powerpc/rtas: introduce rtas_function_token() API

Users of rtas_token() supply a string argument that can't be validated
at build time. A typo or misspelling has to be caught by inspection or
by observing wrong behavior at runtime.

Since the core RTAS code now has consolidated the names of all
possible RTAS functions and mapped them to their tokens, token lookup
can be implemented using symbolic constants to index a static array.

So introduce rtas_function_token(), a replacement API which does that,
along with a rtas_service_present()-equivalent helper,
rtas_function_implemented(). Callers supply an opaque predefined
function handle which is used internally to index the function
table. Typos or other inappropriate arguments yield build errors, and
the function handle is a type that can't be easily confused with RTAS
tokens or other integer types.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-19-26929c8cce78@linux.ibm.com

1 files changed, 27 insertions, 1 deletions

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 1f80f0b8a9ad..bb6f5370c279 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -453,6 +453,32 @@ static struct rtas_function rtas_function_table[] __ro_after_init = {
 	},
 };
 
+/**
+ * rtas_function_token() - RTAS function token lookup.
+ * @handle: Function handle, e.g. RTAS_FN_EVENT_SCAN.
+ *
+ * Context: Any context.
+ * Return: the token value for the function if implemented by this platform,
+ *         otherwise RTAS_UNKNOWN_SERVICE.
+ */
+s32 rtas_function_token(const rtas_fn_handle_t handle)
+{
+	const size_t index = handle.index;
+	const bool out_of_bounds = index >= ARRAY_SIZE(rtas_function_table);
+
+	if (WARN_ONCE(out_of_bounds, "invalid function index %zu", index))
+		return RTAS_UNKNOWN_SERVICE;
+	/*
+	 * Various drivers attempt token lookups on non-RTAS
+	 * platforms.
+	 */
+	if (!rtas.dev)
+		return RTAS_UNKNOWN_SERVICE;
+
+	return rtas_function_table[index].token;
+}
+EXPORT_SYMBOL_GPL(rtas_function_token);
+
 static int rtas_function_cmp(const void *a, const void *b)
 {
 	const struct rtas_function *f1 = a;
@@ -1011,7 +1037,7 @@ static int ibm_errinjct_token;
  * @....: List of @nargs input parameters.
  *
  * Invokes the RTAS function indicated by @token, which the caller
- * should obtain via rtas_token().
+ * should obtain via rtas_function_token().
  *
  * The @nargs and @nret arguments must match the number of input and
  * output parameters specified for the RTAS function.