services: agetty: Add agetty instance to base services. Make its tty optional.

* gnu/services/base.scm (%base-services): Instantiate agetty-service.
(default-serial-port): New variable.
(agetty-shepherd-service): Make tty optional, default to the above.
* doc/guix.texi (agetty-configuration): Update "tty" documentation.
* gnu/system/install.scm (agetty-default-service): Delete variable.
(embedded-installation-os): Remove agetty-default-service instance.
Add "console" kernel-argument.

doc/guix.texi

@@ -9724,7 +9724,20 @@ man page for more information.
 
 @item @code{tty}
 The name of the console this agetty runs on, as a string---e.g.,
-@code{"ttyS0"}. This argument is mandatory.
+@code{"ttyS0"}. This argument is optional, it will default to
+a reasonable default serial port used by the kernel Linux.
+
+For this, if there is a value for an option @code{agetty.tty} in the kernel
+command line, agetty will extract the device name of the serial port
+from it and use that.
+
+If not and if there is a value for an option @code{console} with a tty in
+the Linux command line, agetty will extract the device name of the
+serial port from it and use that.
+
+In both cases, agetty will leave the other serial device settings
+(baud rate etc.) alone---in the hope that Linux pinned them to the
+correct values.
 
 @item @code{baud-rate} (default: @code{#f})
 A string containing a comma-separated list of one or more baud rates, in

gnu/services/base.scm

@@ -817,7 +817,7 @@ (define-record-type* <agetty-configuration>
   agetty-configuration?
   (agetty           agetty-configuration-agetty   ;<package>
                     (default util-linux))
-  (tty              agetty-configuration-tty)     ;string
+  (tty              agetty-configuration-tty)     ;string | #f
   (term             agetty-term                   ;string | #f
                     (default #f))
   (baud-rate        agetty-baud-rate              ;string | #f
@@ -890,6 +890,40 @@ (define-record-type* <agetty-configuration>
 ;;;                 (default #f))
   )
 
+(define (default-serial-port)
+  "Return a gexp that determines a reasonable default serial port
+to use as the tty.  This is primarily useful for headless systems."
+  #~(begin
+      ;; console=device,options
+      ;; device: can be tty0, ttyS0, lp0, ttyUSB0 (serial).
+      ;; options: BBBBPNF. P n|o|e, N number of bits,
+      ;; F flow control (r RTS)
+      (let* ((not-comma (char-set-complement (char-set #\,)))
+             (command (linux-command-line))
+             (agetty-specs (find-long-options "agetty.tty" command))
+             (console-specs (filter (lambda (spec)
+                                     (and (string-prefix? "tty" spec)
+                                          (not (or
+                                                (string-prefix? "tty0" spec)
+                                                (string-prefix? "tty1" spec)
+                                                (string-prefix? "tty2" spec)
+                                                (string-prefix? "tty3" spec)
+                                                (string-prefix? "tty4" spec)
+                                                (string-prefix? "tty5" spec)
+                                                (string-prefix? "tty6" spec)
+                                                (string-prefix? "tty7" spec)
+                                                (string-prefix? "tty8" spec)
+                                                (string-prefix? "tty9" spec)))))
+                                    (find-long-options "console" command)))
+             (specs (append agetty-specs console-specs)))
+        (match specs
+         (() #f)
+         ((spec _ ...)
+          ;; Extract device name from first spec.
+          (match (string-tokenize spec not-comma)
+           ((device-name _ ...)
+            device-name)))))))
+
 (define agetty-shepherd-service
   (match-lambda
     (($ <agetty-configuration> agetty tty term baud-rate auto-login
@@ -900,8 +934,9 @@ (define agetty-shepherd-service
         erase-characters kill-characters chdir delay nice extra-options)
      (list
        (shepherd-service
+         (modules '((ice-9 match) (gnu build linux-boot)))
          (documentation "Run agetty on a tty.")
-         (provision (list (symbol-append 'term- (string->symbol tty))))
+         (provision (list (symbol-append 'term- (string->symbol (or tty "auto")))))
 
          ;; Since the login prompt shows the host name, wait for the 'host-name'
          ;; service to be done.  Also wait for udev essentially so that the tty
@@ -946,6 +981,9 @@ (define agetty-shepherd-service
                                         ('always "--local-line=always")
                                         ('never "-local-line=never")))
                                  #~())
+                          #$@(if tty
+                                 #~()
+                                 #~("--keep-baud"))
                           #$@(if extract-baud?
                                  #~("--extract-baud")
                                  #~())
@@ -1009,7 +1047,7 @@ (define agetty-shepherd-service
                           #$@(if login-pause?
                                  #~("--login-pause")
                                  #~())
-                          #$tty
+                          #$(or tty (default-serial-port))
                           #$@(if baud-rate
                                  #~(#$baud-rate)
                                  #~())
@@ -1058,18 +1096,21 @@ (define mingetty-shepherd-service
        ;; text is not lost in the middle of kernel messages (XXX).
        (requirement '(user-processes host-name udev))
 
-       (start  #~(make-forkexec-constructor
+       (start  #~(let ((tty #$(default-serial-port)))
-                  (list #$(file-append mingetty "/sbin/mingetty")
+                   (if tty
-                        "--noclear" #$tty
+                       (make-forkexec-constructor
-                        #$@(if auto-login
+                        (list #$(file-append mingetty "/sbin/mingetty")
-                               #~("--autologin" #$auto-login)
+                              "--noclear" #$tty
-                               #~())
+                              #$@(if auto-login
-                        #$@(if login-program
+                                     #~("--autologin" #$auto-login)
-                               #~("--loginprog" #$login-program)
+                                     #~())
-                               #~())
+                              #$@(if login-program
-                        #$@(if login-pause?
+                                     #~("--loginprog" #$login-program)
-                               #~("--loginpause")
+                                     #~())
-                               #~()))))
+                              #$@(if login-pause?
+                                     #~("--loginpause")
+                                     #~())))
+                       (const #f)))) ; never start.
        (stop   #~(make-kill-destructor)))))))
 
 (define mingetty-service-type
@@ -2012,6 +2053,11 @@ (define %base-services
                         (cons tty %default-console-font))
                       '("tty1" "tty2" "tty3" "tty4" "tty5" "tty6")))
 
+        (agetty-service (agetty-configuration
+                         (extra-options '("-L")) ; no carrier detect
+                         (term "vt100")
+                         (tty #f))) ; automatic
+
         (mingetty-service (mingetty-configuration
                            (tty "tty1")))
         (mingetty-service (mingetty-configuration

gnu/system/install.scm

@@ -381,14 +381,6 @@ (define installation-os
                      nvi                          ;:wq!
                      %base-packages))))
 
-(define* (agetty-default-service #:optional (tty "ttyS0"))
-  "Return an agetty-service on the given TTY"
-  (agetty-service (agetty-configuration
-                   (extra-options '("-L"))
-                   (baud-rate "115200")
-                   (term "vt100")
-                   (tty tty))))
-
 (define* (embedded-installation-os bootloader bootloader-target tty
                                    #:key (extra-modules '()))
   "Return an installation os for embedded systems.
@@ -401,12 +393,13 @@ (define* (embedded-installation-os bootloader bootloader-target tty
                  (bootloader bootloader)
                  (target bootloader-target)))
     (kernel linux-libre)
+    (kernel-arguments
+     (cons (string-append "console=" tty)
+           (operating-system-user-kernel-arguments installation-os)))
     (initrd (lambda (fs . rest)
               (apply base-initrd fs
                      #:extra-modules extra-modules
-                     rest)))
+                     rest)))))
-    (services (cons* (agetty-default-service tty)
-                     (operating-system-user-services installation-os)))))
 
 (define beaglebone-black-installation-os
   (embedded-installation-os u-boot-beaglebone-black-bootloader