Re: [PATCH v1 5/5] driver core: Set fw_devlink=on by default

[Saravana Kannan <saravanak@google.com>]

On Mon, Jan 11, 2021 at 6:18 AM Marek Szyprowski
<m.szyprowski@samsung.com> 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] -
> >> https://lore.kernel.org/lkml/CAGETcx9PiX==mLxB9PO8Myyk6u2vhPVwTMsA5NkD-ywH5xhusw@mail.gmail.com/
> >> Signed-off-by: Saravana Kannan <saravanak@google.com>
> >
> > 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.

-Saravana