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Date:   Wed, 9 Dec 2020 00:33:52 +0000
From:   "Verma, Vishal L" <vishal.l.verma@...el.com>
To:     "Widawsky, Ben" <ben.widawsky@...el.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>
CC:     "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "jcm@...masters.org" <jcm@...masters.org>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "cbrowy@...ry-design.com" <cbrowy@...ry-design.com>,
        "helgaas@...nel.org" <helgaas@...nel.org>
Subject: Re: [RFC PATCH v2 00/14] CXL 2.0 Support

On Tue, 2020-12-08 at 16:24 -0800, Ben Widawsky wrote:
> Changes since v1 [1]
> 
> A few additions have been made:
>  - IOCTL (UAPI) interface has been added with commands
>  - Kernel docs have been created
>  - A new debug macro is introduced and sprinkled throughout.
> 
> A deletion was made:
>  - Removal of the non-standard _OSC UUID.
> 
> The detailed list of fixes is:
>  - fix cxl_register() no previous prototype warning (0day robot)
>  - s/REGLOG/REGLOC/ (Ben)
>  - Wait for doorbell on cxl_mem_mbox_get() and add comment on why (Ben)
>  - make "type-3" a proper adjective, add spec references, also did the same for
>    the Kconfig (Bjorn)
>  - align some defines (Bjorn)
>  - s/bar/BAR (Bjorn)
>  - rename cxl_bus_prepared() to cxl_bus_acquire() (Bjorn)
>  - move definition of struct cxl_mem to "cxl/mem: Map memory device registers" (Bjorn)
>  - use consistent hex/decimal (Bjorn)
>  - use consistent upper/lower hex values (Bjorn)
>  - get rid of READ_ONCE (Bjorn)
>  - add offsets to debug messages (Bjorn)
>  - cleanup SPDX comment style (Bjorn, Christoph)
>  - change errors returned by case (Bjorn, Dan)
>  - 80 character violation cleanups (Christoph)
>  - cleanup CXL_BUS_PROVIDER dependencies (Christoph, Randy)
>  - remove "raw" from mmio functions (Dan)
>  - rename PCI_DVSEC_VENDOR_CXL to add _ID (Jonathan)
>  - combine introduction of mbox infrastruct and cxl_mem_identify() (Jonathan)
>  - add ABI documentation for sysfs attributes (Jonathan)
>  - document scope of cxl_memdev_lock (Jonathan)
>  - rework cxl_register() to have devm semantics (reaction to comments about
>    cxl_mem_remove() and cxl_mem_add_memdev() semantics) (Jonathan)
>  - fix cxl_mem_exit() ordering (Jonathan)
>  - use GENMASK/GET_FIELD (Jonathan)
>  - fix and add comments for cap ids (Jonathan)
>  - use _OFFSET postfix in definitions (Jonathan)
>  - save pci_set_drvdata for later (Jonathan)

There are a few more change credits for the acpi patches:

- Remove unnecessary helpers and callbacks in drivers/cxl/acpi.c (Christoph, Jonathan)
- Remove some unnecessary variable initalizations (Bjorn, Jonathan)
- Convert to platform_driver (Rafael)

> 
> [1]: https://lore.kernel.org/linux-cxl/20201111054356.793390-1-ben.widawsky@intel.com/
> 
> ---
> 
> Introduce support for “type-3” memory devices defined in the recently released
> Compute Express Link (CXL) 2.0 specification[2]. Specifically, these are the
> memory devices defined by section 8.2.8.5 of the CXL 2.0 spec. A reference
> implementation emulating these devices has been submitted to the QEMU mailing
> list and is available on gitlab [3]. “Type-3” is a CXL device that acts as a
> memory expander for RAM or PMEM.  It might be interleaved with other CXL devices
> in a given physical address range.
> 
> These changes allow for foundational enumeration of CXL 2.0 memory devices as
> well as basic userspace interaction. The functionality present is:
> - Initial driver bring-up
> - Device enumeration and an initial sysfs representation
> - Submit a basic firmware command via ‘mailbox’ to an emulated memory device
>   with non-volatile capacity.
> - Provide an interface to send "raw" commands to the hardware.
> 
> Some of the functionality that is still missing includes:
> - Memory interleaving at the host bridge, root port, or switch level
> - CXL 1.1 Root Complex Integrated Endpoint Support
> - CXL 2.0 Hot plug support
> - A bevy of supported device commands
> 
> In addition to the core functionality of discovering the spec defined registers
> and resources, introduce a CXL device model that will be the foundation for
> translating CXL capabilities into existing Linux infrastructure for Persistent
> Memory and other memory devices. For now, this only includes support for the
> management command mailbox that type-3 devices surface. These control devices
> fill the role of “DIMMs” / nmemX memory-devices in LIBNVDIMM terms.
> 
> Now, while implementing the driver some feedback for the specification was
> generated to cover perceived gaps and address conflicts. The feedback is
> presented as a reference implementation in the driver and QEMU emulation.
> Specifically the following concepts are original to the Linux implementation and
> feedback / collaboration is requested to develop these into specification
> proposals:
> 1. Top level ACPI object (ACPI0017)
> 2. HW imposed address space and interleave constraints
> 
> ACPI0017
> --------
> Introduce a new ACPI namespace device with an _HID of ACPI0017. The purpose of
> this object is twofold, support a legacy OS with a set of out-of-tree CXL
> modules, and establish an attach point for a driver that knows about
> interleaving. Both of these boil down to the same point, to centralize Operating
> System support for resources described by the CXL Early Discovery Table (CEDT).
> 
> The legacy OS problem stems from the spec's description of a host bridge,
> ACPI0016 is denoted as the _HID for host bridges, with a _CID of PNP0A08. In a
> CXL unaware version of Linux, the core ACPI subsystem will bind a driver to
> PNP0A08 and preclude a CXL-aware driver from binding to ACPI0016. An ACPI0017
> device allows a standalone CXL-aware driver to register for handling /
> coordinating CEDT and CXL-specific _OSC control.
> 
> Similarly when managing interleaving there needs to be some management layer
> above the ACPI0016 device that is capable of assembling leaf nodes into
> interleave sets. As is the case with ACPI0012 that does this central
> coordination for NFIT defined resources, ACPI0017 does the same for CEDT
> described resources.
> 
> Memory Windows
> -------
> For CXL.mem capable platforms, there is a need for a mechanism for platform
> firmware to make the Operating System aware of any restrictions that hardware
> might have in address space. For example, in a system with 4 host bridges all
> participating in an interleave set, the firmware needs to provide some
> description of this. That information is missing from the CXL 2.0 spec as of
> today and it also is not implemented in the driver. A variety of ACPI based
> mechanisms, for example _CRS fields on the ACPI0017 device, were considered.
> 
> Next steps after this basic foundation is expanded command support and LIBNVDIMM
> integration. This is the initial “release early / release often” version of the
> Linux CXL enabling.
> 
> [2]: https://www.computeexpresslink.org/
> [3]: https://gitlab.com/bwidawsk/qemu/-/tree/cxl-2.0v2
> 
> 
> Ben Widawsky (10):
>   docs: cxl: Add basic documentation
>   cxl/mem: Map memory device registers
>   cxl/mem: Find device capabilities
>   cxl/mem: Implement polled mode mailbox
>   cxl/mem: Add basic IOCTL interface
>   cxl/mem: Add send command
>   cxl/mem: Add a "RAW" send command
>   cxl: Add basic debugging
>   MAINTAINERS: Add maintainers of the CXL driver
>   WIP/cxl/mem: Add get firmware for testing
> 
> Dan Williams (2):
>   cxl/mem: Introduce a driver for CXL-2.0-Type-3 endpoints
>   cxl/mem: Register CXL memX devices
> 
> Vishal Verma (2):
>   cxl/acpi: Add an acpi_cxl module for the CXL interconnect
>   cxl/acpi: add OSC support
> 
>  Documentation/ABI/testing/sysfs-bus-cxl |   26 +
>  Documentation/cxl/index.rst             |   12 +
>  Documentation/cxl/memory-devices.rst    |   51 ++
>  Documentation/index.rst                 |    1 +
>  MAINTAINERS                             |   10 +
>  drivers/Kconfig                         |    1 +
>  drivers/Makefile                        |    1 +
>  drivers/cxl/Kconfig                     |   58 ++
>  drivers/cxl/Makefile                    |    9 +
>  drivers/cxl/acpi.c                      |  352 ++++++++
>  drivers/cxl/acpi.h                      |   35 +
>  drivers/cxl/bus.c                       |   54 ++
>  drivers/cxl/bus.h                       |    8 +
>  drivers/cxl/cxl.h                       |  188 +++++
>  drivers/cxl/mem.c                       | 1022 +++++++++++++++++++++++
>  drivers/cxl/pci.h                       |   34 +
>  include/acpi/actbl1.h                   |   51 ++
>  include/uapi/linux/cxl_mem.h            |  148 ++++
>  18 files changed, 2061 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-bus-cxl
>  create mode 100644 Documentation/cxl/index.rst
>  create mode 100644 Documentation/cxl/memory-devices.rst
>  create mode 100644 drivers/cxl/Kconfig
>  create mode 100644 drivers/cxl/Makefile
>  create mode 100644 drivers/cxl/acpi.c
>  create mode 100644 drivers/cxl/acpi.h
>  create mode 100644 drivers/cxl/bus.c
>  create mode 100644 drivers/cxl/bus.h
>  create mode 100644 drivers/cxl/cxl.h
>  create mode 100644 drivers/cxl/mem.c
>  create mode 100644 drivers/cxl/pci.h
>  create mode 100644 include/uapi/linux/cxl_mem.h
> 

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