gem5: aarch64 graphic mode works

build: allow passing extra kernel config fragment with -c.
Old -c was renamed to -C, and old -C to -f.

README.adoc

@@ -297,6 +297,7 @@ More concretely:
 ....
 git checkout gem5-arm
 ./build -aa -lg -K linux/arch/arm/configs/gem5_defconfig
+./run -aa -gu
 ....
 
 and then on another shell:
@@ -305,15 +306,30 @@ and then on another shell:
 vinagre localhost:5900
 ....
 
-TODO: without the `-K` it fails even though we have added to link:kernel_config_fragment/default[] on the branch:
+The port `5900` is incremented by one if you already have something running on that port: `gem5` stdout tells us the right port on stdout as:
 
 ....
-CONFIG_DRM=y
+system.vncserver: Listening for connections on port 5900
-CONFIG_DRM_HDLCD=y
-CONFIG_DRM_VIRT_ENCODER=y
 ....
 
-so what other options are missing from `gem5_defconfig`? The failing dmesg is:
+aarch64:
+
+....
+./build -aA -lg -K linux/arch/arm64/configs/gem5_defconfig -c kernel_config_fragment/gem5-aarch64-hdlcd
+./run -aA -gu
+....
+
+We need the extra config fragment because the gem5 aarch64 defconfig does not enable HDLCD like the 32 bit one for some reason.
+
+TODO: without the `-K`:
+
+....
+git checkout gem5-arm
+./build -aa -lg -c kernel_config_fragment/gem5-aarch64-hdlcd
+./run -aa -gu
+....
+
+it does not work with a failing dmesg:
 
 ....
 [    0.121078] [drm] found ARM HDLCD version r0p0
@@ -325,6 +341,8 @@ so what other options are missing from `gem5_defconfig`? The failing dmesg is:
 [    0.121197] hdlcd: probe of 2b000000.hdlcd failed with error -12
 ....
 
+So what other options are missing from `gem5_defconfig`? It would be cool to minimize it out to better understand the options.
+
 === Automatic startup commands
 
 When debugging a module, it becomes tedious to wait for build and re-type:
@@ -2222,7 +2240,7 @@ TODO we were not able to get it working yet: https://stackoverflow.com/questions
 
 === Linux kernel configuration
 
-==== Use your own kernel config
+==== Modify kernel config options
 
 By default, we use a `.config` that is a mixture of:
 
@@ -2237,14 +2255,24 @@ If you want to just use your own exact `.config` instead, do:
 
 Beware that Buildroot can `sed` override some of the configurations we make no matter what, e.g. it forces `CONFIG_BLK_DEV_INITRD=y` when `BR2_TARGET_ROOTFS_CPIO` is on, so you might want to double check as explained at <<find-the-kernel-config>>. TODO check if there is a way to prevent that patching and maybe patch Buildroot for it, it is too fuzzy. People should be able to just build with whatever `.config` they want.
 
-==== Modify a config option
+Modify a single option:
-
-Only effective for the current build:
 
 ....
-./build -c 'CONFIG_FORTIFY_SOURCE=y' -l
+./build -C 'CONFIG_FORTIFY_SOURCE=y' -l
 ....
 
+Use an extra kernel config fragment file:
+
+....
+printf '
+CONFIG_IKCONFIG=y
+CONFIG_IKCONFIG_PROC=y
+' > myconfig
+./build -c 'myconfig' -l
+....
+
+`-K`, `-c`, `-C` can all be used at the same time. Options passed via `-C` take precedence over `-c`, which takes precedence over `-K`.
+
 ==== Find the kernel config
 
 Ge the build config in guest:
@@ -3003,7 +3031,7 @@ TODO: can you get function arguments? https://stackoverflow.com/questions/276087
 ==== Kprobes
 
 ....
-./build -c 'CONFIG_KPROBES=y'
+./build -C 'CONFIG_KPROBES=y'
 ./run -F 'insmod /kprobe_example.ko && sleep 4 & sleep 4 &'
 ....
 
@@ -3282,7 +3310,7 @@ Make it harder to get hacked and easier to notice that you were, at the cost of
 Enable:
 
 ....
-./build -c 'CONFIG_FORTIFY_SOURCE=y'
+./build -C 'CONFIG_FORTIFY_SOURCE=y'
 ....
 
 Test it out:

build

@@ -15,11 +15,12 @@ extra_make_args=
 j="$(nproc)"
 linux_reconfigure=false
 linux_kernel_custom_config_file=
+kernel_config_fragments=
 post_script_args=
 qemu_sdl='--enable-sdl --with-sdlabi=2.0'
 suffix=
 v=0
-while getopts 'a:B:b:Cc:Ggj:hIiK:klp:qSs:v' OPT; do
+while getopts 'a:B:b:C:c:fGgj:hIiK:klp:qSs:v' OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -31,10 +32,13 @@ while getopts 'a:B:b:Cc:Ggj:hIiK:klp:qSs:v' OPT; do
       config_fragments="${config_fragments} $(common_abspath "${OPTARG}")"
       ;;
     C)
-      configure=false
+      echo "$OPTARG" >> "$kernel_config_fragment_cli_file"
       ;;
     c)
-      echo "$OPTARG" >> "$kernel_config_fragment_cli_file"
+      kernel_config_fragments="${kernel_config_fragments} $(common_abspath "${OPTARG}")"
+      ;;
+    f)
+      configure=false
       ;;
     g)
       extra_make_args="${extra_make_args} gem5-reconfigure \\
@@ -145,15 +149,22 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"${post_script_args}\"
       echo "error: -K: file does not exist: ${linux_kernel_custom_config_file}" 1>&2
       exit 1
     fi
+    default_config_fragments=
   else
-    d=../kernel_config_fragment
+    default_config_fragments="${d}/min ${d}/default"
-    f="${d}/min"
-    printf "BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES=\"${f} ${d}/default ${kernel_config_fragment_cli_file}\"\n" >> "$config_file"
-    if "${linux_reconfigure}"; then
-      # https://stackoverflow.com/questions/49260466/why-when-i-change-br2-linux-kernel-custom-config-file-and-run-make-linux-reconfi
-      touch "$f"
-    fi
   fi
+  d=../kernel_config_fragment
+  printf "BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES=\"${default_config_fragments} ${kernel_config_fragments} ${kernel_config_fragment_cli_file}\"\n" >> "$config_file"
+  if "${linux_reconfigure}"; then
+    # https://stackoverflow.com/questions/49260466/why-when-i-change-br2-linux-kernel-custom-config-file-and-run-make-linux-reconfi
+    touch "${d}/min"
+  fi
+  if [ "$arch" = 'mips64' ]; then
+    # Workaround for:
+    # http://lists.busybox.net/pipermail/buildroot/2017-August/201053.html
+    sed -Ei 's/^BR2_PACKAGE_LINUX_TOOLS_GPIO/BR2_PACKAGE_LINUX_TOOLS_GPIO=n/' "$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

build-usage.adoc

@@ -12,13 +12,18 @@
 |`-a` |`ARCH`               |Build for architecture `ARCH`.
 |`-B` |`BR2_CONFIG`         |Add a single Buildroot option to the current build.
                              Example: `-B 'BR2_TARGET_ROOTFS_EXT2_SIZE="512M"'`
-|`-b` |`BR2_FILE`           |Also use the given Buildroot configuration fragment file.
+|`-b` |`BR2_CONFIG_FILE`    |Also use the given Buildroot configuration fragment file.
                              Pass multiple times to use multiple fragment files.
-|`-C` |                     |Skip the Buildroot configuration. Saves a few seconds,
+|`-C` |`CONFIG_SOMETHING`   |Also use the given Linux kernel configuration, example:
-                             but requires you to know what you are doing :-)
-|`-c` |`CONFIG_SOMETHING`   |Also use the given Linux kernel configuration, example:
                              `./build -c 'CONFIG_FORTIFY_SOURCE=y'`
                              Can be used multiple times for multiple configs.
+                             These options take precedence over `-c`.
+|`-c` |`KERNEL_CONFIG_FILE` |Also use the given kernel configuration fragment file.
+                             Pass multiple times to use multiple fragment files.
+                             These options take precedence over `-K`.
+|`-f` |                     |Skip the Buildroot configuration. Saves a few seconds,
+                             but requires you to know what you are doing :-)
+                             Mnemonic: `fast`.
 |`-g` |                     |Enable gem5 build or force its rebuild.
 |`-h` |                     |Show this help message.
 |`-I` |                     |Enable initramfs for the current build.

kernel_config_fragment/gem5-aarch64-hdlcd

@@ -0,0 +1,3 @@
+CONFIG_DRM=y
+CONFIG_DRM_HDLCD=y
+CONFIG_DRM_VIRT_ENCODER=y