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

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Date: Wed, 26 Jun 2024 12:00:47 +0200
From: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
To: Alison Schofield <alison.schofield@...el.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 Friday, June 21, 2024 10:57:32 PM GMT+2 Alison Schofield wrote:
> On Wed, Jun 19, 2024 at 02:59:41PM +0200, Fabio M. De Francesco wrote:
> 
Hi Alison,
>
> 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?
>
I'll add a spec reference with v2.

> While you're spelling things out, please expand RCRB and CHBCR

I'll do it.
>
> > 
> > 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?
> 
Please read Dan on this topic.
I think I'll add a few lines to the commit message to summarize Dan's comment.
>
> > 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.
>
Yes, "nr_versions" is better.

Thank you.

Fabio
>
> -- Alison
> >  };
> >  
> snip
> > 
>