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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 3 Sep 2021 19:37:50 +0200
Message-ID: <CAJZ5v0hiiAv5quZo993+F3RR2y4hTZVauTbYr-KV_vW7igwUbQ@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] of: platform: Mark bus devices nodes with FWNODE_FLAG_NEVER_PROBES
To:     Saravana Kannan <saravanak@google.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Len Brown <lenb@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Android Kernel Team <kernel-team@android.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        "open list:ACPI FOR ARM64 (ACPI/arm64)" <linux-acpi@vger.kernel.org>
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On Fri, Sep 3, 2021 at 3:16 AM Saravana Kannan <saravanak@google.com> wrote:
>
> On Thu, Sep 2, 2021 at 5:53 PM Rob Herring <robh+dt@kernel.org> wrote:
> >
> > On Thu, Sep 2, 2021 at 2:29 PM Saravana Kannan <saravanak@google.com> wrote:
> > >
> > > On Thu, Sep 2, 2021 at 12:03 PM Rob Herring <robh+dt@kernel.org> wrote:
> > > >
> > > > 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.
> > >
> > > I was thinking that initially, but if we fail a probe, the driver core
> > > will permanently give up (won't search further) or might end up
> > > retrying with the same driver and never get to the other driver. I'll
> > > send out a v2 with what I described above. It's not too bad and it
> > > will also allow us to handle the PPC cases (we'll just need to keep
> > > adding the simple-bus equivalent entries to a table).
> >
> > I wasn't sure, but I traced the calls and it looks like based on
> > __driver_attach() that if a driver fails probe another one matching
> > should get to probe:
>
> __driver_attach() is called over every device already in a bus. It's
> called only when a new driver is registered. So it makes sense that
> one ignores the error returned from probe(). You don't want to fail
> driver registration because one specific device needs to defer probe.
>
> The comment is actually from __device_attach_driver()
>
> >
> >         /*
> >          * Ignore errors returned by ->probe so that the next driver can try
> >          * its luck.
> >          */
>
> I saw that comment too, but isn't the comment wrong/stale?
>
> bus_probe_device() -> device_initial_probe() -> __device_attach().
>
> In __device_attach() we have:
> ret = bus_for_each_drv(dev->bus, NULL, &data, __device_attach_driver);
>
> If you look at bus_for_each_drv()'s comment:
>  * ...... If @fn returns anything but 0, we break out
>  * and return it. If @start is not NULL, we use it as the head
>  * of the list.
>
> Inside __device_attach_driver() we see:
>         /*
>          * Ignore errors returned by ->probe so that the next driver can try
>          * its luck.
>          */
>         ret = driver_probe_device(drv, dev);
>         if (ret < 0)
>                 return ret;
>
> So if probe() returned an error, we'd return it right back out. And
> then bus_for_each_drv() will stop searching for more drivers that
> match.

Well, not quite.

If ->probe() returns an error, really_probe() will convert it into a
positive number.  __driver_probe_device() will then return as is and
driver_probe_device() doesn't touch that value.

Bottom line: you'll see a positive number here, so the check above
will not trigger and 0 is returned, so bus_for_each_drv() will
actually continue searching.

> So I don't think one driver can give up after a match and have another
> driver give a device a shot.

Yes, it can.