cxl/core/port: Clarify decoder creation

Add wrappers for the creation of decoder objects at the root level and
switch level, and keep the core helper private to cxl/core/port.c. Root
decoders are static descriptors conveyed from platform firmware (e.g.
ACPI CFMWS). Switch decoders are CXL standard decoders enumerated via
the HDM decoder capability structure. The base address for the HDM
decoder capability structure may be conveyed either by PCIe or platform
firmware (ACPI CEDT.CHBS).

Additionally, the kdoc descriptions for these helpers and their
dependencies is updated.
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
[djbw: fixup changelog, clarify kdoc]
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164366463014.111117.9714595404002687111.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/acpi.c

@@ -102,7 +102,7 @@ static int cxl_parse_cfmws(union acpi_subtable_headers *header, void *arg,
 	for (i = 0; i < CFMWS_INTERLEAVE_WAYS(cfmws); i++)
 		target_map[i] = cfmws->interleave_targets[i];
 
-	cxld = cxl_decoder_alloc(root_port, CFMWS_INTERLEAVE_WAYS(cfmws));
+	cxld = cxl_root_decoder_alloc(root_port, CFMWS_INTERLEAVE_WAYS(cfmws));
 	if (IS_ERR(cxld))
 		return 0;
 
@@ -260,7 +260,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
 	 * dport. Disable the range until the first CXL region is enumerated /
 	 * activated.
 	 */
-	cxld = cxl_decoder_alloc(port, 1);
+	cxld = cxl_switch_decoder_alloc(port, 1);
 	if (IS_ERR(cxld))
 		return PTR_ERR(cxld);
 

drivers/cxl/core/port.c

@@ -495,13 +495,26 @@ static int decoder_populate_targets(struct cxl_decoder *cxld,
 	return rc;
 }
 
-struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets)
+/**
+ * cxl_decoder_alloc - Allocate a new CXL decoder
+ * @port: owning port of this decoder
+ * @nr_targets: downstream targets accessible by this decoder. All upstream
+ *		ports and root ports must have at least 1 target.
+ *
+ * A port should contain one or more decoders. Each of those decoders enable
+ * some address space for CXL.mem utilization. A decoder is expected to be
+ * configured by the caller before registering.
+ *
+ * Return: A new cxl decoder to be registered by cxl_decoder_add()
+ */
+static struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
+					     unsigned int nr_targets)
 {
 	struct cxl_decoder *cxld;
 	struct device *dev;
 	int rc = 0;
 
-	if (nr_targets > CXL_DECODER_MAX_INTERLEAVE || nr_targets < 1)
+	if (nr_targets > CXL_DECODER_MAX_INTERLEAVE || nr_targets == 0)
 		return ERR_PTR(-EINVAL);
 
 	cxld = kzalloc(struct_size(cxld, target, nr_targets), GFP_KERNEL);
@@ -519,20 +532,74 @@ struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets)
 	device_set_pm_not_required(dev);
 	dev->parent = &port->dev;
 	dev->bus = &cxl_bus_type;
-
-	/* root ports do not have a cxl_port_type parent */
-	if (port->dev.parent->type == &cxl_port_type)
-		dev->type = &cxl_decoder_switch_type;
+	if (is_cxl_root(port))
+		cxld->dev.type = &cxl_decoder_root_type;
 	else
-		dev->type = &cxl_decoder_root_type;
+		cxld->dev.type = &cxl_decoder_switch_type;
 
 	return cxld;
 err:
 	kfree(cxld);
 	return ERR_PTR(rc);
 }
-EXPORT_SYMBOL_NS_GPL(cxl_decoder_alloc, CXL);
 
+/**
+ * cxl_root_decoder_alloc - Allocate a root level decoder
+ * @port: owning CXL root of this decoder
+ * @nr_targets: static number of downstream targets
+ *
+ * Return: A new cxl decoder to be registered by cxl_decoder_add(). A
+ * 'CXL root' decoder is one that decodes from a top-level / static platform
+ * firmware description of CXL resources into a CXL standard decode
+ * topology.
+ */
+struct cxl_decoder *cxl_root_decoder_alloc(struct cxl_port *port,
+					   unsigned int nr_targets)
+{
+	if (!is_cxl_root(port))
+		return ERR_PTR(-EINVAL);
+
+	return cxl_decoder_alloc(port, nr_targets);
+}
+EXPORT_SYMBOL_NS_GPL(cxl_root_decoder_alloc, CXL);
+
+/**
+ * cxl_switch_decoder_alloc - Allocate a switch level decoder
+ * @port: owning CXL switch port of this decoder
+ * @nr_targets: max number of dynamically addressable downstream targets
+ *
+ * Return: A new cxl decoder to be registered by cxl_decoder_add(). A
+ * 'switch' decoder is any decoder that can be enumerated by PCIe
+ * topology and the HDM Decoder Capability. This includes the decoders
+ * that sit between Switch Upstream Ports / Switch Downstream Ports and
+ * Host Bridges / Root Ports.
+ */
+struct cxl_decoder *cxl_switch_decoder_alloc(struct cxl_port *port,
+					     unsigned int nr_targets)
+{
+	if (is_cxl_root(port))
+		return ERR_PTR(-EINVAL);
+
+	return cxl_decoder_alloc(port, nr_targets);
+}
+EXPORT_SYMBOL_NS_GPL(cxl_switch_decoder_alloc, CXL);
+
+/**
+ * cxl_decoder_add - Add a decoder with targets
+ * @cxld: The cxl decoder allocated by cxl_decoder_alloc()
+ * @target_map: A list of downstream ports that this decoder can direct memory
+ *              traffic to. These numbers should correspond with the port number
+ *              in the PCIe Link Capabilities structure.
+ *
+ * Certain types of decoders may not have any targets. The main example of this
+ * is an endpoint device. A more awkward example is a hostbridge whose root
+ * ports get hot added (technically possible, though unlikely).
+ *
+ * Context: Process context. Takes and releases the cxld's device lock.
+ *
+ * Return: Negative error code if the decoder wasn't properly configured; else
+ *	   returns 0.
+ */
 int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map)
 {
 	struct cxl_port *port;

drivers/cxl/cxl.h

@@ -278,6 +278,17 @@ struct cxl_dport {
 	struct list_head list;
 };
 
+/*
+ * The platform firmware device hosting the root is also the top of the
+ * CXL port topology. All other CXL ports have another CXL port as their
+ * parent and their ->uport / host device is out-of-line of the port
+ * ancestry.
+ */
+static inline bool is_cxl_root(struct cxl_port *port)
+{
+	return port->uport == port->dev.parent;
+}
+
 struct cxl_port *to_cxl_port(struct device *dev);
 struct cxl_port *devm_cxl_add_port(struct device *host, struct device *uport,
 				   resource_size_t component_reg_phys,
@@ -288,7 +299,10 @@ int cxl_add_dport(struct cxl_port *port, struct device *dport, int port_id,
 
 struct cxl_decoder *to_cxl_decoder(struct device *dev);
 bool is_root_decoder(struct device *dev);
-struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets);
+struct cxl_decoder *cxl_root_decoder_alloc(struct cxl_port *port,
+					   unsigned int nr_targets);
+struct cxl_decoder *cxl_switch_decoder_alloc(struct cxl_port *port,
+					     unsigned int nr_targets);
 int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map);
 int cxl_decoder_autoremove(struct device *host, struct cxl_decoder *cxld);