linux-kernel - Re: [PATCH v4] cxl: docs/driver-api/conventions resolve conflicts between CFMWS, LMH, Decoders

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Message-ID: <aK9pUhETnNgs-7UG@gourry-fedora-PF4VCD3F>
Date: Wed, 27 Aug 2025 16:23:46 -0400
From: Gregory Price <gourry@...rry.net>
To: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
Cc: linux-cxl@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	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>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	ALOK TIWARI <alok.a.tiwari@...cle.com>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v4] cxl: docs/driver-api/conventions resolve conflicts
 between CFMWS, LMH, Decoders

On Wed, Aug 20, 2025 at 05:06:39PM +0200, Fabio M. De Francesco wrote:
> +
> +E.g, a real x86 platform with two CFMWS, 384 GB total memory, and LMH
> +starting at 2 GB:
> +
> +Window | CFMWS Base | CFMWS Size | HDM Decoder Base | HDM Decoder Size | Ways | Granularity
> +  0    |   0 GB     |     2 GB   |      0 GB        |       3 GB       |  12  |    256
> +  1    |   4 GB     |   380 GB   |      0 GB        |     380 GB       |  12  |    256
> +

This may be a dumb question, but... how is validation supposed to work?

Like in theory according to the above something like the following would
also be valid:

Window | CFMWS Base | CFMWS Size | HDM Decoder Base | HDM Decoder Size
  0    |   4 GB     |   380 GB   |      2 GB        |     382 GB      

(ignoring ways/granularity, i didn't adjust those).

The entirety of the CFMWS would be contained within the HDM decoder, but
with carve-outs on either end.  This would be "allowed" according to the
logic here.

This would effectively allow all HDM decoder base/size values to be valid
as long as one CFMWS is contained entirely within it.

As a result, wouldn't it then also be valid to have an HDM Decoder cover
more than one CFMWS range (two full CFMWS described by a single HDM
decoder).

That seems like it could cause issues.

~Gregory