linux-kernel - Re: [PATCH v2 1/3] drivers/cxl: introduce cxl_region_driver field for cxl

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Message-ID: <aWfdQESLWEKDlBL_@aschofie-mobl2.lan>
Date: Wed, 14 Jan 2026 10:15:28 -0800
From: Alison Schofield <alison.schofield@...el.com>
To: Gregory Price <gourry@...rry.net>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<kernel-team@...a.com>, <dave@...olabs.net>, <jonathan.cameron@...wei.com>,
	<dave.jiang@...el.com>, <vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
	<dan.j.williams@...el.com>
Subject: Re: [PATCH v2 1/3] drivers/cxl: introduce cxl_region_driver field
 for cxl_region

On Tue, Jan 13, 2026 at 03:21:36PM -0500, Gregory Price wrote:
> The CXL driver presently has 3 modes of managing a cxl_region:
>   - no specific driver (bios-onlined SystemRAM)
>   - dax_region (all other RAM regions, for now)
>   - pmem_region (all PMEM regions)
> 
> Formalize these into specific "region drivers".
> 
> enum cxl_region_driver {
> 	CXL_REGION_DRIVER_NONE,
> 	CXL_REGION_DRIVER_DAX,
> 	CXL_REGION_DRIVER_PMEM
> };
> 
> $cat regionN/region_driver
> [none,dax,pmem]
> 
> The intent is to clarify how to to add additional drivers (sysram,
> dynamic_capacity, etc) in the future, and to allow switching the
> driver selection via a sysfs entry `regionN/region_driver`.

Needs description in Documentation/ABI/testing/sysfs-bus-cxl
I think that will help me understand the switching we expect
to support.


> All RAM regions will be defaulted to CXL_CONTROL_DAX.

CXL_CONTROL_DAX ?

> 
> 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.  This will be discovered at probe time.
> 
> Signed-off-by: Gregory Price <gourry@...rry.net>

snip

> +static ssize_t region_driver_store(struct device *dev,
> +				   struct device_attribute *attr,
> +				   const char *buf, size_t len)
> +{
> +	struct cxl_region *cxlr = to_cxl_region(dev);
> +	struct cxl_region_params *p = &cxlr->params;
> +	int rc;
> +
> +	ACQUIRE(rwsem_write_kill, rwsem)(&cxl_rwsem.region);
> +	if ((rc = ACQUIRE_ERR(rwsem_write_kill, &rwsem)))
> +		return rc;
> +
> +	if (p->state >= CXL_CONFIG_COMMIT)
> +		return -EBUSY;
> +
> +	/* PMEM drivers cannot be changed */
> +	if (cxlr->mode == CXL_PARTMODE_PMEM)
> +		return -EBUSY;

why isn't above "if (cxlr->driver == CXL_REGION_DRIVER_PMEM)"

> +
> +	/* NONE type is not a valid selection for manually probed regions */
> +	if (sysfs_streq(buf, "dax"))
> +		cxlr->driver = CXL_REGION_DRIVER_DAX;
> +	else
> +		return -EINVAL;
> +
> +	return len;
> +}