powerpc/rtas: Add function return status constants

Not all of the generic RTAS function statuses specified in PAPR have
symbolic constants and descriptions in rtas.h. Fix this, providing a
little more background, slightly updating the existing wording, and
improving the formatting.
Reviewed-by: "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org>
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-4-e9eafd0c8c6c@linux.ibm.com

arch/powerpc/include/asm/rtas.h

@@ -201,12 +201,25 @@ typedef struct {
 /* Memory set aside for sys_rtas to use with calls that need a work area. */
 #define RTAS_USER_REGION_SIZE (64 * 1024)
 
-/* RTAS return status codes */
-#define RTAS_HARDWARE_ERROR	-1    /* Hardware Error */
-#define RTAS_BUSY		-2    /* RTAS Busy */
-#define RTAS_INVALID_PARAMETER	-3    /* Invalid indicator/domain/sensor etc. */
-#define RTAS_EXTENDED_DELAY_MIN	9900
-#define RTAS_EXTENDED_DELAY_MAX	9905
+/*
+ * Common RTAS function return values, derived from the table "RTAS
+ * Status Word Values" in PAPR+ v2.13 7.2.8: "Return Codes". If a
+ * function can return a value in this table then generally it has the
+ * meaning listed here. More extended commentary in the documentation
+ * for rtas_call().
+ *
+ * RTAS functions may use negative and positive numbers not in this
+ * set for function-specific error and success conditions,
+ * respectively.
+ */
+#define RTAS_SUCCESS                     0 /* Success. */
+#define RTAS_HARDWARE_ERROR             -1 /* Hardware or other unspecified error. */
+#define RTAS_BUSY                       -2 /* Retry immediately. */
+#define RTAS_INVALID_PARAMETER          -3 /* Invalid indicator/domain/sensor etc. */
+#define RTAS_UNEXPECTED_STATE_CHANGE    -7 /* Seems limited to EEH and slot reset. */
+#define RTAS_EXTENDED_DELAY_MIN       9900 /* Retry after delaying for ~1ms. */
+#define RTAS_EXTENDED_DELAY_MAX       9905 /* Retry after delaying for ~100s. */
+#define RTAS_ML_ISOLATION_ERROR      -9000 /* Multi-level isolation error. */
 
 /* statuses specific to ibm,suspend-me */
 #define RTAS_SUSPEND_ABORTED     9000 /* Suspension aborted */