Re: [PATCH v6 3/3] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module

[Will Deacon]

On Tue, Nov 17, 2020 at 02:47:54PM +0100, Thierry Reding wrote:
> On Mon, Nov 16, 2020 at 11:48:39AM -0800, John Stultz wrote:
> > On Mon, Nov 16, 2020 at 8:36 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > On Mon, Nov 16, 2020 at 04:59:36PM +0100, Thierry Reding wrote:
> > > > On Fri, Nov 06, 2020 at 04:27:10AM +0000, John Stultz wrote:
> > > > Unfortunately, the ARM SMMU module will eventually end up being loaded
> > > > once the root filesystem has been mounted (for example via SDHCI or
> > > > Ethernet, both with using just plain, non-IOMMU-backed DMA API) and then
> > > > initialize, configuring as "fault by default", which then results from a
> > > > slew of SMMU faults from all the devices that have previously configured
> > > > themselves without IOMMU support.
> > >
> > > I wonder if fw_devlink=on would help here?
> > >
> > > But either way, I'd be more inclined to revert this change if it's causing
> > > problems for !QCOM devices.
> > >
> > > Linus -- please can you drop this one (patch 3/3) for now, given that it's
> > > causing problems?
> > 
> > Agreed. Apologies again for the trouble.
> > 
> > I do feel like the probe timeout to handle optional links is causing a
> > lot of the trouble here. I expect fw_devlink would solve this, but it
> > may be awhile before it can be always enabled.  I may see about
> > pushing the default probe timeout value to be a little further out
> > than init (I backed away from my last attempt as I didn't want to
> > cause long (30 second) delays for cases like NFS root, but maybe 2-5
> > seconds would be enough to make things work better for everyone).
> 
> I think there are two problems here: 1) the deferred probe timeout can
> cause a mismatch between what SMMU masters and the SMMU think is going
> on and 2) a logistical problem of dealing with the SMMU driver being a
> loadable module.
> 
> The second problem can be dealt with by shipping the module in the
> initial ramdisk. That's a bit annoying, but perhaps the right thing to
> do. At least on Tegra we need this because all the devices that carry
> the root filesystem (Ethernet for NFS and SDHCI/USB/SATA/PCI for disk
> boot) are SMMU masters and will start to fault once the SMMU driver is
> loaded.

Realistically, if you're building an IOMMU driver as a module then it needs
to be part of the initrd and fw_devlink needs to be enabled. Relying on
timeouts and the phase of the moon is not going to be reliable.

But back to the original issue, I think we should revert the Kconfig patch
from Linus' tree. Please can you send a revert for that?

Will