Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210902025528.1017391-1-saravanak@google.com>
 <20210902025528.1017391-3-saravanak@google.com> <CAL_JsqJOv7D5nHteGPDKC2+ns1caVNs-NFFJppLuK0OEB8dztQ@mail.gmail.com>
 <CAGETcx-rOakAX_apu2ecu6jWCwzO0RgMkwdfzyF+UaxQfVj4CA@mail.gmail.com>
 <CAL_Jsq+A1T5+KK5xsVVtrMVeuMre3B6sAAroX+a3gQy6wY+r8A@mail.gmail.com>
 <CAGETcx9wY66TsFX_1rFUO+toE-OpkAvWSdcNVK7M=LYwa6xbAw@mail.gmail.com>
 <CAL_JsqJzt_duSkMm43dwpiGCmqW8PAQ1n6SeyiNkECP8CTcY=g@mail.gmail.com>
 <CAGETcx-VkJXTXCwh_ctMg_-VDUb4WFxLLYt0ht8tv8yn+kCH6Q@mail.gmail.com> <CAL_JsqJ4auRZonAnR5CKGyKNs1pVfHK24+av0Ox9L_805v=n6w@mail.gmail.com>
In-Reply-To: <CAL_JsqJ4auRZonAnR5CKGyKNs1pVfHK24+av0Ox9L_805v=n6w@mail.gmail.com>
From:   Saravana Kannan <saravanak@google.com>
Date:   Fri, 3 Sep 2021 10:06:53 -0700
Message-ID: <CAGETcx_UaXLOTwThWocN5ZeszXOj9YRXUFHveCEfY50m6L3f6A@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] of: platform: Mark bus devices nodes with FWNODE_FLAG_NEVER_PROBES
To:     Rob Herring <robh+dt@kernel.org>
Cc:     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>,
        devicetree@vger.kernel.org,
        "open list:ACPI FOR ARM64 (ACPI/arm64)" <linux-acpi@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Sep 3, 2021 at 7:58 AM Rob Herring <robh+dt@kernel.org> wrote:
>
> On Thu, Sep 2, 2021 at 8:16 PM 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 behavior should be the same no matter whether the device or driver
> is registered first.
>
> Deferred probe errors are handled differently AFAICT.
>
> >
> > 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?
>
> I don't know...
>
> >
> > 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.
>
> With the exception of deferred probe, probe errors are made positive
> and then ignored.

Oh I totally missed this!

>
> > So I don't think one driver can give up after a match and have another
> > driver give a device a shot.
>
> I think it just needs to be tried out...

Oh yeah, it would definitely work and would be a lot nicer. I'll rework this.

-Saravana