The System Control and Management Interface (SCMI) [1], which is a set of operating system-independent software interfaces that are used in system management. SCMI currently provides interfaces for:

• Discovery and self-description of the interfaces it supports
• Power domain management
• Clock management
• Reset domain management
• Voltage domain management
• Sensor management
• Performance management
• Power capping and monitoring
• Pin control protocol.

The SCMI compliant firmware could run:

• as part of EL3 secure world software (like Trusted Firmware-A) with ARM SMC shared-memory transport;
• on dedicated System Control Processor (SCP) with HW mailbox shared-memory transport

The major purpose of enabling SCMI support in Xen is to enable guest domains access to the SCMI interfaces for performing management actions on passed-through devices (such as clocks/resets etc) without accessing directly to the System control HW (like clock controllers) which in most cases can't be shared/split between domains. Or, at minimum, allow SCMI access for dom0/hwdom (or guest domain serving as Driver domain).

The below sections describe SCMI support options available for Xen.

1 Simple SCMI over SMC calls forwarding driver (EL3)

The EL3 SCMI firmware (TF-A) with a single SCMI OSPM agent support is pretty generic case for the default vendors SDK and new platforms with SCMI support. Such EL3 SCMI firmware supports only single SCMI OSPM transport (agent) with Shared memory based transport and SMC calls as doorbell.

The SCMI over SMC calls forwarding driver solves major problem for this case by allowing SMC calls to be forwarded from guest to the EL3 SCMI firmware.

By default, the SCMI over SMC calls forwarding is enabled for Dom0/hwdom.

+--------------------------+
|                          |
| EL3 SCMI FW (TF-A)       |
++-------+--^--------------+
 |shmem  |  | smc-id
 +----^--+  |
      |     |
 +----|-+---+---+----------+
 |    | |  FWD  |      Xen |
 |    | +---^---+          |
 +----|-----|--------------+
      |     | smc-id
 +----v-----+--+ +---------+
 |             | |         |
 | Dom0/hwdom  | | DomU    |
 |             | |         |
 |             | |         |
 +-------------+ +---------+

The SCMI messages are passed directly through SCMI shared-memory (zero-copy) and driver only forwards SMC calls.

1.1 Compiling

To build with the SCMI over SMC calls forwarding enabled support, enable Kconfig option

SCMI_SMC

The CONFIG_SCMI_SMC is enabled by default.

1.2 Pass-through SCMI SMC to domain which serves as Driver domain

This section describes how to configure the SCMI over SMC calls forwarding driver to handle use case "thin Dom0 with guest domain, which serves as Driver domain". In this case HW need to be enabled in Driver domain and dom0 is performing only control functions (without accessing FW) and so, the SCMI need to be enabled in Driver domain.

+--------------------------+
|EL3 SCMI FW (TF-A)        |
|                          |
+-------------^--+-------+-+
        smc-id|  |shmem0 |
              |  +----^--+
+-------------++------+|----+
|Xen          |  FWD  ||    |
|             +--^----+|    |
+----------------|-----|----+
         smc-id |     |
+-----------+ +--+-----v-----+
|           | |              |
| Dom0      | |    Driver    |
| Control   | |    domain    |
|           | |              |
+-----------+ +--------------+

The SCMI can be enabled for one and only one guest domain.

First, configure Dom0 to enable SCMI pass-through using Xen Command Line "scmi-smc-passthrough" option. This will disable SCMI for Dom0/hwdom and SCMI nodes will be removed from Dom0/hwdom device tree.

Configure SCMI pass-through for guest domain with toolstack

• In domain's xl.cfg file add "arm_sci" option as below

arm_sci = "type=scmi_smc"

• In domain's xl.cfg file enable access to the "arm,scmi-shmem"

iomem = [
    "47ff0,1@22001",
]

• Add SCMI nodes to the Driver domain partial device tree as in the below example:

passthrough {
   scmi_shm_0: sram@22001000 {
       compatible = "arm,scmi-shmem";
       reg = <0x0 0x22001000 0x0 0x1000>;
   };

   firmware {
        compatible = "simple-bus";
            scmi: scmi {
                compatible = "arm,scmi-smc";
                shmem = <&scmi_shm_0>;
                ...
            }
    }
}

Please refer to [2] for details of SCMI DT bindings.

In general, the configuration is similar to any other HW pass-through, except explicitly enabling SCMI with "arm_sci" xl.cfg option.

Configure SCMI pass-through for predefined domain (dom0less)

• add "xen,sci_type" property for required DomU ("xen,domain") node

xen,sci_type="scmi_smc"

• add scmi nodes to the Driver domain partial device tree the same way as above and enable access to the "arm,scmi-shmem" according to dom0less documentation. For example:

scmi_shm_0: sram@22001000 {
      compatible = "arm,scmi-shmem";
      reg = <0x00 0x22001000 0x00 0x1000>;
->        xen,reg = <0x0 0x47ff0000 0x0 0x1000 0x0 0x22001000>;
->        xen,force-assign-without-iommu;
};