[PATCH xserver] Allow compile-time selection of a fallback input driver

[Peter Hutterer]

A new --with-fallback-input-driver=foo option allows selecting a fallback
driver for the server if the driver configured for the device is not found.
Note that this only applies when the device has a driver assigned and that
module fails to load, devices without a driver are ignored as usual.

This avoids the situation where a configuration assigns e.g. the synaptics
driver but that driver is not available on the system, resulting in a dead
device. A fallback driver can at least provides some functionality.

This becomes more important as we move towards making other driver true leaf
nodes that can be installed/uninstalled as requested. Specifically, wacom and
synaptics, a config that assigns either driver should be viable even when the
driver itself is not (yet) installed on the system.

It is up to the distributions to make sure that the fallback driver is always
installed. The fallback driver can be disabled with
--without-fallback-input-driver and is disabled by default on non-Linux
systems because we don't have generic drivers on those platforms.
Default driver on Linux is libinput, evdev is the only other serious candidate
here.

Sample log output:
[  3274.421] (II) config/udev: Adding input device SynPS/2 Synaptics TouchPad (/dev/input/event4)
[  3274.421] (**) SynPS/2 Synaptics TouchPad: Applying InputClass "touchpad weird driver"
[  3274.421] (II) LoadModule: "banana"
[  3274.422] (WW) Warning, couldn't open module banana
[  3274.422] (II) UnloadModule: "banana"
[  3274.422] (II) Unloading banana
[  3274.422] (EE) Failed to load module "banana" (module does not exist, 0)
[  3274.422] (EE) No input driver matching `banana'
[  3274.422] (II) Falling back to input driver `libinput'
.. server proceeds to assign libinput, init the device, world peace and rainbows
everywhere, truly what a sight. Shame about the banana though.

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 configure.ac                   | 19 +++++++++++++++++++
 hw/xfree86/common/xf86Xinput.c | 39 +++++++++++++++++++++++++++++++--------
 include/xorg-config.h.in       |  3 +++
 3 files changed, 53 insertions(+), 8 deletions(-)

diff --git a/configure.ac b/configure.ac
index 0d3ad5a..f14e088 100644
--- a/configure.ac
+++ b/configure.ac
@@ -486,6 +486,25 @@ AC_ARG_ENABLE(listen-local,  AS_HELP_STRING([--disable-listen-local],
                                             [Listen on local by default (default:enabled)]),
                                 [LISTEN_LOCAL=$enableval], [LISTEN_LOCAL=yes])
 
+case $host_os in
+    linux*)
+        FALLBACK_INPUT_DRIVER="libinput"
+        ;;
+    *)
+        FALLBACK_INPUT_DRIVER=""
+        ;;
+esac
+AC_ARG_WITH(fallback-input-driver,
+            AC_HELP_STRING([--with-fallback-input-driver=$FALLBACK_INPUT_DRIVER],
+                           [Input driver fallback if the requested driver for a device is unavailable]),
+                           [ FALLBACK_INPUT_DRIVER=$withval ], [])
+if test "x$FALLBACK_INPUT_DRIVER" = "xno"; then
+    FALLBACK_INPUT_DRIVER=""
+fi
+AC_MSG_CHECKING([for fallback input driver])
+AC_MSG_RESULT([$FALLBACK_INPUT_DRIVER])
+AC_DEFINE_UNQUOTED(FALLBACK_INPUT_DRIVER, ["$FALLBACK_INPUT_DRIVER"], [ Fallback input driver ])
+
 dnl Determine font path
 XORG_FONTROOTDIR
 XORG_FONTSUBDIR(FONTMISCDIR, fontmiscdir, misc)
diff --git a/hw/xfree86/common/xf86Xinput.c b/hw/xfree86/common/xf86Xinput.c
index 9ed88f2..d18f53e 100644
--- a/hw/xfree86/common/xf86Xinput.c
+++ b/hw/xfree86/common/xf86Xinput.c
@@ -825,6 +825,22 @@ xf86stat(const char *path, int *maj, int *min)
     *min = minor(st.st_rdev);
 }
 
+static inline InputDriverPtr
+xf86LoadInputDriver(const char *driver_name)
+{
+    InputDriverPtr drv = NULL;
+
+    /* Memory leak for every attached device if we don't
+     * test if the module is already loaded first */
+    drv = xf86LookupInputDriver(driver_name);
+    if (!drv) {
+        if (xf86LoadOneModule(driver_name, NULL))
+            drv = xf86LookupInputDriver(driver_name);
+    }
+
+    return drv;
+}
+
 /**
  * Create a new input device, activate and enable it.
  *
@@ -851,16 +867,23 @@ xf86NewInputDevice(InputInfoPtr pInfo, DeviceIntPtr *pdev, BOOL enable)
     int rval;
     char *path = NULL;
 
-    /* Memory leak for every attached device if we don't
-     * test if the module is already loaded first */
-    drv = xf86LookupInputDriver(pInfo->driver);
-    if (!drv)
-        if (xf86LoadOneModule(pInfo->driver, NULL))
-            drv = xf86LookupInputDriver(pInfo->driver);
+    drv = xf86LoadInputDriver(pInfo->driver);
     if (!drv) {
         xf86Msg(X_ERROR, "No input driver matching `%s'\n", pInfo->driver);
-        rval = BadName;
-        goto unwind;
+
+        if (strlen(FALLBACK_INPUT_DRIVER) > 0) {
+            xf86Msg(X_INFO, "Falling back to input driver `%s'\n",
+                    FALLBACK_INPUT_DRIVER);
+            drv = xf86LoadInputDriver(FALLBACK_INPUT_DRIVER);
+            if (drv) {
+                free(pInfo->driver);
+                pInfo->driver = strdup(FALLBACK_INPUT_DRIVER);
+            }
+        }
+        if (!drv) {
+            rval = BadName;
+            goto unwind;
+        }
     }
 
     xf86Msg(X_INFO, "Using input driver '%s' for '%s'\n", drv->driverName,
diff --git a/include/xorg-config.h.in b/include/xorg-config.h.in
index 63dc5b3..a7d80b5 100644
--- a/include/xorg-config.h.in
+++ b/include/xorg-config.h.in
@@ -148,4 +148,7 @@
 /* Support APM/ACPI power management in the server */
 #undef XF86PM
 
+/* Fallback input driver if the assigned driver fails */
+#undef FALLBACK_INPUT_DRIVER
+
 #endif /* _XORG_CONFIG_H_ */
-- 
2.7.4