Only check for for relative path on virtio devices.  Otherwise it could
break driver loading in some circumstances, notably the IceCat sandbox.

https://gitlab.freedesktop.org/mesa/drm/-/issues/39

Taken from upstream:
https://gitlab.freedesktop.org/mesa/drm/-/commit/57df07572ce45a1b60bae6fb89770388d3abd6dd

diff --git a/xf86drm.c b/xf86drm.c
--- a/xf86drm.c
+++ b/xf86drm.c
@@ -3103,15 +3103,18 @@ static int drmParseSubsystemType(int maj, int min)
     int subsystem_type;
 
     snprintf(path, sizeof(path), "/sys/dev/char/%d:%d/device", maj, min);
-    if (!realpath(path, real_path))
-        return -errno;
-    snprintf(path, sizeof(path), "%s", real_path);
 
     subsystem_type = get_subsystem_type(path);
+    /* Try to get the parent (underlying) device type */
     if (subsystem_type == DRM_BUS_VIRTIO) {
+        /* Assume virtio-pci on error */
+        if (!realpath(path, real_path))
+            return DRM_BUS_VIRTIO;
         strncat(path, "/..", PATH_MAX);
         subsystem_type = get_subsystem_type(path);
-    }
+        if (subsystem_type < 0)
+            return DRM_BUS_VIRTIO;
+     }
     return subsystem_type;
 #elif defined(__OpenBSD__) || defined(__DragonFly__) || defined(__FreeBSD__)
     return DRM_BUS_PCI;
@@ -3920,6 +3923,7 @@ process_device(drmDevicePtr *device, const char *d_name,
 
     switch (subsystem_type) {
     case DRM_BUS_PCI:
+    case DRM_BUS_VIRTIO:
         return drmProcessPciDevice(device, node, node_type, maj, min,
                                    fetch_deviceinfo, flags);
     case DRM_BUS_USB:
ce26a8cba9d11de39e4ddb86987200581a86b'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=273ce26a8cba9d11de39e4ddb86987200581a86b'>root</a>/<a href='/guix/log/bootstrap?id=273ce26a8cba9d11de39e4ddb86987200581a86b'>bootstrap</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/bootstrap?id=273ce26a8cba9d11de39e4ddb86987200581a86b&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-06-13 20:46:01 +0200'>2021-06-13</span></td><td><a href='/guix/commit/bootstrap?id=bd02e50a02ceed914135dba1e69e4457e489cf86'>bootstrap: Simplify search for translation languages.</a><span class='msg-avail'>...<span class='msg-tooltip'>Extend the sed script to also behave like "basename",
saving the addtional call of "xargs basename".

* bootstrap (langs): Extend sed scripts, remove running xargs.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2020-02-17 11:49:08 +0100'>2020-02-17</span></td><td><a href='/guix/commit/bootstrap?id=3dd311e3a059131ef245417106d4fb659222ef3c'>bootstrap: Fix typo.</a><span class='msg-avail'>...<span class='msg-tooltip'>* bootstrap: Substitute ‘guix-cookbook’ for copy/pasted ‘guix-manual’.

Reported-by: jetomit on #guix
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-09-18 11:38:14 +0200'>2019-09-18</span></td><td><a href='/guix/commit/bootstrap?id=7bc46ecc34ceb042ff890e5d306d0a2a736e891d'>doc: Add Guix Cookbook.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .gitignore: Update ignore list.
* Makefile.am (assert-no-store-file-names): Exclude the cookbook.
* bootstrap: Generate po files for cookbook translations.
* doc/guix-cookbook.texi: New file.
* doc/local.mk (info_TEXINFOS): Add it; add a rule to build cookbook
translations.
* po/doc/local.mk (DOC_COOKBOOK_PO_FILES): New variable.
(EXTRA_DIST): Add cookbook pot file and po files.
(doc-po-update-cookbook-%): New target.
(doc-pot-update): Also update cookbook pot file.
(doc-po-update): Also update cookbook po files.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-04-26 11:21:32 +0200'>2019-04-26</span></td><td><a href='/guix/commit/bootstrap?id=9ca5ff882e2ac4eaab02eb0fde545bd784af478b'>bootstrap: Break automake dependency on generated files.</a><span class='msg-avail'>...<span class='msg-tooltip'>* bootstrap: Generate stub files for the manual translations whose
generated files are not included in the VCS.
* doc/contributing.de.texi: Remove file.
* doc/contributing.es.texi: Remove file.
* doc/contributing.fr.texi: Remove file.
* doc/contributing.zh_CN.texi: Remove file.
* doc/guix.de.texi: Remove file.
* doc/guix.es.texi: Remove file.
* doc/guix.fr.texi: Remove file.
* doc/guix.zh_CN.texi: Remove file.
* .gitignore: Add them.

Signed-off-by: Julien Lepiller &lt;julien@lepiller.eu&gt;
</span></span></td><td>Miguel Ángel Arruga Vivas</td></tr>