Re: [PATCH v4 01/26] dt-bindings: interrupt-controller: Add Arm GICv5

On Thu, 29 May 2025 at 15:21, Lorenzo Pieralisi [off-list ref] wrote:

On Thu, May 29, 2025 at 02:17:26PM +0100, Peter Maydell wrote:

quoted

The dt bindings do allow for describing Secure-world devices:
Documentation/devicetree/bindings/arm/secure.txt has the
details. We use this in QEMU so we can provide a DTB to
guest EL3 firmware that tells it where the hardware is
(and which EL3 can then pass on to an NS kernel). It would
be helpful for the GICv5 binding to be defined in a way that
we can do this for a GICv5 system too.

quoted

Two questions:

- I don't have to spell out the IRS/ITS config frame (and SETLPI, by
  the way) as non-secure, since that's implicit, is that correct ?

Do you want the DT binding to handle the case of "CPU and GIC do not
implement EL3, and the only implemented security state is Secure"
without the kernel needing to do something different from "ditto ditto
but the only implemented security state is Nonsecure" ?

Not sure I follow you here sorry :)

In a hypothetical system like that the dt could either
define the (only) IRS frame in reg[], or in secure-reg[].
The former would let the kernel not care about the fact it was
in Secure, but would be a bit weird. But I think we can probably
ignore this hypothetical in favour of keeping the binding simple.

quoted

(Currently booting.html says you must be in NS, so we effectively
say we don't support booting on this particular unicorn :-)
But the secure.txt bindings envisage "kernel got booted in S",
mostly for the benefit of aarch32.)

quoted

- How can the schema describe, if present, EL3, Secure and Realm frames ?

The tempting thing to do is to have regs[] list the frames
in some given order, but the spec makes them not simple
supersets, allowing all of:
 * NS
 * S
 * NS, S, EL3
 * NS, Realm, EL3
 * NS, Realm, S, EL3

secure.txt says:
# The general principle of the naming scheme for Secure world bindings
# is that any property that needs a different value in the Secure world
# can be supported by prefixing the property name with "secure-". So for
# instance "secure-foo" would override "foo".

So maybe we could have
 reg : the NS frame(s)
 secure-reg : the S frame(s)
 realm-reg : the Realm frame(s)
 root-reg : the EL3 frame(s)

??

I assume someone has to write the root/realm binding extensions.

In Documentation/devicetree/bindings/arm/secure.txt I don't think that
reg is a contemplated property - I don't know if the list of properties
is up-to-date.

It's up to date in the sense that so far we've only needed
to have the 'status' property have a secure- variant. My
suggestion here is that we might extend that to also allow
secure-reg, and to have root- and realm- prefixes too.
Though I don't think we would want to permit secure-reg for
any old device, so maybe something more-GICv5-specific would
work better.

thanks
-- PMM