linux-kernel - Re: [PATCH v2] cxl: core/region - ignore interleave granularity when ways=1

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Message-ID: <20250406183804.wvrnuahoszefhgj5@offworld>
Date: Sun, 6 Apr 2025 11:38:04 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: Gregory Price <gourry@...rry.net>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, dan.j.williams@...el.com,
	vishal.l.verma@...el.com, dave.jiang@...el.com,
	jonathan.cameron@...wei.com, alison.schofield@...el.com,
	ira.weiny@...el.com
Subject: Re: [PATCH v2] cxl: core/region - ignore interleave granularity when
 ways=1

On Wed, 02 Apr 2025, Gregory Price wrote:

>When validating decoder IW/IG when setting up regions, the granularity
>is irrelevant when iw=1 - all accesses will always route to the only
>target anyway - so all ig values are "correct". Loosen the requirement
>that `ig = (parent_iw * parent_ig)` when iw=1.
>
>On some Zen5 platforms, the platform BIOS specifies a 256-byte
>interleave granularity window for host bridges when there is only
>one target downstream.  This leads to Linux rejecting the configuration
>of a region with a x2 root with two x1 hostbridges.
>
>Decoder Programming:
>   root - iw:2 ig:256
>   hb1  - iw:1 ig:256  (Linux expects 512)
>   hb2  - iw:1 ig:256  (Linux expects 512)
>   ep1  - iw:2 ig:256
>   ep2  - iw:2 ig:256
>
>This change allows all decoders downstream of a passthrough decoder to
>also be configured as passthrough (iw:1 ig:X), but still disallows
>downstream decoders from applying subsequent interleaves.
>
>e.g. in the above example if there was another decoder south of hb1
>attempting to interleave 2 endpoints - Linux would enforce hb1.ig=512
>because the southern decoder would have iw:2 and require ig=pig*piw.
>
>Signed-off-by: Gregory Price <gourry@...rry.net>
>Reviewed-by: Dave Jiang <dave.jiang@...el.com>

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>