On 8/31/2025 11:21 PM, Zhijian Li (Fujitsu) wrote:
On 22/08/2025 11:42, Smita Koralahalli wrote:
Prevent cxl_region_probe() from unconditionally calling into
devm_cxl_add_dax_region() when the DEV_DAX_CXL driver is not enabled.
Wrap the call with IS_ENABLED(CONFIG_DEV_DAX_CXL) so region probe skips
DAX setup cleanly if no consumer is present.
A question came to mind:
Why is the case of `CXL_REGION && !DEV_DAX_CXL` necessary? It appears to fall back to the hmem driver in that scenario.
If so, could we instead simplify it as follows?
--- a/drivers/cxl/Kconfig
+++ b/drivers/cxl/Kconfig
@@ -200,6 +200,7 @@ config CXL_REGION
depends on SPARSEMEM
select MEMREGION
select GET_FREE_REGION
+ select DEV_DAX_CXL
I’m not entirely sure about the full implications of disabling
CXL_REGION when DEV_DAX_CXL is disabled.
The primary intent of this patch was to address the scenario where
DEV_DAX_HMEM=y and CXL=m, which results in DEV_DAX_CXL being disabled.
In that configuration, ownership of the soft-reserved ranges incorrectly
falls back to hmem instead of being managed by CXL. This leads to
misleading output in /proc/iomem, as I illustrated earlier.
That said, as you pointed out, dax_hmem is not exclusive to CXL, so I
will drop this patch in v2. The next revision will therefore not cover
the case of DEV_DAX_HMEM=y and CXL=m. I would appreciate input on how
best to handle this scenario efficiently.
Thanks
Smita
In parallel, update DEV_DAX_HMEM’s Kconfig to depend on
!CXL_BUS || (CXL_ACPI && CXL_PCI) || m. This ensures:
Built-in (y) HMEM is allowed when CXL is disabled, or when the full
CXL discovery stack is built-in. Module (m) HMEM remains always possible.
Hmm,IIUC, `dax_hmem` isn't exclusively designed for CXL. It could support other
special memory types (e.g., HBM).
Thanks
Zhijian
Signed-off-by: Smita Koralahalli <[email protected]>
---
I did not want to override Dan’s original approach, so I am posting this
as an RFC.
This patch addresses a corner case when applied on top of Patches 1–5.
When DEV_DAX_HMEM=y and CXL=m, the DEV_DAX_CXL option ends up disabled.
In that configuration, with Patches 1–5 applied, ownership of the Soft
Reserved ranges falls back to dax_hmem. As a result, /proc/iomem looks
like this:
850000000-284fffffff : CXL Window 0
850000000-284fffffff : region3
850000000-284fffffff : Soft Reserved
850000000-284fffffff : dax0.0
850000000-284fffffff : System RAM (kmem)
2850000000-484fffffff : CXL Window 1
2850000000-484fffffff : region4
2850000000-484fffffff : Soft Reserved
2850000000-484fffffff : dax1.0
2850000000-484fffffff : System RAM (kmem)
4850000000-684fffffff : CXL Window 2
4850000000-684fffffff : region5
4850000000-684fffffff : Soft Reserved
4850000000-684fffffff : dax2.0
4850000000-684fffffff : System RAM (kmem)
In this case the dax devices are created by dax_hmem, not by dax_cxl.
Consequently, a "cxl disable-region <regionx>" operation does not
unregister these devices. In addition, the dmesg output can be misleading
to users, since it looks like the CXL region driver created the devdax
devices:
devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.2: created region5
..
..
This patch addresses those situations. I am not entirely sure how clean
the approach of using “|| m” is, so I am sending it as RFC for feedback.
---
drivers/cxl/core/region.c | 4 +++-
drivers/dax/Kconfig | 1 +
2 files changed, 4 insertions(+), 1 deletion(-)
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 71cc42d05248..6a2c21e55dbc 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -3617,7 +3617,9 @@ static int cxl_region_probe(struct device *dev)
p->res->start, p->res->end, cxlr,
is_system_ram) > 0)
return 0;
- return devm_cxl_add_dax_region(cxlr);
+ if (IS_ENABLED(CONFIG_DEV_DAX_CXL))
+ return devm_cxl_add_dax_region(cxlr);
+ return 0;
default:
dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
cxlr->mode);
diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 3683bb3f2311..fd12cca91c78 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -30,6 +30,7 @@ config DEV_DAX_PMEM
config DEV_DAX_HMEM
tristate "HMEM DAX: direct access to 'specific purpose' memory"
depends on EFI_SOFT_RESERVE
+ depends on !CXL_BUS || (CXL_ACPI && CXL_PCI) || m
select NUMA_KEEP_MEMINFO if NUMA_MEMBLKS
default DEV_DAX
help
