xfree86: Fix autoconfig secondary GPU devices

The code path added by commit 69e4b8e6 (xfree86: attempt to autoconfig
gpu slave devices (v3)) assumes that it will only be run if the primary
device on the screen is the first device in xf86configptr->conf_device_lst.
While this is true most of the time, there are two specific cases where
this assumption fails.

First, if the first device in conf_device_lst is assigned to a different
seat than the running X server, it will be skipped by the previous
FIND_SUITABLE macro usage. Second, if the primary device was explicitly
assigned to the screen but auto_gpu_device is still set and no secondary
devices were explicitly listed, that device may not be the first device
in conf_device_lst.

When the first device in conf_device_lst is not the primary device
assigned to the screen, two problems emerge. First, the first device in
conf_device_lst will never be assigned to the screen as a secondary
device. Second, the primary device is additionally assigned to the
screen as a secondary device. The combination of these problems causes
certain otherwise valid configurations to be invalid. For example, if a
primary device is assigned to a screen and a secondary device is listed
in the config but not explicitly assigned to the screen, then one order
of the device sections results in a usable PRIME or Reverse PRIME setup
and the other order does not.

This commit removes the assumption that the primary device is the first
device in conf_device_lst by starting the loop from the start of
conf_device_lst and skipping the primary device when it is encountered.

Signed-off-by: Jacob Cherry <jcherry@nvidia.com>

hw/xfree86/common/xf86Config.c

@@ -1738,15 +1738,34 @@ configScreen(confScreenPtr screenp, XF86ConfScreenPtr conf_screen, int scrnum,
 
     if (auto_gpu_device && conf_screen->num_gpu_devices == 0 &&
         xf86configptr->conf_device_lst) {
-        XF86ConfDevicePtr sdevice = xf86configptr->conf_device_lst->list.next;
+        /* Loop through the entire device list and skip the primary device
+         * assigned to the screen. This is important because there are two
+         * cases where the assigned primary device is not the first device in
+         * the device list. Firstly, if the first device in the list is assigned
+         * to a different seat than this X server, it will not have been picked
+         * by the previous FIND_SUITABLE. Secondly, if the device was explicitly
+         * assigned in the config but there is still only one screen, this code
+         * path is executed but the explicitly assigned device may not be the
+         * first device in the list. */
+        XF86ConfDevicePtr ptmp, sdevice = xf86configptr->conf_device_lst;
 
         for (i = 0; i < MAX_GPUDEVICES; i++) {
             if (!sdevice)
                 break;
 
-            FIND_SUITABLE (XF86ConfDevicePtr, sdevice, conf_screen->scrn_gpu_devices[i]);
+            FIND_SUITABLE (XF86ConfDevicePtr, sdevice, ptmp);
-            if (!conf_screen->scrn_gpu_devices[i])
+            if (!ptmp)
                 break;
+
+            /* skip the primary device on the screen */
+            if (ptmp != conf_screen->scrn_device) {
+                conf_screen->scrn_gpu_devices[i] = ptmp;
+            } else {
+                sdevice = ptmp->list.next;
+                i--; /* run the next iteration with the same index */
+                continue;
+            }
+
             screenp->gpu_devices[i] = xnfcalloc(1, sizeof(GDevRec));
             if (configDevice(screenp->gpu_devices[i], conf_screen->scrn_gpu_devices[i], TRUE, TRUE)) {
                 screenp->gpu_devices[i]->myScreenSection = screenp;