Re: [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties

[Saravana Kannan]

On Tue, Feb 9, 2021 at 1:33 PM Rob Herring <robh@xxxxxxxxxx> wrote:
>
> On Fri, Feb 05, 2021 at 02:26:40PM -0800, Saravana Kannan wrote:
> > Not all DT bindings are mandatory bindings. Add support for optional DT
> > bindings and mark iommus, iommu-map, dmas as optional DT bindings.
>
> I don't think we can say these are optional or not. It's got to be a
> driver decision somehow.

Right, so maybe the word "optional" isn't a good name for it. I can
change that if you want.

The point being, fw_devlink can't block the probe of this driver based
on iommu property. We let the driver decide if it wants to
-EPROBE_DEFER or not or however it wants to handle this.

> For example, if IOMMU is optional, what happens with this sequence:
>
> driver probes without IOMMU
> driver calls dma_map_?()
> IOMMU driver probes
> h/w accesses DMA buffer --> BOOM!

Right. But how is this really related to fw_devlink? AFAICT, this is
an issue even today. If the driver needs the IOMMU, then it needs to
make sure the IOMMU has probed? What am I missing?

-Saravana