linux-kernel - Re: [PATCH] cxl/acpi: Warn on unsupported platform config detection

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Date: Fri, 21 Jun 2024 13:57:32 -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-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cxl/acpi: Warn on unsupported platform config detection

On Wed, Jun 19, 2024 at 02:59:41PM +0200, Fabio M. De Francesco wrote:

Hi Fabio,
You've written such a detailed commit msg, that it pulls me in,
and now I want to understand more....


> Each Host Bridge instance has a corresponding CXL Host Bridge Structure
> (CHBS) ACPI table that identifies its capabilities. CHBS tables can be
> two types: RCRB and CHBCR.

Is there a spec reference for this?
While you're spelling things out, please expand RCRB and 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.
> 
> However, the new (CXL 2.0+) Component registers (e.g., Extended Security
> Capability), can only be accessed by means of a base address published
> with a CHBCR.
> 
> 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.
> 


Were users seeing this and confused by this silent failure?
What did it look like before?


> 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.
> 
> Signed-off-by: Fabio M. De Francesco <fabio.m.de.francesco@...ux.intel.com>
> ---
>  drivers/cxl/acpi.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 571069863c62..9e226a65a5ea 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -482,6 +482,7 @@ struct cxl_chbs_context {
>  	unsigned long long uid;
>  	resource_size_t base;
>  	u32 cxl_version;
> +	int count;

Maybe s/count/nr_versions to be more explicit of what it counts.

-- Alison
>  };
>  
snip
>