linux-kernel - Re: [PATCH 1/6] drivers/cxl: add cxl_memctrl

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Message-ID: <696560c0979e0_20718100b8@dwillia2-mobl4.notmuch>
Date: Mon, 12 Jan 2026 12:59:44 -0800
From: <dan.j.williams@...el.com>
To: Gregory Price <gourry@...rry.net>, <linux-cxl@...r.kernel.org>
CC: <linux-kernel@...r.kernel.org>, <kernel-team@...a.com>,
	<dave@...olabs.net>, <jonathan.cameron@...wei.com>, <dave.jiang@...el.com>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>, <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/6] drivers/cxl: add cxl_memctrl_mode and region->memctrl

Gregory Price wrote:
> The CXL driver presently hands policy management over to DAX subsystem
> for sysram regions, which makes building policy around the entire region
> clunky and at time difficult (e.g. multiple actions to offline and
> hot-unplug memory reliably).
> 
> To support multiple backend controllers for memory regions (for example
> dax vs direct hotplug), implement a memctrl field in cxl_region allows
> switching uncomitted regions between different "memory controllers".
> 
> CXL_CONTROL_NONE: No selected controller, probe will fail.
> CXL_CONTROL_AUTO: If memory is already online as SysRAM, no controller
>                   otherwise register a dax_region
> CXL_CONTROL_DAX : register a dax_region
> 
> Auto regions will either be static sysram (BIOS-onlined) and has no
> region controller associated with it - or if the SP bit was set a
> DAX device will be created.
> 
> Rather than default all regions to the auto-controller, only default
> auto-regions to the auto controller.
> 
> Non-auto regions will be defaulted to CXL_CONTROL_NONE, which will cause
> a failure to probe unless a controller is selected.
> 
> Signed-off-by: Gregory Price <gourry@...rry.net>
> ---
>  drivers/cxl/core/Makefile             |   1 +
>  drivers/cxl/core/core.h               |   2 +
>  drivers/cxl/core/memctrl/Makefile     |   4 +
>  drivers/cxl/core/memctrl/dax_region.c |  79 +++++++++++++++
>  drivers/cxl/core/memctrl/memctrl.c    |  42 ++++++++
>  drivers/cxl/core/region.c             | 136 ++++++++++----------------
>  drivers/cxl/cxl.h                     |  14 +++
>  7 files changed, 192 insertions(+), 86 deletions(-)
>  create mode 100644 drivers/cxl/core/memctrl/Makefile
>  create mode 100644 drivers/cxl/core/memctrl/dax_region.c
>  create mode 100644 drivers/cxl/core/memctrl/memctrl.c
> 
> diff --git a/drivers/cxl/core/Makefile b/drivers/cxl/core/Makefile
> index 5ad8fef210b5..79de20e3f8aa 100644
> --- a/drivers/cxl/core/Makefile
> +++ b/drivers/cxl/core/Makefile
> @@ -17,6 +17,7 @@ cxl_core-y += cdat.o
>  cxl_core-y += ras.o
>  cxl_core-$(CONFIG_TRACING) += trace.o
>  cxl_core-$(CONFIG_CXL_REGION) += region.o
> +include $(src)/memctrl/Makefile

Not sure this merits its own directory, but if it does just do the
canonical:

obj-y += memctrl/

...to add an object-sub-directory.
 
>  cxl_core-$(CONFIG_CXL_MCE) += mce.o
>  cxl_core-$(CONFIG_CXL_FEATURES) += features.o
>  cxl_core-$(CONFIG_CXL_EDAC_MEM_FEATURES) += edac.o
> diff --git a/drivers/cxl/core/core.h b/drivers/cxl/core/core.h
> index 1fb66132b777..1156a4bd0080 100644
> --- a/drivers/cxl/core/core.h
> +++ b/drivers/cxl/core/core.h
> @@ -42,6 +42,8 @@ int cxl_get_poison_by_endpoint(struct cxl_port *port);
>  struct cxl_region *cxl_dpa_to_region(const struct cxl_memdev *cxlmd, u64 dpa);
>  u64 cxl_dpa_to_hpa(struct cxl_region *cxlr, const struct cxl_memdev *cxlmd,
>  		   u64 dpa);
> +int cxl_enable_memctrl(struct cxl_region *cxlr);

This is a "probe" operation not an "enable" in terms of runtime ABI and
presentation that starts decorating the region. In that respect it also
is not a "control" as much as an "operation model / driver". So no need
for a "control" concept, i.e.:

s/CXL_CONTROL_{NONE,AUTO,DAX}/CXL_DRIVER_{NONE,AUTO,DAX}/
s/enum cxl_memctrl_mode/enum cxl_region_driver/

...otherwise there is nothing in this proposal that makes me want to
abandon the traditional meaning of a "driver" probing a "resource" in a
certain way to make it usable with the rest of the kernel.

Rest of this looks fine. With that fixup if we are going to have a set
of different region driver modes then the directory can be:

drivers/cxl/core/region/