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

[Geert Uytterhoeven]

Hi Saravana,

CC iommu

On Tue, Feb 9, 2021 at 10:55 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
> 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.

The driver cannot make that decision, cfr. below.

> > 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!

Does it really behave that way? Or does it continue without IOMMU?

> 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?

Individual I/O (IOMMU slave) drivers are completely unaware of the
presence or absence of an IOMMU; they just use the DMA API, which is the
same regardless of an IOMMU being used or not.
While for GPIO/IRQ/CLK/DMA/... have request/get_{gpio,irq,clk,dma,...}
APIs for a driver to get a reference, which can return -EPROBE_DEFER, no
such thing exists for IOMMUs.  This is handled by the IOMMU core
instead.

Using the IOMMU or not is more like a system policy decision.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds