linux-kernel - Re: [PATCH v3] cxl: docs/driver-api/conventions resolve conflicts btw CFMWS, LMH, ED

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Message-ID: <687ea20d2e508_34e0f2941@iweiny-mobl.notmuch>
Date: Mon, 21 Jul 2025 15:24:45 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Gregory Price <gourry@...rry.net>, "Fabio M. De Francesco"
	<fabio.m.de.francesco@...ux.intel.com>
CC: Dave Jiang <dave.jiang@...el.com>, <linux-cxl@...r.kernel.org>, "Davidlohr
 Bueso" <dave@...olabs.net>, Jonathan Cameron <jonathan.cameron@...wei.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>
Subject: Re: [PATCH v3] cxl: docs/driver-api/conventions resolve conflicts
 btw CFMWS, LMH, ED

Gregory Price wrote:
> On Thu, Jul 17, 2025 at 04:14:13PM +0200, Fabio M. De Francesco wrote:
> > The table above shows a real configuration copied from an x86 platform 
> > where the Low Memory Hole (LMH) starts at 2GB. 
> > 
> > The"HDM Decoder Base/Size" refers specifically to the CXL Endpoint 
> > Decoders HPA range Base/Size. The first row of the table describes the 
> > first window (CFMWS[0]), whose HPA rage base/size is 0/2GB, and the 
> > Endpoint Decoder that the CXL driver should match with that CFMWS, 
> > whose HPA range base/size is 0/3GB.
> 
> The only thing i ask is being more precise with decoder references.
> 
> HDM Decoder can refer to any of: root, switch, hb, or endpoint decoders.
> 
> Below you make this distinct in the explanation, but in the table it's
> simply general "HDM Decoder".  All I ask is for a bit more clarity on
> what decoder will contain what values to avoid further ambiguity.
> 
> > The driver expects that the Endpoint Decoders HPA ranges to be contained 
> > into their corresponding Root Decoders. Furthermore, Linux fails to 
> > attach Endpoint decoders to already constructed CXL Regions because of 
> > the same size discrepancy issue. 
> > > 
> > > I think you need to describe what the expected behavior is for what linux
> > > will produce in terms of the decoder objects given the above.
> > >
> > The expected behavior is that Linux should be able to match the Endpoint 
> > Decoder with the Root Decoder range even if the CFMWS size is smaller 
> > than the Decoder's, as long as the latter adheres to the 256MB * interleave 
> > ways rule. Furthermore, Linux should be able to match the Endpoint decoders 
> > with already constructed CXL Regions and allow the attachment process to 
> > succeed. 
> > 
> 
> You may also need to describe more than just the contents of the
> endpoint decoder.  What would the content of any intermediate decoders
> be (matching the root or matching the endpoint?).

I wonder if the explanation is lacking intermediate decoders because the
test system does not have them?

Is this true Fabio?

Regardless I think Gregory is correct here.  We should try and document
the general case.

Ira

> 
> > If this explanation suffices, I will incorporate it into the next version
> > of this patch and also explain that "HDM Decoder" stands for Endpoint Decoder 
> > and that the CFMWS HPA base/size describes the System Physical Address (SPA) 
> > which the CXL driver uses to make Root Decoders HPA range base/size. 
> > 
> 
> This explanation is better, just need a few more bits of data and I
> think you're good to go.
> 
> ~Gregory