Re: [PATCH v1 2/2] of: platform: Mark bus devices nodes with FWNODE_FLAG_NEVER_PROBES

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

On Thu, Sep 2, 2021 at 11:57 AM Saravana Kannan <saravanak@google.com> wrote:
>
> On Thu, Sep 2, 2021 at 7:24 AM Rob Herring <robh+dt@kernel.org> wrote:
> >
> > On Wed, Sep 1, 2021 at 9:55 PM Saravana Kannan <saravanak@google.com> wrote:
> > >
> > > We don't want fw_devlink creating device links for bus devices as
> > > they'll never probe. So mark those device node with this flag.
> > >
> > > Signed-off-by: Saravana Kannan <saravanak@google.com>
> > > ---
> > >  drivers/of/platform.c | 16 ++++++++++++++++
> > >  1 file changed, 16 insertions(+)
> > >
> > > diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> > > index 74afbb7a4f5e..42b3936d204a 100644
> > > --- a/drivers/of/platform.c
> > > +++ b/drivers/of/platform.c
> > > @@ -392,6 +392,22 @@ static int of_platform_bus_create(struct device_node *bus,
> > >         if (!dev || !of_match_node(matches, bus))
> > >                 return 0;
> > >
> > > +       /*
> > > +        * If the bus node has only one compatible string value and it has
> > > +        * matched as a bus node, it's never going to get probed by a device
> > > +        * driver. So flag it as such so that fw_devlink knows not to create
> > > +        * device links with this device.
> > > +        *
> > > +        * This doesn't catch all devices that'll never probe, but this is good
> > > +        * enough for now.
> > > +        *
> > > +        * This doesn't really work for PPC because of how it uses
> > > +        * of_platform_bus_probe() to add normal devices. So ignore PPC cases.
> > > +        */
> > > +       if (!IS_ENABLED(CONFIG_PPC) &&
> > > +           of_property_count_strings(bus, "compatible") == 1)
> > > +               bus->fwnode.flags |= FWNODE_FLAG_NOT_DEVICE;
> >
> > This looks fragile relying on 1 compatible string, and the DT flags in
> > this code have been fragile too. I'm pretty sure we have cases of
> > simple-bus or simple-mfd that also have another compatible.
> >
> > Couldn't we solve this with a simple driver?
>
> Oh, I didn't think you'd like that. I'd lean towards that option too
> if we can address some of the other concerns below.
>
> > Make 'simple-pm-bus'
> > driver work for other cases?
>
> > BTW, this patch doesn't even work for
> > simple-pm-bus.
>
> How do you mean? Because simple-pm-bus already has a driver and
> doesn't set "matches" param when it calls of_platform_populate() and
> this flag won't be set. So at least for simple-pm-bus I don't see any
> issue.

You're right.

> I was trying to reuse of_default_bus_match_table without explicitly
> referring to it, but if it's confusing I can add a separate list of
> compatible strings and use those here instead of using "matches".

What happens with a non-default table? I'm not sure we can assume the
same behavior.

> > A driver for simple-bus may cause issues if there's a
> > more specific driver to bind to as we don't handle that. It's simply
> > whichever matches first.
>
> Right, this is my worry. Especially for devices like this (there are
> plenty of cases like this) which have a driver that probes them but
> also lists simple-bus
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/arm-realview-pb11mp.dts?id=73f3af7b4611d77bdaea303fb639333eb28e37d7#n299

Uhh, that one is certainly a leakage of wanting an soc_device in the
hierarchy, not any real bus structure reflecting the h/w. I'm not a
fan of the soc_device stuff and its optional nature. Everything is an
SoC, so it should always be there? Or your device hierarchy should
change when you decide to add a soc_device?

> So as long as there's a compatible string that's not one of the
> "transparent" busses, this driver shouldn't match. So, I don't think I
> can get away from checking the compatible strings.
>
> How about I check here to make sure all the "compatible" strings are
> from an approved transparent bus list, and if it's true, I use
> driver_override to force match it to a transparent bus driver? Would
> you be okay with that?

Can't we do that within a driver? We check this and fail probe if
there's a more specific compatible.  Then another driver can match and
probe.

Rob