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Message-ID: <20260129191039.kodgkum445ws5rhs@offworld>
Date: Thu, 29 Jan 2026 11:10:39 -0800
From: Davidlohr Bueso <dave@...olabs.net>
To: Gregory Price <gourry@...rry.net>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...a.com, 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, anisa.su@...sung.com,
dongjoo.seo1@...sung.com, s.neeraj@...sung.com
Subject: Re: [PATCH v2 1/3] drivers/cxl: introduce cxl_region_driver field
for cxl_region
On Tue, 13 Jan 2026, 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".
I think your overall idea of these region driver backends to isolate use
cases makes a lot of sense and helps untangle things. This series is a good
step and nicely moves ad-hoc complexity out of region.c.
>
>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`.
>
>All RAM regions will be defaulted to CXL_CONTROL_DAX.
s/CXL_CONTROL_DAX/CXL_REGION_DRIVER_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>
Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
>---
> drivers/cxl/core/region.c | 113 ++++++++++++++++++++++++++++++--------
> drivers/cxl/cxl.h | 8 +++
> 2 files changed, 98 insertions(+), 23 deletions(-)
>
>diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>index ae899f68551f..f8262d2169ea 100644
>--- a/drivers/cxl/core/region.c
>+++ b/drivers/cxl/core/region.c
>@@ -626,6 +626,57 @@ static ssize_t mode_show(struct device *dev, struct device_attribute *attr,
> }
> static DEVICE_ATTR_RO(mode);
>
>+static ssize_t region_driver_show(struct device *dev,
>+ struct device_attribute *attr,
>+ char *buf)
>+{
>+ struct cxl_region *cxlr = to_cxl_region(dev);
>+ const char *desc;
>+
>+ switch (cxlr->driver) {
>+ case CXL_REGION_DRIVER_NONE:
>+ desc = "none";
>+ break;
>+ case CXL_REGION_DRIVER_DAX:
>+ desc = "dax";
>+ break;
>+ case CXL_REGION_DRIVER_PMEM:
>+ desc = "pmem";
>+ break;
>+ }
>+
>+ return sysfs_emit(buf, "%s\n", desc);
>+}
>+
>+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;
>+
>+ /* 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;
>+}
>+static DEVICE_ATTR_RW(region_driver);
>+
> static int alloc_hpa(struct cxl_region *cxlr, resource_size_t size)
> {
> struct cxl_root_decoder *cxlrd = to_cxl_root_decoder(cxlr->dev.parent);
>@@ -772,6 +823,7 @@ static struct attribute *cxl_region_attrs[] = {
> &dev_attr_size.attr,
> &dev_attr_mode.attr,
> &dev_attr_extended_linear_cache_size.attr,
>+ &dev_attr_region_driver.attr,
> NULL,
> };
>
>@@ -2599,6 +2651,16 @@ static struct cxl_region *devm_cxl_add_region(struct cxl_root_decoder *cxlrd,
> cxlr->mode = mode;
> cxlr->type = type;
>
>+ /*
>+ * PMEM regions only have 1 driver: pmem_region
>+ * RAM regions default to DAX, but if the memory is already onlined by
>+ * BIOS as 'System-RAM', the DAX driver will be dropped during probe.
>+ */
>+ if (mode == CXL_PARTMODE_PMEM)
>+ cxlr->driver = CXL_REGION_DRIVER_PMEM;
>+ else
>+ cxlr->driver = CXL_REGION_DRIVER_DAX;
>+
> dev = &cxlr->dev;
> rc = dev_set_name(dev, "region%d", id);
> if (rc)
>@@ -3951,33 +4013,38 @@ static int cxl_region_probe(struct device *dev)
> return rc;
> }
>
>- switch (cxlr->mode) {
>- case CXL_PARTMODE_PMEM:
>- rc = devm_cxl_region_edac_register(cxlr);
>- if (rc)
>- dev_dbg(&cxlr->dev, "CXL EDAC registration for region_id=%d failed\n",
>- cxlr->id);
>+ if (cxlr->mode > CXL_PARTMODE_PMEM) {
>+ dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
>+ cxlr->mode);
>+ return -ENXIO;
>+ }
>
>- return devm_cxl_add_pmem_region(cxlr);
>- case CXL_PARTMODE_RAM:
>- rc = devm_cxl_region_edac_register(cxlr);
>- if (rc)
>- dev_dbg(&cxlr->dev, "CXL EDAC registration for region_id=%d failed\n",
>- cxlr->id);
>+ /*
>+ * The region can not be managed by CXL if any portion of
>+ * it is already online as 'System RAM'.
>+ */
>+ if (walk_iomem_res_desc(IORES_DESC_NONE,
>+ IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY,
>+ p->res->start, p->res->end, cxlr,
>+ is_system_ram) > 0) {
>+ cxlr->driver = CXL_REGION_DRIVER_NONE;
>+ return 0;
>+ }
>
>- /*
>- * The region can not be manged by CXL if any portion of
>- * it is already online as 'System RAM'
>- */
>- if (walk_iomem_res_desc(IORES_DESC_NONE,
>- IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY,
>- p->res->start, p->res->end, cxlr,
>- is_system_ram) > 0)
>- return 0;
>+ rc = devm_cxl_region_edac_register(cxlr);
>+ dev_dbg(&cxlr->dev, "CXL EDAC registration for region_id=%d %s\n",
>+ cxlr->id, rc ? "failed" : "succeeded");
>+
>+ switch (cxlr->driver) {
>+ case CXL_REGION_DRIVER_NONE:
>+ return 0;
>+ case CXL_REGION_DRIVER_DAX:
> return devm_cxl_add_dax_region(cxlr);
>+ case CXL_REGION_DRIVER_PMEM:
>+ return devm_cxl_add_pmem_region(cxlr);
> default:
>- dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
>- cxlr->mode);
>+ dev_dbg(&cxlr->dev, "unsupported region driver: %d\n",
>+ cxlr->driver);
> return -ENXIO;
> }
> }
>diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>index ba17fa86d249..e8256099de29 100644
>--- a/drivers/cxl/cxl.h
>+++ b/drivers/cxl/cxl.h
>@@ -502,6 +502,13 @@ enum cxl_partition_mode {
> CXL_PARTMODE_PMEM,
> };
>
>+
>+enum cxl_region_driver {
>+ CXL_REGION_DRIVER_NONE,
>+ CXL_REGION_DRIVER_DAX,
>+ CXL_REGION_DRIVER_PMEM,
>+};
>+
> /*
> * Indicate whether this region has been assembled by autodetection or
> * userspace assembly. Prevent endpoint decoders outside of automatic
>@@ -543,6 +550,7 @@ struct cxl_region {
> struct device dev;
> int id;
> enum cxl_partition_mode mode;
>+ enum cxl_region_driver driver;
> enum cxl_decoder_type type;
> struct cxl_nvdimm_bridge *cxl_nvb;
> struct cxl_pmem_region *cxlr_pmem;
>--
>2.52.0
>
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