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Date:   Tue, 12 Jan 2021 12:51:58 -0800
From:   Saravana Kannan <>
To:     Marek Szyprowski <>
Cc:     Greg Kroah-Hartman <>,
        "Rafael J. Wysocki" <>,
        Android Kernel Team <>,
        LKML <>,
        Jisheng Zhang <>,
        Kevin Hilman <>,
        John Stultz <>,
        Nicolas Saenz Julienne <>,
        Marc Zyngier <>,
        Linux Samsung SOC <>,
        Krzysztof Kozlowski <>,
        Bartlomiej Zolnierkiewicz <>
Subject: Re: [PATCH v1 5/5] driver core: Set fw_devlink=on by default

On Mon, Jan 11, 2021 at 11:11 PM Marek Szyprowski
<> wrote:
> On 11.01.2021 22:47, Saravana Kannan wrote:
> > On Mon, Jan 11, 2021 at 6:18 AM Marek Szyprowski
> > <> wrote:
> >> On 11.01.2021 12:12, Marek Szyprowski wrote:
> >>> On 18.12.2020 04:17, Saravana Kannan wrote:
> >>>> Cyclic dependencies in some firmware was one of the last remaining
> >>>> reasons fw_devlink=on couldn't be set by default. Now that cyclic
> >>>> dependencies don't block probing, set fw_devlink=on by default.
> >>>>
> >>>> Setting fw_devlink=on by default brings a bunch of benefits (currently,
> >>>> only for systems with device tree firmware):
> >>>> * Significantly cuts down deferred probes.
> >>>> * Device probe is effectively attempted in graph order.
> >>>> * Makes it much easier to load drivers as modules without having to
> >>>>     worry about functional dependencies between modules (depmod is still
> >>>>     needed for symbol dependencies).
> >>>>
> >>>> If this patch prevents some devices from probing, it's very likely due
> >>>> to the system having one or more device drivers that "probe"/set up a
> >>>> device (DT node with compatible property) without creating a struct
> >>>> device for it.  If we hit such cases, the device drivers need to be
> >>>> fixed so that they populate struct devices and probe them like normal
> >>>> device drivers so that the driver core is aware of the devices and their
> >>>> status. See [1] for an example of such a case.
> >>>>
> >>>> [1] -
> >>>>
> >>>> Signed-off-by: Saravana Kannan <>
> >>> This patch landed recently in linux next-20210111 as commit
> >>> e590474768f1 ("driver core: Set fw_devlink=on by default"). Sadly it
> >>> breaks Exynos IOMMU operation, what causes lots of devices being
> >>> deferred and not probed at all. I've briefly checked and noticed that
> >>> exynos_sysmmu_probe() is never called after this patch. This is really
> >>> strange for me, as the SYSMMU controllers on Exynos platform are
> >>> regular platform devices registered by the OF code. The driver code is
> >>> here: drivers/iommu/exynos-iommu.c, example dts:
> >>> arch/arm/boot/dts/exynos3250.dtsi (compatible = "samsung,exynos-sysmmu").
> >> Okay, I found the source of this problem. It is caused by Exynos power
> >> domain driver, which is not platform driver yet. I will post a patch,
> >> which converts it to the platform driver.
> > Thanks Marek! Hopefully the debug logs I added were sufficient to
> > figure out the reason.
> Frankly, it took me a while to figure out that device core waits for the
> power domain devices. Maybe it would be possible to add some more debug
> messages or hints? Like the reason of the deferred probe in
> /sys/kernel/debug/devices_deferred ?

There's already a /sys/devices/.../<device>/waiting_for_supplier file
that tells you if the device is waiting for a supplier device to be
added. That file goes away once the device probes. If the file has 1,
then it's waiting for the supplier device to be added (like your
case). If it's 0, then the device is just waiting on one of the
existing suppliers to probe. You can find the existing suppliers
through /sys/devices/.../<device>/supplier:*/supplier. Also, flip
these dev_dbg() to dev_info() if you need more details about deferred

Hopefully this meets what you are looking for?


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