kernel/dtm: Add an example on the kernel DT API

This comes in the form of a simple kernel module which iterates over
CPUs and prints the contents of some device tree nodes.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 114 insertions, 0 deletions

diff --git a/kernel/dtm/dtm.c b/kernel/dtm/dtm.c
new file mode 100644
index 0000000..adda89a
--- /dev/null
+++ b/kernel/dtm/dtm.c
@@ -0,0 +1,114 @@
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/of.h>
+
+MODULE_DESCRIPTION("Kernel module to show off the DeviceTree Kernel API");
+MODULE_AUTHOR("Miquel Sabaté Solà");
+MODULE_LICENSE("GPL");
+MODULE_VERSION("0.0.1");
+
+// `compatible` value for QEMU/KVM.
+static const char *const qemu[] = {
+	"riscv-virtio",
+	NULL,
+};
+
+// `compatible` values that will match on Starfive VisionFive2 v1.3b (plus a
+// boogie value).
+static const char *const vf2[] = {
+	"starfive,visionfive-2-v1.3b",
+	"starfive,jh7110",
+	"this is bullshit but it won't interfere",
+	NULL,
+};
+
+static int __init dtm_init(void)
+{
+	unsigned int cpu;
+	struct device_node *node, *parent;
+	const char *isa;
+	int rc;
+
+	/*
+	 * `of_compatible_match` returns true if there is _one_ element of the given
+	 * array which matches a string on the `compatible` file. That is, not all
+	 * of them have to match.
+	 */
+	if (of_machine_compatible_match(qemu)) {
+		pr_info("Inside of QEMU\n");
+	} else if (of_machine_compatible_match(vf2)) {
+		pr_info("Inside of VisionFive2\n");
+	} else {
+		pr_info("Somewhere over the rainbow\n");
+	}
+
+	/*
+	 * The `for_each_possible_cpu` is a function-like macro provided by the
+	 * Kernel which allows `cpu` to be an iterator over CPU device nodes.
+	 */
+	for_each_possible_cpu(cpu) {
+		/* isa = NULL; */
+
+		/*
+		 * Given a CPU identifier, `of_cpu_device_node_get` returns a `struct
+		 * device_node` related to the specified CPU (or NULL on error).
+		 */
+		node = of_cpu_device_node_get(cpu);
+		if (!node) {
+			pr_warn("Unable to find cpu node\n");
+			continue;
+		}
+
+		/*
+		 * `of_property_present` returns a boolean on whether the given node
+		 * property is present for the given device node.
+		 */
+		if (!of_property_present(node, "riscv,isa")) {
+			pr_warn("Unable to find 'riscv,isa' for this node\n");
+			continue;
+		}
+
+		/*
+		 * `of_property_read_string` initializes the given `isa` string with the
+		 * contents of the property we are trying to fetch. Returns 0 on
+		 * success, and a kernel error otherwise. Note that the previous
+		 * `of_property_present` call was actually not needed, we would've get
+		 * here an `-EINVAL` error eitherway.
+		 */
+		rc = of_property_read_string(node, "riscv,isa", &isa);
+		if (rc) {
+			pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
+			continue;
+		}
+		pr_info("RISC-V ISA for '%s': %s\n", node->full_name, isa);
+
+		/*
+		 * `of_get_parent` returns the parent device node for the one provided
+		 * or NULL if there's none.
+		 */
+		parent = of_get_parent(node);
+		if (!node) {
+			pr_warn("No parent for the given node!\n");
+			continue;
+		}
+		pr_info("parent: %s\n", node->full_name);
+
+		/*
+		 * `of_node_put` is used to decrement the reference counter on device
+		 * nodes. We are done using these instances, so we should decrement
+		 * their life times.
+		 */
+		of_node_put(node);
+		of_node_put(parent);
+	}
+	return 0;
+}
+
+static void __exit dtm_exit(void)
+{
+	pr_info("Exiting from DTM module\n");
+}
+
+module_init(dtm_init);
+module_exit(dtm_exit);