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TriForceX
137
board/chromebook/snow/readme.txt
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board/chromebook/snow/readme.txt
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Samsung XE303C12 aka Chromebook Snow
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====================================
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This file describes booting the Chromebook from an SD card containing
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Buildroot kernel and rootfs, using the original bootloader. This is
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the least invasive way to get Buildroot onto the devices and a good
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starting point.
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The bootloader will only boot a kernel from a GPT partition marked
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bootable with cgpt tool from vboot-utils package.
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The kernel image must be signed using futility from the same package.
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The signing part is done by sign.sh script in this directory.
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It does not really matter where rootfs is as long as the kernel is able
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to find it, but this particular configuration assumes the kernel is on
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partition 1 and rootfs is on partition 2 of the SD card.
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Make sure to check kernel.args if you change this.
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Making the boot media
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---------------------
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Start by configuring and building the images.
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make chromebook_snow_defconfig
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make menuconfig # if necessary
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make
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The important files are:
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uImage.kpart (kernel and device tree, signed)
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rootfs.tar
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bootsd.img (SD card image containing both kernel and rootfs)
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Write the image directly to some SD card.
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WARNING: make sure there is nothing important on that card,
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and double-check the device name!
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SD=/dev/mmcblk1 # may be /dev/sdX on some hosts
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dd if=output/images/bootsd.img of=$SD
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Switching to developer mode and booting from SD
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-----------------------------------------------
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Power Chromebook down, then power it up while holding Esc+F3.
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BEWARE: switching to developer mode deletes all user data.
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Create backups if you need them.
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While in developer mode, Chromebook will boot into a white screen saying
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"OS verification is off".
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Press Ctrl-D at this screen to boot Chromium OS from eMMC.
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Press Ctrl-U at this screen to boot from SD (or USB)
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Press Power to power it off.
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Do NOT press Space unless you mean it.
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This will switch it back to normal mode.
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The is no way to get rid of the white screen without re-flashing the bootloader.
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Troubleshooting
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---------------
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Loud *BEEP* after pressing Ctrl-U means there's no valid partition to boot from.
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Which in turn means either bad GPT or improperly signed kernel.
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Return to the OS verification screen without any sounds means the code managed
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to reboot the board. May indicate properly signed but invalid image.
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Blank screen means the image is valid and properly signed but cannot boot
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for some reason, like missing or incorrect DT.
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In case the board becomes unresponsive:
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* Press Esc+F3+Power. The board should reboot instantly.
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Remove SD card to prevent it from attempting a system recovery.
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* Hold Power button for around 10s. The board should shut down into
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its soft-off mode. Press Power button again or open the lid to turn in on.
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* If that does not work, disconnect the charger and push a hidden
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button on the underside with a pin of some sort. The board should shut
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down completely. Opening the lid and pressing Power button will not work.
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To turn it back on, connect the charger.
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Partitioning SD card manually
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-----------------------------
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Check mksd.sh for partitioning commands.
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Use parted and cgpt on a real device, and calculate the partition
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sizes properly. The kernel partition may be as small as 4MB, but
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you will probably want the rootfs to occupy the whole remaining space.
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cgpt may be used to check current layout:
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output/host/bin/cgpt show $SD
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All sizes and all offsets are in 512-byte blocks.
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Writing kernel and rootfs to a partitioned SD card
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--------------------------------------------------
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Write .kpart directly to the bootable partition:
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dd if=output/images/uImage.kpart of=${SD}1
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Make a new filesystem on the rootfs partition, and unpack rootfs.tar there:
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mkfs.ext4 ${SD}2
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mount ${SD2} /mnt/<ROOTFS-PARTITION>
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tar -xvf output/images/rootfs.tar -C /mnt/<ROOTFS-PARTITION>
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umount /mnt/<ROOTFS-PARTITION>
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This will require root permissions even if you can write to $SD.
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Kernel command line
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-------------------
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The command line is taken from board/chromebook/snow/kernel.args and stored
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in the vboot header (which also holds the signature).
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The original bootloader prepends "cros_secure console= " to the supplied
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command line. The only way to suppress this is to enable CMDLINE_FORCE
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in the kernel config, disabling external command line completely.
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That's not necessary however. The mainline kernel ignores cros_secure,
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and supplying console=tty1 in kernel.args undoes the effect of console=
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Booting with console= suppresses all kernel output.
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As a side effect, it makes /dev/console unusable, which the init in use must
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be able to handle.
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WiFi card
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---------
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Run modprobe mwifiex_sdio to load the driver.
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The name of the device should be mlan0.
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Further reading
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---------------
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https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/samsung-arm-chromebook
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http://linux-exynos.org/wiki/Samsung_Chromebook_XE303C12/Installing_Linux
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http://archlinuxarm.org/platforms/armv7/samsung/samsung-chromebook
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http://www.de7ec7ed.com/2013/05/application-processor-ap-uart-samsung.html
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http://www.de7ec7ed.com/2013/05/embedded-controller-ec-uart-samsung.html
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