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

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Message-ID: <aPaNzeGqUHf6gGIu@aschofie-mobl2.lan>
Date: Mon, 20 Oct 2025 12:30:21 -0700
From: Alison Schofield <alison.schofield@...el.com>
To: Gregory Price <gourry@...rry.net>
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 10:19:09AM -0400, Gregory Price wrote:
> 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.

Really need to hear more about spec here. You mention quirk, but is it
really a quirk or spec defined behavior?

> 
> 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.

Users see a memdev in the topology and want to use it but find no
available endpoint decoder. We'll probably want a mechanism to show why
that is so, hence the suggestion to add to topology and show as locked.

> 
> ~Gregory