gbm: Allow user to provide device path in environment

If waffle_display_connect() is passed name=NULL, then Waffle's GBM backend will
now open the device path defined by environment variable WAFFLE_GBM_DEVICE. If
WAFFLE_GBM_DEVICE is unset, then Waffle's behavior remains as before: it uses
udev to search for a sensible DRM device node.

Requested-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Chad Versace <chad.versace@intel.com>
(cherry picked from commit cef67496e379b660601f64ab2ddf7cf06307b8aa)

2 files changed, 22 insertions, 9 deletions

diff --git a/man/waffle_display.3.xml b/man/waffle_display.3.xml
index 824a01a..9896247 100644
--- a/man/waffle_display.3.xml
+++ b/man/waffle_display.3.xml
@@ -105,10 +105,15 @@ struct waffle_display;
             uses the value of the environment variable <envar>WAYLAND_DISPLAY</envar>.
           </para>
           <para>
-            On GBM, the function opens the device at the filepath <parameter>name</parameter>. If
-            <parameter>name</parameter> is null, then it iterates using udev through the set of card devices in the drm
-            subsystem, which are usually located in <filename>/dev/dri</filename>, and attempts to open each in turn
-            with <code>open(O_RDWR | O_CLOEXEC)</code> until successful.
+            On GBM, the function opens the device at the filepath <parameter>name</parameter>.
+
+            If <parameter>name</parameter> is NULL, then the function uses the value of environment variable
+            <envar>WAFFLE_GBM_DEVICE</envar>.
+
+            If <parameter>name</parameter> is null and <envar>WAFFLE_GBM_DEVICE</envar> is unset, then the function
+            iterates using udev through the set of card devices in the drm subsystem, which are usually located in
+            <filename>/dev/dri</filename>, and attempts to open each in turn with <code>open(O_RDWR | O_CLOEXEC)</code>
+            until successful.
           </para>
         </listitem>
       </varlistentry>
diff --git a/src/waffle/gbm/wgbm_display.c b/src/waffle/gbm/wgbm_display.c
index 76e6c32..edb714f 100644
--- a/src/waffle/gbm/wgbm_display.c
+++ b/src/waffle/gbm/wgbm_display.c
@@ -128,15 +128,23 @@ wgbm_display_connect(struct wcore_platform *wc_plat,
     if (self == NULL)
         return NULL;
 
+    if (name == NULL) {
+        name = getenv("WAFFLE_GBM_DEVICE");
+    }
+
     if (name != NULL) {
         fd = open(name, O_RDWR | O_CLOEXEC);
+        if (fd < 0) {
+            wcore_errorf(WAFFLE_ERROR_UNKNOWN,
+                    "failed to open gbm device \"%s\"", name);
+            goto error;
+        }
     } else {
         fd = wgbm_get_default_fd();
-    }
-
-    if (fd < 0) {
-        wcore_errorf(WAFFLE_ERROR_UNKNOWN, "open drm file for gbm failed");
-        goto error;
+        if (fd < 0) {
+            wcore_errorf(WAFFLE_ERROR_UNKNOWN, "open drm file for gbm failed");
+            goto error;
+        }
     }
 
     dlopen("libglapi.so.0", RTLD_LAZY | RTLD_GLOBAL);