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

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Message-ID: <6403027.OV4Wx5bFTl@fdefranc-mobl3>
Date: Mon, 01 Sep 2025 14:26:27 +0200
From: "Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>
To: Gregory Price <gourry@...rry.net>
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 Wednesday, August 27, 2025 10:23:46 PM Central European Summer Time Gregory Price wrote:
> 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      
> 
I'm sorry, it seems that a mistake with copy/pasting I made has led you to 
hypothesize a case that is out of scope of this document.

A case like the one you described will still lead the CXL driver to fail.

Please refer to my reply to Robert and to an old email from Dan.[1][2]

Thanks,

Fabio

[1] https://lore.kernel.org/linux-cxl/4179950.vuYhMxLoTh@fdefranc-mobl3/
[2] https://lore.kernel.org/linux-cxl/67ec4d61c3fd6_288d2947b@dwillia2-xfh.jf.intel.com.notmuch/
>
> (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
> 
>