Re: [PATCH v6 3/7] of/platform: Add functional dependency link from DT bindings

[Rob Herring <robh+dt@kernel.org>]

On Tue, Jul 23, 2019 at 2:49 PM Saravana Kannan <saravanak@google.com> wrote:
>
> On Tue, Jul 23, 2019 at 11:06 AM Rob Herring <robh+dt@kernel.org> wrote:
> >
> > On Sat, Jul 20, 2019 at 12:17 AM Saravana Kannan <saravanak@google.com> wrote:
> > >
> > > Add device-links after the devices are created (but before they are
> > > probed) by looking at common DT bindings like clocks and
> > > interconnects.
> >
> > The structure now looks a lot better to me. A few minor things below.
>
> Thanks.
>
> > >
> > > Automatically adding device-links for functional dependencies at the
> > > framework level provides the following benefits:
> > >
> > > - Optimizes device probe order and avoids the useless work of
> > >   attempting probes of devices that will not probe successfully
> > >   (because their suppliers aren't present or haven't probed yet).
> > >
> > >   For example, in a commonly available mobile SoC, registering just
> > >   one consumer device's driver at an initcall level earlier than the
> > >   supplier device's driver causes 11 failed probe attempts before the
> > >   consumer device probes successfully. This was with a kernel with all
> > >   the drivers statically compiled in. This problem gets a lot worse if
> > >   all the drivers are loaded as modules without direct symbol
> > >   dependencies.
> > >
> > > - Supplier devices like clock providers, interconnect providers, etc
> > >   need to keep the resources they provide active and at a particular
> > >   state(s) during boot up even if their current set of consumers don't
> > >   request the resource to be active. This is because the rest of the
> > >   consumers might not have probed yet and turning off the resource
> > >   before all the consumers have probed could lead to a hang or
> > >   undesired user experience.
> > >
> > >   Some frameworks (Eg: regulator) handle this today by turning off
> > >   "unused" resources at late_initcall_sync and hoping all the devices
> > >   have probed by then. This is not a valid assumption for systems with
> > >   loadable modules. Other frameworks (Eg: clock) just don't handle
> > >   this due to the lack of a clear signal for when they can turn off
> > >   resources. This leads to downstream hacks to handle cases like this
> > >   that can easily be solved in the upstream kernel.
> > >
> > >   By linking devices before they are probed, we give suppliers a clear
> > >   count of the number of dependent consumers. Once all of the
> > >   consumers are active, the suppliers can turn off the unused
> > >   resources without making assumptions about the number of consumers.
> > >
> > > By default we just add device-links to track "driver presence" (probe
> > > succeeded) of the supplier device. If any other functionality provided
> > > by device-links are needed, it is left to the consumer/supplier
> > > devices to change the link when they probe.
> > >
> > > Signed-off-by: Saravana Kannan <saravanak@google.com>
> > > ---
> > >  .../admin-guide/kernel-parameters.txt         |   5 +
> > >  drivers/of/platform.c                         | 158 ++++++++++++++++++
> > >  2 files changed, 163 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 138f6664b2e2..109b4310844f 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -3141,6 +3141,11 @@
> > >                         This can be set from sysctl after boot.
> > >                         See Documentation/sysctl/vm.txt for details.
> > >
> > > +       of_devlink      [KNL] Make device links from common DT bindings. Useful
> > > +                       for optimizing probe order and making sure resources
> > > +                       aren't turned off before the consumer devices have
> > > +                       probed.
> > > +
> > >         ohci1394_dma=early      [HW] enable debugging via the ohci1394 driver.
> > >                         See Documentation/debugging-via-ohci1394.txt for more
> > >                         info.
> > > diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> > > index 04ad312fd85b..88a2086e26fa 100644
> > > --- a/drivers/of/platform.c
> > > +++ b/drivers/of/platform.c
> > > @@ -509,6 +509,163 @@ int of_platform_default_populate(struct device_node *root,
> > >  }
> > >  EXPORT_SYMBOL_GPL(of_platform_default_populate);
> > >
> > > +bool of_link_is_valid(struct device_node *con, struct device_node *sup)
> > > +{
> > > +       of_node_get(sup);
> > > +       /*
> > > +        * Don't allow linking a device node as a consumer of one of its
> > > +        * descendant nodes. By definition, a child node can't be a functional
> > > +        * dependency for the parent node.
> > > +        */
> > > +       while (sup) {
> > > +               if (sup == con) {
> > > +                       of_node_put(sup);
> > > +                       return false;
> > > +               }
> > > +               sup = of_get_next_parent(sup);
> > > +       }
> > > +       return true;
> > > +}
> > > +
> > > +static int of_link_to_phandle(struct device *dev, struct device_node *sup_np)
> > > +{
> > > +       struct platform_device *sup_dev;
> > > +       u32 dl_flags = DL_FLAG_AUTOPROBE_CONSUMER;
> > > +       int ret = 0;
> > > +
> > > +       /*
> > > +        * Since we are trying to create device links, we need to find
> > > +        * the actual device node that owns this supplier phandle.
> > > +        * Often times it's the same node, but sometimes it can be one
> > > +        * of the parents. So walk up the parent till you find a
> > > +        * device.
> > > +        */
> > > +       while (sup_np && !of_find_property(sup_np, "compatible", NULL))
> > > +               sup_np = of_get_next_parent(sup_np);
> > > +       if (!sup_np)
> > > +               return 0;
> > > +
> > > +       if (!of_link_is_valid(dev->of_node, sup_np)) {
> > > +               of_node_put(sup_np);
> > > +               return 0;
> > > +       }
> > > +       sup_dev = of_find_device_by_node(sup_np);
> > > +       of_node_put(sup_np);
> > > +       if (!sup_dev)
> > > +               return -ENODEV;
> > > +       if (!device_link_add(dev, &sup_dev->dev, dl_flags))
> > > +               ret = -ENODEV;
> > > +       put_device(&sup_dev->dev);
> > > +       return ret;
> > > +}
> > > +
> > > +static struct device_node *parse_prop_cells(struct device_node *np,
> > > +                                           const char *prop, int i,
> >
> > I like 'i' for for loops, but less so for function params. Perhaps
> > 'index' instead like of_parse_phandle_with_args.
>
> Sounds good.
>
> >
> > > +                                           const char *binding,
> > > +                                           const char *cell)
> > > +{
> > > +       struct of_phandle_args sup_args;
> > > +
> > > +       if (!i && strcmp(prop, binding))
> >
> > Why the '!i' test?
>
> To avoid a string comparison for every index. It's kinda wasteful once
> the first index passes.

That's not very obvious and pretty fragile though this is a static
function. Perhaps we should split to match() and parse() functions. At
least put a comment here as to what we're doing.

>
> > > +               return NULL;
> > > +
> > > +       if (of_parse_phandle_with_args(np, binding, cell, i, &sup_args))
> > > +               return NULL;
> > > +
> > > +       return sup_args.np;
> > > +}
> > > +
> > > +static struct device_node *parse_clocks(struct device_node *np,
> > > +                                       const char *prop, int i)
> > > +{
> > > +       return parse_prop_cells(np, prop, i, "clocks", "#clock-cells");
> > > +}
> > > +
> > > +static struct device_node *parse_interconnects(struct device_node *np,
> > > +                                              const char *prop, int i)
> > > +{
> > > +       return parse_prop_cells(np, prop, i, "interconnects",
> > > +                               "#interconnect-cells");
> > > +}
> > > +
> > > +static int strcmp_suffix(const char *str, const char *suffix)
> > > +{
> > > +       unsigned int len, suffix_len;
> > > +
> > > +       len = strlen(str);
> > > +       suffix_len = strlen(suffix);
> > > +       if (len <= suffix_len)
> > > +               return -1;
> > > +       return strcmp(str + len - suffix_len, suffix);
> > > +}
> > > +
> > > +static struct device_node *parse_regulators(struct device_node *np,
> > > +                                           const char *prop, int i)
> > > +{
> > > +       if (i || strcmp_suffix(prop, "-supply"))
> > > +               return NULL;
> > > +
> > > +       return of_parse_phandle(np, prop, 0);
> > > +}
> > > +
> > > +/**
> > > + * struct supplier_bindings - Information for parsing supplier DT binding
> > > + *
> > > + * @parse_prop:                If the function cannot parse the property, return NULL.
> > > + *                     Otherwise, return the phandle listed in the property
> > > + *                     that corresponds to index i.
> > > + */
> > > +struct supplier_bindings {
> > > +       struct device_node *(*parse_prop)(struct device_node *np,
> > > +                                         const char *name, int i);
> > > +};
> > > +
> > > +struct supplier_bindings bindings[] = {
> >
> > static const
>
> Will do.
>
> >
> > > +       { .parse_prop = parse_clocks, },
> > > +       { .parse_prop = parse_interconnects, },
> > > +       { .parse_prop = parse_regulators, },
> > > +       { },
> > > +};
> > > +
> > > +static bool of_link_property(struct device *dev, struct device_node *con_np,
> > > +                            const char *prop)
> > > +{
> > > +       struct device_node *phandle;
> > > +       struct supplier_bindings *s = bindings;
> > > +       unsigned int i = 0;
> > > +       bool done = true;
> > > +
> > > +       while (!i && s->parse_prop) {
> >
> > Using 'i' is a little odd. Perhaps a 'matched' bool would be easier to read.
>
> That's how I wrote it first (locally) and then redid it this way
> because the bool felt very superfluous. I don't think this is that
> hard to understand.

Alright...

> > > +               while ((phandle = s->parse_prop(con_np, prop, i))) {
> > > +                       i++;
> > > +                       if (of_link_to_phandle(dev, phandle))
> > > +                               done = false;
> >
> > Just return here. No point in continuing as 'done' is never set back to true.
>
> Actually, there is a point for this. Say Device-C depends on suppliers
> Device-S1 and Device-S2 and they are listed in DT in that order.
>
> Say, S1 gets populated after late_initcall_sync but S2 is probes way
> before that. If I don't continue past a "failed linking" to S1 and
> also link up to S2, then S2 will get a sync_state() callback before C
> is probed. So I have to go through all possible suppliers and as many
> as possible.
>
> Let me add a comment about this somewhere in the code (probably the
> header that defines the add_links() ops).

Okay, makes sense.

Rob