tpm: Clean up driver registration & lookup

We have a strict separation between enum TpmType and be_drivers[]:

* TpmType may have any number of members.  It just happens to have one.

* tpm_register_driver() uses the first empty slot in be_drivers[].

  If you register more than tpm_models[] has space,
  tpm_register_driver() fails.  Its caller silently ignores the
  failure.

  If you register more than one with a given TpmType,
  tpm_display_backend_drivers() will shows all of them, but
  tpm_driver_find_by_type() and tpm_get_backend_driver() will find
  only the one one that registered first.

Since we only ever register one driver, and be_drivers[] has space for
just that one, this contraption even works.

Turn be_drivers[] into a straight map from enum TpmType to driver.
Much simpler, and has a decent chance to actually work should we ever
acquire additional drivers.

While there, use qapi_enum_parse() in tpm_get_backend_driver().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-8-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased, superfluous initializer dropped, commit message rewritten]
Cc: Stefan Berger <stefanb@us.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-4-git-send-email-armbru@redhat.com>

include/sysemu/tpm_backend.h

@@ -227,6 +227,6 @@ TPMBackend *qemu_find_tpm(const char *id);
 
 const TPMDriverOps *tpm_get_backend_driver(const char *type);
 int tpm_register_model(enum TpmModel model);
-int tpm_register_driver(const TPMDriverOps *tdo);
+void tpm_register_driver(const TPMDriverOps *tdo);
 
 #endif

tpm.c

@@ -14,6 +14,7 @@
 #include "qemu/osdep.h"
 
 #include "qapi/qmp/qerror.h"
+#include "qapi/util.h"
 #include "sysemu/tpm_backend.h"
 #include "sysemu/tpm.h"
 #include "qemu/config-file.h"
@@ -25,11 +26,8 @@ static QLIST_HEAD(, TPMBackend) tpm_backends =
 
 
 #define TPM_MAX_MODELS      1
-#define TPM_MAX_DRIVERS     1
 
-static TPMDriverOps const *be_drivers[TPM_MAX_DRIVERS] = {
-    NULL,
-};
+static TPMDriverOps const *be_drivers[TPM_TYPE__MAX];
 
 static enum TpmModel tpm_models[TPM_MAX_MODELS] = {
     TPM_MODEL__MAX,
@@ -63,31 +61,18 @@ static bool tpm_model_is_registered(enum TpmModel model)
 
 const TPMDriverOps *tpm_get_backend_driver(const char *type)
 {
-    int i;
-
-    for (i = 0; i < TPM_MAX_DRIVERS && be_drivers[i] != NULL; i++) {
-        if (!strcmp(TpmType_lookup[be_drivers[i]->type], type)) {
-            return be_drivers[i];
-        }
-    }
+    int i = qapi_enum_parse(TpmType_lookup, type, -1, NULL);
 
-    return NULL;
+    return i >= 0 ? be_drivers[i] : NULL;
 }
 
 #ifdef CONFIG_TPM
 
-int tpm_register_driver(const TPMDriverOps *tdo)
+void tpm_register_driver(const TPMDriverOps *tdo)
 {
-    int i;
+    assert(!be_drivers[tdo->type]);
 
-    for (i = 0; i < TPM_MAX_DRIVERS; i++) {
-        if (!be_drivers[i]) {
-            be_drivers[i] = tdo;
-            return 0;
-        }
-    }
-    error_report("Could not register TPM driver");
-    return 1;
+    be_drivers[tdo->type] = tdo;
 }
 
 /*
@@ -100,9 +85,12 @@ static void tpm_display_backend_drivers(void)
 
     fprintf(stderr, "Supported TPM types (choose only one):\n");
 
-    for (i = 0; i < TPM_MAX_DRIVERS && be_drivers[i] != NULL; i++) {
+    for (i = 0; i < TPM_TYPE__MAX; i++) {
+        if (be_drivers[i] == NULL) {
+            continue;
+        }
         fprintf(stderr, "%12s   %s\n",
-                TpmType_lookup[be_drivers[i]->type], be_drivers[i]->desc());
+                TpmType_lookup[i], be_drivers[i]->desc());
     }
     fprintf(stderr, "\n");
 }
@@ -239,14 +227,7 @@ int tpm_config_parse(QemuOptsList *opts_list, const char *optarg)
 
 static const TPMDriverOps *tpm_driver_find_by_type(enum TpmType type)
 {
-    int i;
-
-    for (i = 0; i < TPM_MAX_DRIVERS && be_drivers[i] != NULL; i++) {
-        if (be_drivers[i]->type == type) {
-            return be_drivers[i];
-        }
-    }
-    return NULL;
+    return be_drivers[type];
 }
 
 static TPMInfo *qmp_query_tpm_inst(TPMBackend *drv)