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

Ira Weiny
2025-07-21T20:23:20Z

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