Re: [PATCH v3 1/2] dt-bindings: arm: xen: document Xen iommu device

[Rob Herring <robh@xxxxxxxxxx>]

On Wed, Jan 26, 2022 at 10:56:39AM -0800, Stefano Stabellini wrote:
> On Wed, 26 Jan 2022, Robin Murphy wrote:
> > On 2022-01-26 15:09, Sergiy Kibrik wrote:
> > > Hi Robin,
> > > 
> > > > 
> > > > This could break Linux guests, since depending on the deferred probe
> > > > timeout setting it could lead to drivers never probing because the 
> > > > "IOMMU"
> > > > never becomes available.
> > > > 
> > > 
> > > I've noticed no deferred probe timeouts when booting with this patch. 
> > > Could
> > > you please explain more on how this would break guests?
> > 
> > Right now I think it would actually require command-line intervention, e.g.
> > "fw_devlink=on" or "deferred_probe_timeout=3600" (with modules enabled for 
> > the
> > latter to take full effect), but I'm wary of the potential for future config
> > options to control those behaviours by default.

fw_devlink=on is now the default (for at least a couple of cycles).

> 
> If deferred_probe_timeout=3600 was specified, we would just need an
> IOMMU driver in Linux for the "xen,iommu-el2-v1" node to solve the
> problem, right? I guess I am trying to say that it wouldn't be a device
> tree interface problem but rather a Linux implementation discussion.

You would have to add that IOMMU driver to old, existing kernels if you 
want compatibility with a new DT. Otherwise, that kernel would stop 
booting with a new DT.

Rob