linux-kernel - Re: [PATCH v1 1/2] cxl_test: enable zero sized decoders under hb0

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Message-ID: <aPZE3Spas-IvHmfd@gourry-fedora-PF4VCD3F>
Date: Mon, 20 Oct 2025 10:19:09 -0400
From: Gregory Price <gourry@...rry.net>
To: Alison Schofield <alison.schofield@...el.com>
Cc: Vishal Aslot <vaslot@...dia.com>, Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Li Ming <ming.li@...omail.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"open list:COMPUTE EXPRESS LINK (CXL)" <linux-cxl@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/2] cxl_test: enable zero sized decoders under hb0

On Mon, Oct 20, 2025 at 12:09:34AM -0700, Alison Schofield wrote:
> > This patch updates cxl_test to enable decoders 1 and 2
> > in the host-bridge 0 port, in a switch uport under hb0,
> > and the endpoints ports with size zero simulating
> > committed zero sized decoders.
> 
> Decoders 1 & 2 - those are after decoder 0, the autoregion.
> That's a problem ATM, when we try to teardown the autoregion we
> get out of order resets. Like I asked in the other patch, if there
> are rules about where these zero size decoders may appear, that
> may make the solution here simpler.
> 

I think this is going to require a quirk-doc like other deviations.

A committed decoder must have a base address, and with 0-size subsequent
or previous decoders would also have an address that covers that address
as well.  This is on top of the ordering issue if the 0-side decoders
come after a programmable decoder.

I'm not convinced this even makes sense as a security thing if you can
reset the bus and re-activate everything (after a graceful teardown).

Seems easier to just report the decoders as unavailable and then not
probe them.

~Gregory