linux-kernel - RE: [PATCH 00/15] cxl: Add support for Restricted CXL hosts (RCD mode)

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Date:   Wed, 7 Sep 2022 22:43:27 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ira Weiny" <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "Dan Williams" <dan.j.williams@...el.com>
CC:     <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, Robert Richter <rrichter@....com>
Subject: RE: [PATCH 00/15] cxl: Add support for Restricted CXL hosts (RCD
 mode)

Apologies for the delay in getting to this I had hoped to be able to
finish up some other DAX work to focus on this, but time is getting
short so I will need to do both in parallel.

Robert Richter wrote:
> In Restricted CXL Device (RCD) mode (formerly referred to as CXL 1.1)
> the PCIe enumeration hierarchy is different from CXL VH Enumeration
> (formerly referred to as 2.0, for both modes see CXL spec 3.0: 9.11
> and 9.12, [1]). This series adds support for RCD mode. It implements
> the detection of Restricted CXL Hosts (RCHs) and its corresponding
> Restricted CXL Devices (RCDs). It does the necessary enumeration of
> ports and connects the endpoints. With all the plumbing an RCH/RCD
> pair is registered at the Linux CXL bus and becomes visible in sysfs
> in the same way as CXL VH hosts and devices do already. RCDs are
> brought up as CXL endpoints and bound to subsequent drivers such as
> cxl_mem.
> 
> For CXL VH the host driver (cxl_acpi) starts host bridge discovery
> once the ACPI0017 CXL root device is detected and then searches for
> ACPI0016 host bridges to enable CXL. In RCD mode an ACPI0017 device
> might not necessarily exist 

That's a broken BIOS as far as I can see. No ACPI0017 == no OS CXL
services and the CXL aspects of the device need to be 100% managed by
the BIOS. You can still run the cxl_pci driver in that case for mailbox
operation, but error handling must be firmware-first without ACPI0017.

> PCIe host bridge PNP0A08 ID, there aren't any CXL port or switches in
> the PCIe hierarchy visible. As such the RCD mode enumeration and host
> discovery is very different from CXL VH. See patch #5 for
> implementation details.
> 
> This implementation expects the host's downstream and upstream port
> RCRBs base address being reported by firmware using the optional CEDT
> CHBS entry of the host bridge (see CXL spec 3.0, 9.17.1.2).
> 
> RCD mode does not support hot-plug, so host discovery is at boot time
> only.
> 
> Patches #1 to #4 are prerequisites of the series with fixes needed and
> a rework of debug messages for port enumeration. Those are general
> patches and could be applied earlier and independently from the rest
> assuming there are no objections with them. Patches #5 to #15 contain
> the actual implementation of RCD mode support.
> 
> [1] https://www.computeexpresslink.org/spec-landing
> 
> Robert Richter (15):
>   cxl/core: Remove duplicate declaration of devm_cxl_iomap_block()
>   cxl/core: Check physical address before mapping it in
>     devm_cxl_iomap_block()
>   cxl: Unify debug messages when calling devm_cxl_add_port()
>   cxl: Unify debug messages when calling devm_cxl_add_dport()
>   cxl/acpi: Add probe function to detect restricted CXL hosts in RCD
>     mode
>   PCI/ACPI: Link host bridge to its ACPI fw node
>   cxl/acpi: Check RCH's PCIe Host Bridge ACPI ID
>   cxl/acpi: Check RCH's CXL DVSEC capabilities
>   cxl/acpi: Determine PCI host bridge's ACPI UID
>   cxl/acpi: Extract the RCH's RCRB base address from CEDT
>   cxl/acpi: Extract the host's component register base address from RCRB
>   cxl/acpi: Skip devm_cxl_port_enumerate_dports() when in RCD mode
>   cxl/acpi: Rework devm_cxl_enumerate_ports() to support RCD mode
>   cxl/acpi: Enumerate ports in RCD mode to enable RCHs and RCDs
>   cxl/acpi: Specify module load order dependency for the cxl_acpi module
> 
>  drivers/acpi/pci_root.c      |   1 +
>  drivers/cxl/acpi.c           | 311 ++++++++++++++++++++++++++++++++++-
>  drivers/cxl/core/pci.c       |  22 ++-
>  drivers/cxl/core/port.c      | 103 ++++++++----
>  drivers/cxl/core/regs.c      |   3 +
>  drivers/cxl/cxl.h            |   2 -
>  drivers/cxl/mem.c            |   1 +
>  tools/testing/cxl/test/cxl.c |   8 +-
>  8 files changed, 400 insertions(+), 51 deletions(-)
> 
> -- 
> 2.30.2
>