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

[Saravana Kannan]

On Thu, Sep 2, 2021 at 7:24 AM Rob Herring <robh+dt@xxxxxxxxxx> wrote:
>
> On Wed, Sep 1, 2021 at 9:55 PM Saravana Kannan <saravanak@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >  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.

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".

> 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

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?

-Saravana