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

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Message-ID: <2393460e-aed6-44ac-9f11-f5b9a1f29e6b@intel.com>
Date: Mon, 20 Oct 2025 14:22:36 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Alison Schofield <alison.schofield@...el.com>,
 Gregory Price <gourry@...rry.net>
Cc: Vishal Aslot <vaslot@...dia.com>, Davidlohr Bueso <dave@...olabs.net>,
 Jonathan Cameron <jonathan.cameron@...wei.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 10/20/25 12:30 PM, Alison Schofield wrote:
> 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.

I think the kernel driver should be fully aware of what is and isn't fully present and handle them appropriately. And on the user side, 'cxl list' should show a decoder in a zero size state so the admin knows why things are the way they are.  > 
>>
>> ~Gregory