specification: domain-specific access flags are undefined

[mbolivar-nordic]

This statement appears to be incorrect:

Access flags are domain specific and have default values defined in
the System Device Tree specification for each domain type. Different
domains types have different compatible strings, in addition to
"openamp,domain".

https://github.com/devicetree-org/lopper/blob/0da7cbd70bc2c3417e5e909bdd90eea2a0a0c912/specification/source/chapter3-execution-domains.rst

I don't see any place where these are defined.

It's also not clear what flags are being spoken about here:

• device access flags in the access property of a domain?
• memory access flags in the memory property?
• sram access flags in sram property?

All three? None? Just some? Etc.

[mbolivar-nordic]

@sstabellini

[sstabellini]

All three, but especially the flags in the access property of a domain. We have a few of those access flags defined for "xilinx,subsystem-v1" which are a type of domains supported by Xilinx firmware. They are used internally and not defined in this repository today, but we can certainly make them public.

The idea is that a Xen hypervisor domain is going to have different access flags compared to a xilinx,subsystem domain. Xen is using the IOMMU for isolation and Xilinx subsystems use the XPPU/XMPU which works differently and exposes different properties. These are xilinx,subsystem access flags:

timeshare: bool
type: boolean
definition: bit 0
description:
Time sharing or unique sharing. It sets the "Usage Policy" in PLM
CDO commands.

allow-secure:
type: boolean
definition: bit 2
description:
If true, the device is allowed to generate secure accesses
only applicable to masters. allow-secure is "Security Policy" in
PLM CDO commands.

read-only:
type: boolean
definition: bit 4
description:
If true, mapping is read-only, otherwise read-write.
It configures the "Write Policy" in PLM CDO commands.

requested:
type: boolean
definition: bit 6
description:
Requested at boot.

requested-secure:
type: boolean
definition: bit 36
description:
The device is secure or non-secure at boot.
Requested need to be set to true to use this flag.

coherent: bool
type: boolean
definition: bit 37
description:
Routed via cache coherent interconnect.
Requested need to be set to true to use this flag.

virtualized: bool
type: boolean
definition: bit 38
description:
Requested to be routed via SMMU.
Requested need to be set to true to use this flag.

qos:
type:
definition: values [0-100] expressed using bits [64-95]
description:
Device specific QoS setting.
Requested need to be set to true to use this flag.

[mbolivar-nordic]

They are used internally and not defined in this repository today, but we can certainly make them public.

I think from the perspective of the system DT spec, it might make sense to adjust the language to say that it depends on the binding for the domain, and just leave it vague. Not sure if we want to start including vendor-specific bindings in the spec at this point in time; what do you think?

[sstabellini]

I completely agree.