Re: [PATCH] Documentation: dt: Add bindings for Secure-only devices

[Rob Herring <robh@kernel.org>]

On Fri, Oct 30, 2015 at 08:07:34PM +0000, Peter Maydell wrote:
> On 30 October 2015 at 18:28, Rob Herring <robh+dt@kernel.org> wrote:
> > On Thu, Oct 29, 2015 at 9:01 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
> >> +Valid Secure world properties:
> >> +
> >> +- secure-status : specifies whether the device is present and usable
> >> +  in the secure world. The combination of this with "status" allows
> >> +  the various possible combinations of device visibility to be
> >> +  specified:
> >> +   status = "okay";                             // visible in S and NS
> >
> > I assume neither property present or both okay also mean the same.
> >
> > status = "okay"; secure-status = "okay";
> >
> > We should be explicit.
> 
> Yes; status defaults to "okay" (presumably this is listed in
> the overal DT binding spec somewhere), and secure-status
> defaults to "same as status, which might in turn be defaulted".
> We can list the complete set of options (neither present,
> both 'okay', status not present but secure-status present, etc),
> though it gets a bit long-winded, especially if we later add
> more secure- properties (they'd all have to have verbiage about
> "if not present, same as non-prefixed property; if both not
> present, both take the default the non-prefixed property takes
> if it's not present; if prefixed property not present, it
> defaults to same as non-prefixed property", which we already
> say in the introductory section). Still, for just status it
> would be easy enough to add a couple of lines:
> 
> +    status = "okay"; secure-status = "okay"; // ditto
> +    secure-status = "okay";                  // ditto
> +    // neither explicitly defined: ditto
> 
> (Do you want the full set of 9 options you get from multiplying
> out "okay" vs "disabled" vs not-set for each property?)

Better to err on completeness. The inheritance is easily missed.

Also, one nit. Use C style comments so when people copy-n-paste this it 
is the correct style.

> 
> >> +   status = "disabled"; secure-status = "okay"; // S-only
> >> +   status = "okay"; secure-status = "disabled"; // NS-only
> >
> > In HKG when we discussed this, 'status = "secure"' was the proposal.
> > That would be simpler:
> >
> > S world can use "okay" or "secure"
> > NS world can use "okay" or no property.
> >
> > That leaves out the case of disabled in S and enabled for NS. We could
> > want that for s/w reasons, but can we have h/w like that?
> 
> It's perfectly possible to design hardware like that (though
> I can't think of a reason to do so offhand). I think it's the desire
> to be able to describe all the possible valid h/w combinations
> that brought us to this secure- prefix design. Plus it
> extends nicely to cover other possibilities as we need it;
> for instance "device A is at S-0x10000 but NS-0x20000" can be
> done by specifying a device like:
>    status = "okay";
>    secure-status = "okay";
>    reg = < 0x20000 0x1000 >;
>    secure-regs = < 0x10000 0x1000 >;
> (apologies if I've messed the syntax up there).
> 
> Just going for 'status=secure' would deal with the immediate
> requirement, but my preference is for a description that
> lets us describe all the possible configurations, not just
> the ones we think are common, and secure-* is a neat way
> of doing that (IIRC it was Grant's suggestion; speaking of
> whom, I just noticed I forgot to cc him on the original patch).

I don't really want to give blanket approval to use secure-* prefix, but 
I'm okay with secure-status I guess.

For secure-reg in particular, I would do something different and 
similar to PCI where we add another address cell to define the address 
space as secure or non-secure. So your example becomes:

    reg = <0 0x20000 0x1000 >,
          <1 0x10000 0x1000 >;

The bus compatible would also have to be something like 
"arm,secure-bus". No point in debating this now. We can worry about it 
when we have an example.

Rob