linux-kernel - Re: [PATCH v4 6/9] cxl/region: Add register

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Message-ID: <6930d45fb836f_1981100b7@dwillia2-mobl4.notmuch>
Date: Wed, 3 Dec 2025 16:22:55 -0800
From: <dan.j.williams@...el.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>
CC: 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>, Jonathan Cameron <jonathan.cameron@...wei.com>,
	Yazen Ghannam <yazen.ghannam@....com>, Dave Jiang <dave.jiang@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming
	<ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, "Ying
 Huang" <huang.ying.caritas@...il.com>, Yao Xingtao <yaoxt.fnst@...itsu.com>,
	Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org>,
	Nathan Fontenot <nathan.fontenot@....com>, Terry Bowman
	<terry.bowman@....com>, Robert Richter <rrichter@....com>, Benjamin Cheatham
	<benjamin.cheatham@....com>, Zhijian Li <lizhijian@...itsu.com>, "Borislav
 Petkov" <bp@...en8.de>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v4 6/9] cxl/region: Add register_dax flag to defer DAX
 setup

Smita Koralahalli wrote:
> Stop creating cxl_dax during cxl_region_probe(). Early DAX registration
> can online memory before ownership of Soft Reserved ranges is finalized.
> This makes it difficult to tear down regions later when HMEM determines
> that a region should not claim that range.
> 
> Introduce a register_dax flag in struct cxl_region_params and gate DAX
> registration on this flag. Leave probe time registration disabled for
> regions discovered during early CXL enumeration; set the flag only for
> regions created dynamically at runtime to preserve existing behaviour.
> 
> This patch prepares the region code for later changes where cxl_dax
> setup occurs from the HMEM path only after ownership arbitration
> completes.

This seems backwards to me. The dax subsystem knows when it wants to
move ahead with CXL or not, dax_cxl_mode is that indicator. So, just
share that variable with drivers/dax/cxl.c, arrange for
cxl_dax_region_probe() to fail while waiting for initial CXL probing to
succeed.

Once that point is reached move dax_cxl_mode to DAX_CXL_MODE_DROP, which
means drop the hmem alias, and go with the real-deal CXL region. Rescan
the dax-bus to retry cxl_dax_region_probe(). No need to bother 'struct
cxl_region' with a 'dax' flag, it just registers per normal and lets the
dax-subsystem handle accepting / rejecting.

Now, we do need a mechanism from dax-to-cxl to trigger region removal in
the DAX_CXL_MODE_REGISTER case (proceed with the hmem registration), but
that is separate from blocking the attachment of dax to CXL regions.
Keep all that complexity local to dax.