gpio: move documentation to README, document that it broke on -M virt

README.adoc

@@ -3555,6 +3555,36 @@ Looks like a recompile is needed to modify the image...
 * https://superuser.com/questions/736423/changing-kernel-bootsplash-image
 * https://unix.stackexchange.com/questions/153975/how-to-change-boot-logo-in-linux-mint
 
+=== GPIO
+
+TODO: this was working before we moved `arm` from `-M versatilepb` to `-M virt` around af210a76711b7fa4554dcc2abd0ddacfc810dfd4. Either make it work on `-M virt` if that is possible, or document precisely how to make it work with `versatilepb`, or hopefully `vexpress` which is newer.
+
+QEMU does not have a very nice mechanism to observe GPIO activity: https://raspberrypi.stackexchange.com/questions/56373/is-it-possible-to-get-the-state-of-the-leds-and-gpios-in-a-qemu-emulation-like-t/69267#69267
+
+The best you can do is to hack our link:build[] script to add:
+
+....
+HOST_QEMU_OPTS='--extra-cflags=-DDEBUG_PL061=1'
+....
+
+where link:http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0190b/index.html[PL061] is the dominating ARM Holdings hardware that handles GPIO.
+
+Then compile with:
+
+....
+./build -aa -b br2_gpio -c kernel_config_fragment/gpio -l
+....
+
+then test it out with:
+
+....
+/gpio.sh
+....
+
+Buildroot's Linux tools package provides some GPIO CLI tools: `lsgpio`, `gpio-event-mon`, `gpio-hammer`, TODO document them here.
+
+Those broke MIPS build in 2017-02: https://bugs.busybox.net/show_bug.cgi?id=10276 and so we force disable them in our MIPS build currently.
+
 === Linux kernel hardening
 
 Make it harder to get hacked and easier to notice that you were, at the cost of some (small?) runtime overhead.

br2

@@ -62,12 +62,6 @@ BR2_PTHREAD_DEBUG=y
 # ftrace
 BR2_PACKAGE_TRACE_CMD=y
 
-# GPIO: lsgpio, gpio-event-mon, gpio-hammer
-# Breask MIPS build in 2017-02:
-# https://bugs.busybox.net/show_bug.cgi?id=10276
-BR2_PACKAGE_LINUX_TOOLS=y
-BR2_PACKAGE_LINUX_TOOLS_GPIO=y
-
 # DTC
 BR2_PACKAGE_DTC=y
 BR2_PACKAGE_DTC_PROGRAMS=y

br2_gpio

@@ -0,0 +1,2 @@
+BR2_PACKAGE_LINUX_TOOLS=y
+BR2_PACKAGE_LINUX_TOOLS_GPIO=y

build

@@ -160,14 +160,12 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"${post_script_args}\"
     touch "${kernel_config_fragment_dir}/min"
   fi
   if [ "$arch" = 'mips64' ]; then
-    # Workaround for:
-    # http://lists.busybox.net/pipermail/buildroot/2017-August/201053.html
+    # Workaround for: https://bugs.busybox.net/show_bug.cgi?id=10276
     sed -Ei 's/^BR2_PACKAGE_LINUX_TOOLS_GPIO/BR2_PACKAGE_LINUX_TOOLS_GPIO=n/' "$config_file"
   fi
   make O="$buildroot_out_dir" olddefconfig
   if [ "$arch" = 'mips64' ]; then
-    # Workaround for:
-    # http://lists.busybox.net/pipermail/buildroot/2017-August/201053.html
+    # Workaround for: https://bugs.busybox.net/show_bug.cgi?id=10276
     sed -Ei 's/^BR2_PACKAGE_LINUX_TOOLS_GPIO/BR2_PACKAGE_LINUX_TOOLS_GPIO=n/' "$config_file"
   fi
   make O="$buildroot_out_dir" olddefconfig
@@ -192,7 +190,7 @@ env \\
 -u LD_LIBRARY_PATH \\
 make \\
 O='${buildroot_out_dir}' \\
-HOST_QEMU_OPTS='--enable-debug --extra-cflags=-DDEBUG_PL061=1 --enable-trace-backends=simple ${qemu_sdl}' \\
+HOST_QEMU_OPTS='--enable-debug --enable-trace-backends=simple ${qemu_sdl}' \\
 V='${v}' \\
 ${extra_make_args} \
 all \\

kernel_config_fragment/default

@@ -79,6 +79,8 @@ CONFIG_VIRTIO_NET=y
 # Misc
 CONFIG_DUMMY_IRQ=m
 CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y
+# Requirement for LOGO.
+CONFIG_FB=y
 CONFIG_LOGO=y
 
 ## Networking
@@ -158,8 +160,9 @@ CONFIG_UIO_PCI_GENERIC=m
 #         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
+# to observe 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
@@ -173,12 +176,6 @@ CONFIG_LEDS_TRIGGER_ONESHOT=y
 CONFIG_LEDS_TRIGGER_TIMER=y
 CONFIG_NEW_LEDS=y
 
-# GPIO
-CONFIG_ARM_AMBA=y
-CONFIG_GPIOLIB=y
-CONFIG_GPIO_SYSFS=y
-CONFIG_GPIO_PL061=y
-
 # Like CONFIG_X86_PTDUMP for ARM.
 CONFIG_ARM64_PTDUMP=y
 

kernel_config_fragment/gpio

@@ -0,0 +1,4 @@
+CONFIG_ARM_AMBA=y
+CONFIG_GPIOLIB=y
+CONFIG_GPIO_SYSFS=y
+CONFIG_GPIO_PL061=y

rootfs_overlay/gpio.sh

@@ -1,6 +1,4 @@
 #!/bin/sh
-# ARM only.
-# https://raspberrypi.stackexchange.com/questions/56373/is-it-possible-to-get-the-state-of-the-leds-and-gpios-in-a-qemu-emulation-like-t/69267#69267
 set -e
 cd /sys/class/gpio
 echo 480 > export