services: xorg: Allow users to specify the drivers to use.

* gnu/services/xorg.scm (xorg-start-command): Add #:drivers parameter.
  Add 'device-section' procedure, and use it in 'xserver.conf'.
* doc/guix.texi (X Window): Document it.

doc/guix.texi

@@ -3953,6 +3953,15 @@ password.  When @var{auto-login?} is true, log in automatically as
 @var{default-user}.
 @end deffn
 
+@deffn {Monadic Procedure} xorg-start-command [#:guile] @
+  [#:drivers '()] [#:xorg-server @var{xorg-server}]
+Return a derivation that builds a @var{guile} script to start the X server
+from @var{xorg-server}.  Usually the X server is started by a login manager.
+
+@var{drivers} must be either the empty list, in which case Xorg chooses a
+graphics driver automatically, or a list of driver names that will be tried in
+this order---e.g., @code{("modesetting" "vesa")}.
+@end deffn
 
 @node Setuid Programs
 @subsection Setuid Programs

gnu/services/xorg.scm

@@ -42,9 +42,21 @@ (define-module (gnu services xorg)
 
 (define* (xorg-start-command #:key
                              (guile (canonical-package guile-2.0))
-                             (xorg-server xorg-server))
-  "Return a derivation that builds a GUILE script to start the X server from
-XORG-SERVER.  Usually the X server is started by a login manager."
+                             (xorg-server xorg-server)
+                             (drivers '()))
+  "Return a derivation that builds a @var{guile} script to start the X server
+from @var{xorg-server}.  Usually the X server is started by a login manager.
+
+@var{drivers} must be either the empty list, in which case Xorg chooses a
+graphics driver automatically, or a list of driver names that will be tried in
+this order---e.g., @code{(\"modesetting\" \"vesa\")}."
+
+  (define (device-section driver)
+    (string-append "
+Section \"Device\"
+  Identifier \"device-" driver "\"
+  Driver \"" driver "\"
+EndSection"))
 
   (define (xserver.conf)
     (text-file* "xserver.conf" "
@@ -69,7 +81,8 @@ (define (xserver.conf)
 Section \"ServerFlags\"
   Option \"AllowMouseOpenFail\" \"on\"
 EndSection
-"))
+"
+  (string-join (map device-section drivers) "\n")))
 
   (mlet %store-monad ((config (xserver.conf)))
     (define script