[PATCH-for-5.1 1/2] tpm: Display when no backend is available

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Display "No TPM backend available in this binary." error when
no backend is available.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 tpm.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tpm.c b/tpm.c
index fe03b24858..e36803a64d 100644
--- a/tpm.c
+++ b/tpm.c
@@ -41,6 +41,22 @@ tpm_be_find_by_type(enum TpmType type)
     return TPM_BACKEND_CLASS(oc);
 }
 
+/*
+ * Walk the list of available TPM backend drivers and count them.
+ */
+static int tpm_backend_drivers_count(void)
+{
+    int count = 0, i;
+
+    for (i = 0; i < TPM_TYPE__MAX; i++) {
+        const TPMBackendClass *bc = tpm_be_find_by_type(i);
+        if (bc) {
+            count++;
+        }
+    }
+    return count;
+}
+
 /*
  * Walk the list of available TPM backend drivers and display them on the
  * screen.
@@ -87,6 +103,11 @@ static int tpm_init_tpmdev(void *dummy, QemuOpts *opts, Error **errp)
     TPMBackend *drv;
     int i;
 
+    if (!tpm_backend_drivers_count()) {
+        error_setg(errp, "No TPM backend available in this binary.");
+        return 1;
+    }
+
     if (!QLIST_EMPTY(&tpm_backends)) {
         error_setg(errp, "Only one TPM is allowed.");
         return 1;
-- 
2.21.3