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

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Message-ID: <aBGIMglj8r5MNAmN@gourry-fedora-PF4VCD3F>
Date: Tue, 29 Apr 2025 22:17:22 -0400
From: Gregory Price <gourry@...rry.net>
To: Li Zhijian <lizhijian@...itsu.com>
Cc: 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
Subject: Re: [RFC PATCH] cxl: Allow reprogramming misconfigured hdm decoders

On Wed, Apr 30, 2025 at 09:29:15AM +0800, Li Zhijian wrote:
> During kernel booting, CXL drivers will attempt to construct the CXL region
> according to the pre-programed(firmware provisioning) HDM decoders.
> 
> This construction process will fail for some reasons, in this case, the
> userspace cli like ndctl/cxl cannot destroy nor create regions upon the
> existing decoders.
> 
> Introuce a new flag CXL_DECODER_F_NEED_RESET tell the driver to reset
> the decoder during `cxl destroy-region regionN`, so that region can be
> create again after that.
> 

My best understanding of why this is disallowed is that firmware/bios
programmed decoders need to be locked because there is an assumption
that the platform programmed it that way *for a reason* - and that
changing the programming would break it (cause MCEs for other reasons,
etc).

So the appropriate solution here is for the platform vendor to fix their
firmware.

But I am not a platform people - so I will defer to them on whether my
understanding is correct.

~Gregory