Re: [PATCH v1 4/4] of: platform: Batch fwnode parsing when adding all top level devices

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Saravana,

On Wed, Jun 17, 2020 at 8:36 PM Saravana Kannan <saravanak@google.com> wrote:
> On Wed, Jun 17, 2020 at 5:20 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Fri, May 15, 2020 at 7:38 AM Saravana Kannan <saravanak@google.com> wrote:
> > > The fw_devlink_pause() and fw_devlink_resume() APIs allow batching the
> > > parsing of the device tree nodes when a lot of devices are added. This
> > > will significantly cut down parsing time (as much a 1 second on some
> > > systems). So, use them when adding devices for all the top level device
> > > tree nodes in a system.
> > >
> > > Signed-off-by: Saravana Kannan <saravanak@google.com>
> >
> > This is now commit 93d2e4322aa74c1a ("of: platform: Batch fwnode parsing
> > when adding all top level devices") in v5.8-rc1, and I have bisected a
> > regression to it: on r8a7740/armadillo and sh73a0/kzm9g, the system can
> > no longer be woken up from s2ram by a GPIO key. Reverting the commit
> > fixes the issue.
> >
> > On these systems, the GPIO/PFC block has its interrupt lines connected
> > to intermediate interrupt controllers (Renesas INTC), which are in turn
> > connected to the main interrupt controller (ARM GIC).  The INTC block is
> > part of a power and clock domain.  Hence if a GPIO is enabled as a
> > wake-up source, the INTC is part of the wake-up path, and thus must be
> > kept enabled when entering s2ram.
> >
> > While this commit has no impact on probe order for me (unlike in Marek's
> > case), it does have an impact on suspend order:
> >   - Before this commit:
> >       1. The keyboard (gpio-keys) is suspended, and calls
> >          enable_irq_wake() to inform the upstream interrupt controller
> >          (INTC) that it is part of the wake-up path,
> >       2. INTC is suspended, and calls device_set_wakeup_path() to inform
> >          the device core that it must be kept enabled,
> >       3. The system is woken by pressing a wake-up key.
> >
> >   - After this commit:
> >       1. INTC is suspended, and is not aware it is part of the wake-up
> >          path, so it is disabled by the device core,
> >       2. gpio-keys is suspended, and calls enable_irq_wake() in vain,
> >       3. Pressing a wake-up key has no effect, as INTC is disabled, and
> >          the interrupt does not come through.
> >
> > It looks like no device links are involved, as both gpio-keys and INTC have
> > no links.
> > Do you have a clue?
> >
> > Thanks!
>
> That patch of mine defers probe on all devices added by the
> of_platform_default_populate() call, and then once the call returns,
> it immediately triggers a deferred probe.
>
> So all these devices are being probed in parallel in the deferred
> probe workqueue while the main "initcall thread" continues down to
> further initcalls. It looks like some of the drivers in subsequent
> initcalls are assuming that devices in the earlier initcalls always
> probe and can't be deferred?
>
> There are two options.
> 1. Fix these drivers.
> 2. Add a "flush deferred workqueue" in fw_devlink_resume()
>
> I'd rather we fix the drivers so that they handle deferred probes
> correctly. Thoughts?

While the affected drivers should handle deferred probe fine, none of
the affected drivers is subject to deferred probing: they all probe
successfully on first try (I had added debug prints to
platform_drv_probe() to be sure).
The affected drivers are still probed in the same order (INTC is one of
the earliest drivers probed, gpio-keys is the last). However, during
system suspend, gpio-keys is suspended before INTC, which is wrong, as
gpio-keys uses an interrupt provided by INTC.

Perhaps the "in parallel" is the real culprit, and there is a race
condition somewhere?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds