cxl: Add handling of locked CXL decoder

When a decoder is locked, it means that its configuration cannot be
changed. CXL spec r3.2 8.2.4.20.13 discusses the details regarding
locked decoders. Locking happens when bit 8 of the decoder control
register is set and then the decoder is committed afterwards (CXL
spec r3.2 8.2.4.20.7).

Given that the driver creates a virtual decoder for each CFMWS, the
Fixed Device Configuration (bit 4) of the Window Restriction field is
considered as locking for the virtual decoder by the driver.

The current driver code disregards the locked status and a region can
be destroyed regardless of the locking state.

Add a region flag to indicate the region is in a locked configuration.
The driver will considered a region locked if the CFMWS or any decoder
is configured as locked. The consideration is all or nothing regarding
the locked state. It is reasonable to determine the region "locked"
status while the region is being assembled based on the decoders.

Add a check in region commit_store() to intercept when a 0 is written
to the commit sysfs attribute in order to prevent the destruction of a
region when in locked state. This should be the only entry point from user
space to destroy a region.

Add a check is added to cxl_decoder_reset() to prevent resetting a locked
decoder within the kernel driver.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251105201826.2901915-1-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/cxl/core/hdm.c

@@ -905,6 +905,9 @@ static void cxl_decoder_reset(struct cxl_decoder *cxld)
 	if ((cxld->flags & CXL_DECODER_F_ENABLE) == 0)
 		return;
 
+	if (test_bit(CXL_DECODER_F_LOCK, &cxld->flags))
+		return;
+
 	if (port->commit_end == id)
 		cxl_port_commit_reap(cxld);
 	else

drivers/cxl/core/region.c

@@ -245,6 +245,9 @@ static void cxl_region_decode_reset(struct cxl_region *cxlr, int count)
 	struct cxl_region_params *p = &cxlr->params;
 	int i;
 
+	if (test_bit(CXL_REGION_F_LOCK, &cxlr->flags))
+		return;
+
 	/*
 	 * Before region teardown attempt to flush, evict any data cached for
 	 * this region, or scream loudly about missing arch / platform support
@@ -419,6 +422,9 @@ static ssize_t commit_store(struct device *dev, struct device_attribute *attr,
 		return len;
 	}
 
+	if (test_bit(CXL_REGION_F_LOCK, &cxlr->flags))
+		return -EPERM;
+
 	rc = queue_reset(cxlr);
 	if (rc)
 		return rc;
@@ -1059,6 +1065,16 @@ static int cxl_rr_assign_decoder(struct cxl_port *port, struct cxl_region *cxlr,
 	return 0;
 }
 
+static void cxl_region_set_lock(struct cxl_region *cxlr,
+				struct cxl_decoder *cxld)
+{
+	if (!test_bit(CXL_DECODER_F_LOCK, &cxld->flags))
+		return;
+
+	set_bit(CXL_REGION_F_LOCK, &cxlr->flags);
+	clear_bit(CXL_REGION_F_NEEDS_RESET, &cxlr->flags);
+}
+
 /**
  * cxl_port_attach_region() - track a region's interest in a port by endpoint
  * @port: port to add a new region reference 'struct cxl_region_ref'
@@ -1170,6 +1186,8 @@ static int cxl_port_attach_region(struct cxl_port *port,
 		}
 	}
 
+	cxl_region_set_lock(cxlr, cxld);
+
 	rc = cxl_rr_ep_add(cxl_rr, cxled);
 	if (rc) {
 		dev_dbg(&cxlr->dev,
@@ -2439,6 +2457,7 @@ static struct cxl_region *cxl_region_alloc(struct cxl_root_decoder *cxlrd, int i
 	dev->bus = &cxl_bus_type;
 	dev->type = &cxl_region_type;
 	cxlr->id = id;
+	cxl_region_set_lock(cxlr, &cxlrd->cxlsd.cxld);
 
 	return cxlr;
 }

drivers/cxl/cxl.h

@@ -517,6 +517,14 @@ enum cxl_partition_mode {
  */
 #define CXL_REGION_F_NEEDS_RESET 1
 
+/*
+ * Indicate whether this region is locked due to 1 or more decoders that have
+ * been locked. The approach of all or nothing is taken with regard to the
+ * locked attribute. CXL_REGION_F_NEEDS_RESET should not be set if this flag is
+ * set.
+ */
+#define CXL_REGION_F_LOCK 2
+
 /**
  * struct cxl_region - CXL region
  * @dev: This region's device