Linux DVB: run-time fallback from 5.5 to 5.0 frontend probing

This enables running VLC to run on a kernel that does not support
version 5.5 of the Linux DVB API (i.e. Linux < 3.3), even if VLC was
compiled with more recent kernel headers.

Unfortunately, this is of limited use. In practice, the running kernel
version is more likely to be newer than the kernel headers, than to be
older. In that case, VLC will print an explicit error.

modules/access/dtv/linux.c

@@ -447,7 +447,7 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
     if (ioctl (fd, FE_GET_PROPERTY, &props) < 0)
     {
          msg_Err (d->obj, "cannot enumerate frontend systems: %m");
-         return 0;
+         goto legacy;
     }
 
     static const unsigned systab[] = {
@@ -489,6 +489,10 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
         msg_Dbg (d->obj, " system %u", sys);
         systems |= systab[sys];
     }
+
+    return systems;
+legacy:
+    props.num = 1;
 #else
     struct dtv_property prop[1] = {
         { .cmd = DTV_API_VERSION },
@@ -497,7 +501,8 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
         .num = 1,
         .props = prop
     };
-
+    unsigned systems = 0;
+#endif
     if (ioctl (fd, FE_GET_PROPERTY, &props) < 0)
     {
         msg_Err (d->obj, "unsupported kernel DVB version 3 or older (%m)");
@@ -507,6 +512,15 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
     msg_Dbg (d->obj, "probing frontend (kernel API v%u.%u, user API v%u.%u)",
              prop[0].u.data >> 8, prop[0].u.data & 0xFF,
              DVB_API_VERSION, DVB_API_VERSION_MINOR);
+#if !DVBv5(5)
+    /* Some delivery systems cannot be detected without the DVB API v5.5.
+     * To run correctly on recent kernels (Linux >= 3.3),
+     * VLC needs to be compiled with up-to-date kernel headers. */
+    if ((prop[0].u.data >> 8) > 5
+     || ((prop[0].u.data >> 8) == 5 && (prop[0].u.data & 0xFF) >= 5))
+        msg_Err (d->obj, "obsolete user API version running on a new kernel");
+        msg_Info (d->obj, "please recompile "PACKAGE_NAME" "PACKAGE_VERSION);
+#endif
     struct dvb_frontend_info info;
     if (ioctl (fd, FE_GET_INFO, &info) < 0)
     {
@@ -523,8 +537,6 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
              info.symbol_rate_min, info.symbol_rate_max);
     msg_Dbg (d->obj, " (%"PRIu32" tolerance)", info.symbol_rate_tolerance);
 
-    unsigned systems;
-
     /* DVB first generation and ATSC */
     switch (info.type)
     {
@@ -533,7 +545,6 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
         case FE_OFDM: systems = DVB_T; break;
         case FE_ATSC: systems = ATSC | CQAM; break;
         default:
-            systems = 0;
             msg_Err (d->obj, "unknown frontend type %u", info.type);
     }
 
@@ -552,7 +563,7 @@ static unsigned dvb_probe_frontend (dvb_device_t *d, int fd)
     /* ISDB (only terrestrial before DVBv5.5)  */
     if (info.type == FE_OFDM)
         systems |= ISDB_T;
-#endif
+
     return systems;
 }