linux-kernel - Re: [RFC PATCH] cxl: Allow reprogramming misconfigured hdm decoders

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Message-ID: <8f7019ed-a7a2-4cc8-9a8b-e7f96cf5c8d7@fujitsu.com>
Date: Wed, 7 May 2025 06:42:22 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: Alejandro Lucero Palau <alucerop@....com>, Gregory Price
	<gourry@...rry.net>
CC: "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, Alison
 Schofield <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Dan Williams
	<dan.j.williams@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] cxl: Allow reprogramming misconfigured hdm decoders



On 30/04/2025 16:42, Alejandro Lucero Palau wrote:
>>
>> Hi Gregory,
>>
>> Thank you for the feedback. Based on current CXL driver behavior, user-space tools
>> can indeed reprogram firmware-provisioned HDM decoders in practice.
>>
>> For example, after a successful boot, one may destroy the auto-constructed region
>> via cxl destroy-region and create a new different region.
>> This indicates that the kernel does not inherently lock down these decoders.
>>
>> As for the locking rationale you mentioned, platform vendors might enforce their policies
>> through mechanisms like the *Lock-On-Commit* in CXL HDM Decoder n Control Register
>>
>> While platform vendors may have valid considerations (as you noted), from a driver and
>> end-user perspective, depending solely on firmware updates to fix transient failures
>> is not smooth sometimes 🙂.
>>
> 
> Hi Zhijan,
> 
> 
>  From my current effort trying to get a Type2 device properly initialized by the kernel after the BIOS/platform firmware doing whatever it needs to do, I really think we should have a wider discussion regarding this sync, and maybe to have first something from the kernel expectation of what the BIOS should and should not do.
> 
> 
> If this makes sense, I could work on a initial draft about the outline or points to discuss about this.
> 

Hi Alejandro,

Thanks for sharing this concrete pain point. Your experience highlights a critical gap in
"defining clear handoff protocols between firmware and the kernel" for CXL device initialization.
I agree that we need a community-driven effort to establish these expectations.


I’m happy to see your draft or thread for deeper discussion.

Best,
Zhijian


> 
> Thank you