linux-kernel - Re: [PATCH v2] cxl/acpi: Warn on mixed CXL VH and RCH/RCD Hierarchy

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Date: Mon, 1 Jul 2024 08:49:32 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
Cc: Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org
Subject: Re: [PATCH v2] cxl/acpi: Warn on mixed CXL VH and RCH/RCD Hierarchy

On Fri, 28 Jun 2024, Fabio M. De Francesco wrote:

>Each Host Bridge instance has a corresponding CXL Host Bridge Structure
>(CHBS) ACPI table that identifies its capabilities. CHBS tables can be
>two types (CXL 3.1 Table 9-21): The PCIe Root Complex Register Block
>(RCRB) and CXL Host Bridge Component Registers (CHBCR).
>
>If a Host Bridge is attached to a device that is operating in Restricted
>CXL Device Mode (RCD), BIOS publishes an RCRB with the base address of
>registers that describe its capabilities (CXL 3.1 sec. 9.11).
>
>Instead, the new (CXL 2.0+) Component registers can only be accessed
>by means of a base address published with a CHBCR (CXL 3.1 sec. 9.12).
>
>If an eRCD (a device that forces the host-bridge into CXL 1.1 Restricted
>CXL Host mode) is attached to a CXL 2.0+ Host-Bridge, the current CXL
>specification does not define a mechanism for finding CXL-2.0-only
>root-port component registers like HDM decoders and Extended Security
>capability.
>
>An algorithm to locate a CHBCR associated with an RCRB, would be too
>invasive to land without some concrete motivation.
>
>Therefore, just print a message to inform of unsupported config.
>
>Count how many different CHBS "Version" types are detected by
>cxl_get_chbs_iter(). Then make cxl_get_chbs() print a warning if that sum
>is greater than 1.
>
>Tested-by: Alison Schofield <alison.schofield@...el.com>
>Signed-off-by: Fabio M. De Francesco <fabio.m.de.francesco@...ux.intel.com>

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>