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

[Saravana Kannan]

On Mon, Jan 31, 2022 at 7:18 PM Kevin Hilman <khilman at baylibre.com> wrote:
>
> Hi Saravana,
>
> Saravana Kannan <saravanak at google.com> writes:
>
> > 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.
> >
> > [1] - https://lore.kernel.org/lkml/CAPDyKFo9Bxremkb1dDrr4OcXSpE0keVze94Cm=zrkOVxHHxBmQ@mail.gmail.com/
> > Fixes: c442a0d18744 ("driver core: Set fw_devlink to "permissive" behavior by default")
> > Signed-off-by: Saravana Kannan <saravanak at google.com>
> > Tested-by: Saravana Kannan <saravanak at google.com>
> > Tested-by: Ulf Hansson <ulf.hansson at linaro.org>
>
> This patch landed in stable/linux-5.10.y as commit d5f13bbb5104 and it
> broke suspend/resume on at least one TI AM335x board I'm testing on:
> upstream dts: arch/arm/boot/dts/am335x-icev2.dts, upstream defconfig:
> arch/arm/configs/omap2plus_defconfig.
>
> Bisecting between vanilla v5.10 (good) and stable/linux-5.10.y (bad)
> pointed me to this patch, and I confirmed that reverting just this patch
> on top of stable/linux-5.10.y makes it work again.
>
> Also interesting, this same platform works fine on vanilla v5.15, which
> also includes this patch.  That suggests that either 1) this patch
> should not have been backported to v5.10 stable or 2) there are some
> other dependencies that are missing in v5.10.
>
> Since vanilla v5.10 works fine, I'm leaning towards (1), but if you have
> any ideas for deps that need backporting, I'm happy to try.

Oh wow! I didn't realize I made so many changes AFTER 5.10! Unless I'm
doing something wrong with my git commands.
$ git log v5.10..v5.15 --oneline -- drivers/of/property.c
$ git log v5.10..v5.15 --oneline --author=saravanak -- drivers/base/

If you don't think I got my git command completely wrong, yeah, way
too many patches are missing on 5.10. I'd go with the option of
dropping this patch on 5.10.

> I haven't debugged exactly where it's hanging yet, but, enabling
> CONFIG_DEBUG_DRIVER=y, and suspending with "no_console_suspend" on the
> command line, the last line before it hangs is:
>
>    [   28.129966] simple-pm-bus ocp: noirq power domain suspend
>
> Any ideas?

I'd guess it's either a sync_state() happening too soon since some of
the dependencies aren't tracked. Or some dependency cycle that'd be
handled correctly if the rest of the patches were picked up. Yeah, a
pretty broad/vague answer.

-Saravana