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docs: arm: add docs for SCMI over SMC calls forwarding driver
Add documentation section for Simple Arm SCMI over SMC calls forwarding driver (EL3). Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com> Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com> [stefano: code style, 80 chars limit per line] Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
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.. SPDX-License-Identifier: CC-BY-4.0
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ARM System Control and Management Interface (SCMI)
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==================================================
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The System Control and Management Interface (SCMI) [1], which is a set
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of operating system-independent software interfaces that are used in
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system management. SCMI currently
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provides interfaces for:
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- Discovery and self-description of the interfaces it supports
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- Power domain management
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- Clock management
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- Reset domain management
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- Voltage domain management
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- Sensor management
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- Performance management
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- Power capping and monitoring
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- Pin control protocol.
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The SCMI compliant firmware could run:
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- as part of EL3 secure world software (like Trusted Firmware-A) with
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ARM SMC shared-memory transport;
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- on dedicated System Control Processor (SCP) with HW mailbox
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shared-memory transport
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The major purpose of enabling SCMI support in Xen is to enable guest
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domains access to the SCMI interfaces for performing management actions
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on passed-through devices (such as clocks/resets etc) without accessing
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directly to the System control HW (like clock controllers) which in most
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cases can't be shared/split between domains. Or, at minimum, allow SCMI
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access for dom0/hwdom (or guest domain serving as Driver domain).
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The below sections describe SCMI support options available for Xen.
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| [1] `Arm SCMI <https://developer.arm.com/documentation/den0056/latest/>`_
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| [2] `System Control and Management Interface (SCMI) bindings <https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/firmware/arm,scmi.yaml>`_
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Simple SCMI over SMC calls forwarding driver (EL3)
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------------------------------------------------------
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The EL3 SCMI firmware (TF-A) with a single SCMI OSPM agent support is
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pretty generic case for the default vendors SDK and new platforms with
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SCMI support. Such EL3 SCMI firmware supports only single SCMI OSPM
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transport (agent) with Shared memory based transport and SMC calls as
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doorbell.
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The SCMI over SMC calls forwarding driver solves major problem for this
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case by allowing SMC calls to be forwarded from guest to the EL3 SCMI
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firmware.
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By default, the SCMI over SMC calls forwarding is enabled for
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Dom0/hwdom.
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::
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+--------------------------+
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| |
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| EL3 SCMI FW (TF-A) |
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++-------+--^--------------+
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|shmem | | smc-id
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+----^--+ |
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| |
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+----|-+---+---+----------+
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| | | FWD | Xen |
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| | +---^---+ |
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+----|-----|--------------+
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| | smc-id
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+----v-----+--+ +---------+
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| | | |
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| Dom0/hwdom | | DomU |
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| | | |
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| | | |
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+-------------+ +---------+
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The SCMI messages are passed directly through SCMI shared-memory
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(zero-copy) and driver only forwards SMC calls.
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Compiling
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^^^^^^^^^
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To build with the SCMI over SMC calls forwarding enabled support, enable
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Kconfig option
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::
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SCMI_SMC
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The ``CONFIG_SCMI_SMC`` is enabled by default.
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Pass-through SCMI SMC to domain which serves as Driver domain
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This section describes how to configure the SCMI over SMC calls
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forwarding driver to handle use case "thin Dom0 with guest domain, which
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serves as Driver domain". In this case HW need to be enabled in Driver
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domain and dom0 is performing only control functions (without accessing
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FW) and so, the SCMI need to be enabled in Driver domain.
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::
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+--------------------------+
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|EL3 SCMI FW (TF-A) |
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| |
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+-------------^--+-------+-+
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smc-id| |shmem0 |
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| +----^--+
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+-------------++------+|----+
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|Xen | FWD || |
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| +--^----+| |
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+----------------|-----|----+
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smc-id | |
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+-----------+ +--+-----v-----+
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| | | |
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| Dom0 | | Driver |
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| Control | | domain |
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| | | |
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+-----------+ +--------------+
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The SCMI can be enabled for one and only one guest domain.
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First, configure Dom0 to enable SCMI pass-through using Xen Command Line
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**"scmi-smc-passthrough"** option. This will disable SCMI for Dom0/hwdom
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and SCMI nodes will be removed from Dom0/hwdom device tree.
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**Configure SCMI pass-through for guest domain with toolstack**
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* In domain's xl.cfg file add **"arm_sci"** option as below
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::
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arm_sci = "type=scmi_smc"
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* In domain's xl.cfg file enable access to the "arm,scmi-shmem"
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::
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iomem = [
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"47ff0,1@22001",
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]
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.. note:: It's up to the user to select guest IPA for mapping SCMI shared-memory.
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* Add SCMI nodes to the Driver domain partial device tree as in the
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below example:
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.. code::
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passthrough {
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scmi_shm_0: sram@22001000 {
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compatible = "arm,scmi-shmem";
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reg = <0x0 0x22001000 0x0 0x1000>;
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};
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firmware {
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compatible = "simple-bus";
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scmi: scmi {
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compatible = "arm,scmi-smc";
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shmem = <&scmi_shm_0>;
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...
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}
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}
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}
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Please refer to [2] for details of SCMI DT bindings.
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In general, the configuration is similar to any other HW pass-through,
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except explicitly enabling SCMI with "arm_sci" xl.cfg option.
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**Configure SCMI pass-through for predefined domain (dom0less)**
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* add "xen,sci_type" property for required DomU ("xen,domain") node
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::
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xen,sci_type="scmi_smc"
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* add scmi nodes to the Driver domain partial device tree the same way
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as above and enable access to the "arm,scmi-shmem" according to
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dom0less documentation. For example:
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.. code::
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scmi_shm_0: sram@22001000 {
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compatible = "arm,scmi-shmem";
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reg = <0x00 0x22001000 0x00 0x1000>;
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-> xen,reg = <0x0 0x47ff0000 0x0 0x1000 0x0 0x22001000>;
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-> xen,force-assign-without-iommu;
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};
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@@ -0,0 +1,9 @@
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.. SPDX-License-Identifier: CC-BY-4.0
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ARM
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===
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.. toctree::
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:maxdepth: 2
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firmware/arm-scmi
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@@ -9,3 +9,4 @@ Hypervisor documentation
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code-coverage
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x86/index
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arm/index
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