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Message-ID: <ZoMq9TQoLTMaEPlJ@aschofie-mobl2>
Date: Mon, 1 Jul 2024 15:17:25 -0700
From: Alison Schofield <alison.schofield@...el.com>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
Cc: Davidlohr Bueso <dave@...olabs.net>,
Jonathan Cameron <jonathan.cameron@...wei.com>,
Dave Jiang <dave.jiang@...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, Jun 28, 2024 at 07:48:07PM +0200, 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>
> ---
>
> --- Changes for v2 ---
>
> - Rewrite the Subject line (Alison)
> - Address a bug found by Alison while testing (thanks!)
'Address' a bug doesn't descibe what changed from v1 especially for
folks who reviewed v1.
I'm not asking you to rev this patch, just explain the saved_version
change.
Thanks
-- Alison
> - Add reference to CXL 3.1 Spec. (Alison)
> - Extend the commit messages by borrowing comments to v1 (Dan)
> - Rename field "count" to "nr_versions" (Alison)
> - Add brackets to oneline 'if' statement in precence of comments
> (Dan)
>
> --- Link to v1 ---
>
> https://lore.kernel.org/linux-cxl/20240619125949.167936-1-fabio.m.de.francesco@linux.intel.com/
>
> drivers/cxl/acpi.c | 34 +++++++++++++++++++++++++++-------
> 1 file changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 571069863c62..f9035dbabb1c 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -482,6 +482,8 @@ struct cxl_chbs_context {
> unsigned long long uid;
> resource_size_t base;
> u32 cxl_version;
> + int nr_versions;
> + u32 saved_version;
> };
>
> static int cxl_get_chbs_iter(union acpi_subtable_headers *header, void *arg,
> @@ -490,22 +492,31 @@ static int cxl_get_chbs_iter(union acpi_subtable_headers *header, void *arg,
> struct cxl_chbs_context *ctx = arg;
> struct acpi_cedt_chbs *chbs;
>
> - if (ctx->base != CXL_RESOURCE_NONE)
> - return 0;
> -
> chbs = (struct acpi_cedt_chbs *) header;
>
> - if (ctx->uid != chbs->uid)
> + if (chbs->cxl_version == ACPI_CEDT_CHBS_VERSION_CXL11 &&
> + chbs->length != CXL_RCRB_SIZE)
> return 0;
>
> - ctx->cxl_version = chbs->cxl_version;
> if (!chbs->base)
> return 0;
>
> - if (chbs->cxl_version == ACPI_CEDT_CHBS_VERSION_CXL11 &&
> - chbs->length != CXL_RCRB_SIZE)
> + if (ctx->saved_version != chbs->cxl_version) {
> + /*
> + * cxl_version cannot be overwritten before the next two
> + * checks, then use saved_version
> + */
> + ctx->saved_version = chbs->cxl_version;
> + ctx->nr_versions++;
> + }
> +
> + if (ctx->base != CXL_RESOURCE_NONE)
> + return 0;
> +
> + if (ctx->uid != chbs->uid)
> return 0;
>
> + ctx->cxl_version = chbs->cxl_version;
> ctx->base = chbs->base;
>
> return 0;
> @@ -529,10 +540,19 @@ static int cxl_get_chbs(struct device *dev, struct acpi_device *hb,
> .uid = uid,
> .base = CXL_RESOURCE_NONE,
> .cxl_version = UINT_MAX,
> + .saved_version = UINT_MAX,
> };
>
> acpi_table_parse_cedt(ACPI_CEDT_TYPE_CHBS, cxl_get_chbs_iter, ctx);
>
> + if (ctx->nr_versions > 1) {
> + /*
> + * Disclaim eRCD support given some component register may
> + * only be found via CHBCR
> + */
> + dev_info(dev, "Unsupported platform config, mixed Virtual Host and Restricted CXL Host hierarchy.");
> + }
> +
> return 0;
> }
>
> --
> 2.45.2
>
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