Subject: [PATCH 2/3] Docs: dt: Add PCI MSI map bindings

From: [email-masked] (Mark Rutland)
Date: Thu, 6 Aug 2015 19:14:53 +0100

[...]

> > +PCI root complex
> > +================
> > +
> > +Optional properties
> > +-------------------
> > +
> > +- msi-map: Maps a Requester ID to an MSI controller and associated
> > +  msi-specifier data. The property is an arbitrary number of tuples of
> > +  (rid-base,msi-controller,msi-base,length), where:
> > +
> > +  * rid-base is a single cell describing the first RID matched by the entry.
> > +
> > +  * msi-controller is a single phandle to an MSI controller
> > +
> > +  * msi-base is an msi-specifier describing the msi-specifier produced for the
> > +    first RID matched by the entry.
> > +
> > +  * length is a single cell describing how many consecutive RIDs are matched
> > +    following the rid-base.
> > +
> > +  Any RID r in the interval [rid-base, rid-base + length) is associated with
> > +  the listed msi-controller, with the msi-specifier (r - rid-base + msi-base).
> > +
> > +- msi-map-mask: A mask to be applied to each Requester ID prior to being mapped
> > +  to an msi-specifier per the msi-map property.
> 
> Can we extend the msi-map-mask definition to say:  "A mask value of 0x0 is valid
> and indicates that no RIDs are _currently_ mapped to any msi-specifier."

That would break a valid case of the mask being all zeroes.

Consider the case that all RIDs get mapped to a single msi-specifier;
the obvious way to write that is:

msi-map-mask = <0x0000>;
msi-map = <0x0000 &msi (msi-specifier) 1>;

In this case all RIDS are always mapped to the single msi-specifier.

> We have an SoC with a programmable hardware table in the PCI controller that maps
> requester ID to stream ID, so the overall msi-map (and iommu-map) definition fit
> into that scheme.  But, we would like to be able make the RID->stream-ID mapping
> decision _lazily_, in Linux, based on actual usage of PCI devices.

Dynamically programming the mapping is at odds to this binding. I don't
see how that can fit.

Why can the RID->SID mapping not be statically configured prior to
entering the OS?

Thanks,
Mark.