linux-kernel - Re: [PATCH v4 1/2] drivers: bus: simple-pm-bus: Add support for probing simple bus only devices

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Date:   Tue, 16 Nov 2021 07:19:43 +0200
From:   Tony Lindgren <tony@...mide.com>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Abel Vesa <abelvesa@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Neil Armstrong <narmstrong@...libre.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Damien Le Moal <damien.lemoal@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, kernel-team@...roid.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-oxnas@...ups.io, linux-renesas-soc@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 1/2] drivers: bus: simple-pm-bus: Add support for
 probing simple bus only devices

* Saravana Kannan <saravanak@...gle.com> [211115 20:19]:
> On Wed, Nov 10, 2021 at 12:24 PM Saravana Kannan <saravanak@...gle.com> wrote:
> >
> > On Wed, Nov 10, 2021 at 4:16 AM Abel Vesa <abelvesa@...nel.org> wrote:
> > >
> > > On 21-09-28 17:07:33, Saravana Kannan wrote:
> > > > fw_devlink could end up creating device links for bus only devices.
> > > > However, bus only devices don't get probed and can block probe() or
> > > > sync_state() [1] call backs of other devices. To avoid this, probe these
> > > > devices using the simple-pm-bus driver.
> > > >
> > > > However, there are instances of devices that are not simple buses (they get
> > > > probed by their specific drivers) that also list the "simple-bus" (or other
> > > > bus only compatible strings) in their compatible property to automatically
> > > > populate their child devices. We still want these devices to get probed by
> > > > their specific drivers. So, we make sure this driver only probes devices
> > > > that are only buses.
...
> > >
> > > This change is breaking the expected behavior for the already existent
> > > simple-bus nodes. All the simple-bus compatibles should be replaced now
> > > to simple-pm-bus. In my case, on some i.MX8 platforms, without the
> > > devlink, the devices suspend sequence changes (and even breaks).
> > >
> > > To avoid breaking the already existent simple-bus nodes, maybe the logic
> > > should've been reversed: keep the simple-bus as is and add another
> > > compatible, IDK, something like simple-trasnparent-bus, or something.
> >
> > The intent of this change IS to affect existing simple-bus nodes (but
> > not in the way it's affecting you). But if it's breaking stuff, we
> > obviously need to fix it.
> >
> > I have a hunch on what's going on in your case, but can you point me
> > to the specific simple-bus node that's getting affected? I'm expecting
> > it to be a simple-bus node that gets added AFTER this driver is
> > registered at device_initcall (module_init gets converted to
> > device_initcall).
> >
> > Also, can you try this hack patch to see if it helps your case?
> > https://lore.kernel.org/lkml/CAGETcx9U130Oq-umrvXME4JhEpO0Wadoki3kNxx=0-YvTR6PtQ@mail.gmail.com/
> >
> > I have some thoughts on how I could fix this, but I need to think
> > about a few cases.

Not sure if this is related.. Some drivers need to be updated from
builtin_platform_driver_probe() to use builtin_platform_driver() when
switching to simple-pm-bus because of deferred probe. See more info
in commit e259c2926c01 ("PCI: pci-dra7xx: Prepare for deferred probe
with module_platform_driver").

Regards,

Tony