kind regards
> > > > Ulf Hansson
> > > >
> > > > ---
> > > >  drivers/of/property.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/of/property.c b/drivers/of/property.c
> > > > index 2babb1807228..4d607fdbea24 100644
> > > > --- a/drivers/of/property.c
> > > > +++ b/drivers/of/property.c
> > > > @@ -1356,7 +1356,7 @@ static const struct supplier_bindings of_supplier_bindings[] = {
> > > >         { .parse_prop = parse_io_channels, },
> > > >         { .parse_prop = parse_interrupt_parent, },
> > > >         { .parse_prop = parse_dmas, .optional = true, },
> > > > -       { .parse_prop = parse_power_domains, },
> > > > +       { .parse_prop = parse_power_domains, .optional_con_dev = true, },
> > >
> > > This change is just shooting in dark/completely unrelated to the
> > > commit text. This is just saying the actual consumer is a level up
> > > from where the property is listed (eg: remote-endpoint). It just
> > > happens to fix your case for unrelated reasons.
> >
> > Again, see above.
> >
> > >
> > > Definite Nak as this *will* break other cases.
> >
> > In what way will this break other cases?
> >
> > Would you mind elaborating for my understanding and perhaps point me
> > to an example where it will break?
>
> So if you did this, it'll break:
> (1) the probe of device-A/device-C due to cyclic dependencies. Really
> no, because fw_devlink will just stop enforcing ordering between
> device-A and device-C if it detects a cycle. But if there was a real
> dependency (can me multiple links deep) between device-A -> device-C,
> that would no longer get enforced.

As I said above, cyclic dependencies don't exist for power-domains.

> (2) It'd break sync_state() correctness for device-B -> device-C dependency.

I don't see that. Again, because power-domain providers can't be
described in a cyclic way in DT.

>
> Hope that helps.
>

Perhaps, renaming the flag to "non-cyclic" would be an option? As it
seems like that is what this boils done to, right?

> -Saravana

Thanks a lot for your reply and for taking the time to explain things!

Kind regards
Uffe

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