This is a public RFC of an ACPI proposal to add a new CXL Root Object that indicates the presence of a CXL hierarchy. The proposal is being made in the new 'Code First' workflow of the ACPI Spec Working Group. The subject matter here has been explored in previously posted patches to QEMU[1] as well as Linux[2]. These patches have served as a proof-of- concept test vehicle to help ensure that the proposal works in practice. The cover letter in series [2] goes into additional detail (section titled 'ACPI0017') for the reasoning behind the proposal, and what is needed beyond what's described in the CXL 2.0 spec. The change proposal that follows is in markdown format. [1]: https://lore.kernel.org/qemu-devel/20201111054724.794888-1-ben.widawsky@xxxxxxxxx/ [2]: https://lore.kernel.org/linux-cxl/20201111054356.793390-1-ben.widawsky@xxxxxxxxx/ ---- # Title: Add CXL Root Object _HID # Status: Draft # Document: ACPI Specification 6.4 # License SPDX-License Identifier: CC-BY-4.0 # Submitter: * Sponsor: Vishal Verma, Intel * Creators/Contributors: * Chet Douglas, Intel * Ben Widawsky, Intel * Dan Williams, Intel * Vishal Verma, Intel # Summary of the Change Add “Compute Express Link Root Object” enumeration value to the ACPI Device IDs Table (5.160). # Benefits of the Change Compute Express Link (CXL) is a new high-speed CPU-to-Device and CPU-to- Memory interconnect. The new ACPI Device ID represents a singleton device that allows the OSPM to trigger the enumeration of the CXL.mem address space, and indicates the presence of cross Host Bridge (ACPI0016) interleaved CXL.mem resources. # Impact of the Change One new row added to the ACPI Device IDs table (5.160). # References * Compute Express Link Specification v2.0, <https://www.computeexpresslink.org/> # Detailed Description of the Change Add “Compute Express Link Root Object” enumeration device ### 5.6.7 Device Class-Specific Objects Most device objects are controlled through generic objects and ... Table 5.160 ACPI Device IDs | Value | Description | | :--- | :---: | :---: | :--------------------------- | | .. | .. | | ACPI0017 | **Compute Express Link Root Object:** This device represents the root of a CXL capable device hierarchy. It shall be present whenever the platform allows OSPM to dynamically assign CXL endpoints to a platform address space. | ## Special Instructions
