DTC tools, correct SYSCON LEDs

kernel_config_fragment

@@ -58,17 +58,11 @@ CONFIG_TRACER_SNAPSHOT=y
 # cat /sys/kernel/debug/kernel_page_tables
 CONFIG_X86_PTDUMP=y
 
-# LEDs.
+# LEDs. ARM only.
 #
-#     modprobe led-class
-#     modprobe leds-versatile
-#     ls /sys/class/leds
-#
-# TODO: the LEDs don't appear there. There are some entires under:
-#
-#     /sys/devices/platform/10000000.core-module/10000000.core-module:led@08.0/
-#
-# but they don't have brightness file.
+#     cd /sys/class/leds/versatile:0
+#     cat max_brightness
+#     echo 255 >brightness
 #
 # https://raspberrypi.stackexchange.com/questions/697/how-do-i-control-the-system-leds-using-my-software
 #
@@ -81,10 +75,24 @@ CONFIG_X86_PTDUMP=y
 #
 # - arch/arm/boot/dts/versatile-pb.dts
 # - drivers/leds/led-class.c
-# - drivers/leds/leds-versatile.c
+# - drivers/leds/leds-sysctl.c
 #
+# Try hacking QEMU's `hw/misc/arm_sysctl.c` with a printf:
+#
+#     static void arm_sysctl_write(void *opaque, hwaddr offset,
+#                                 uint64_t val, unsigned size)
+#     {
+#         arm_sysctl_state *s = (arm_sysctl_state *)opaque;
+#
+#         switch (offset) {
+#         case 0x08: /* LED */
+#             printf("LED val = %llx\n", (unsigned long long)val);
+#
+# to obeserve when the callback is made. But beware that one of the LEDs
+# has a heartbeat trigger by default (specified on dts), so it will produce a lot of output.
 CONFIG_LEDS_CLASS=y
 CONFIG_LEDS_CLASS_FLASH=y
+CONFIG_LEDS_SYSCON=y
 CONFIG_LEDS_TRIGGERS=y
 CONFIG_LEDS_TRIGGER_BACKLIGHT=y
 CONFIG_LEDS_TRIGGER_CPU=y
@@ -93,7 +101,6 @@ CONFIG_LEDS_TRIGGER_GPIO=y
 CONFIG_LEDS_TRIGGER_HEARTBEAT=y
 CONFIG_LEDS_TRIGGER_ONESHOT=y
 CONFIG_LEDS_TRIGGER_TIMER=y
-CONFIG_LEDS_VERSATILE=y
 CONFIG_NEW_LEDS=y
 
 # GPIO.