151 lines
6.5 KiB
Plaintext
151 lines
6.5 KiB
Plaintext
Falcon boot option
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------------------
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Falcon boot is a short cut boot method for SD/eMMC targets. It skips loading the
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RAM version U-Boot. Instead, it loads FIT image and boot directly to Linux.
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CONFIG_SPL_OS_BOOT enables falcon boot. CONFIG_SPL_LOAD_FIT enables the FIT
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image support (also need CONFIG_SPL_OF_LIBFDT, CONFIG_SPL_FIT and optionally
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CONFIG_SPL_GZIP).
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To enable falcon boot, a hook function spl_start_uboot() returns 0 to indicate
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booting U-Boot is not the first choice. The kernel FIT image needs to be put
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at CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR. SPL mmc driver reads the header to
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determine if this is a FIT image. If true, FIT image components are parsed and
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copied or decompressed (if applicable) to their destinations. If FIT image is
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not found, normal U-Boot flow will follow.
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An important part of falcon boot is to prepare the device tree. A normal U-Boot
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does FDT fixups when booting Linux. For falcon boot, Linux boots directly from
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SPL, skipping the normal U-Boot. The device tree has to be prepared in advance.
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A command "spl export" should be called under the normal RAM version U-Boot.
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It is equivalent to go through "bootm" step-by-step until device tree fixup is
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done. The device tree in memory is the one needed for falcon boot. Falcon boot
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flow suggests to save this image to SD/eMMC at the location pointed by macro
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CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTOR, with maximum size specified by macro
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CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTORS. However, when FIT image is used for
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Linux, the device tree stored in FIT image overwrites the memory loaded by spl
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driver from these sectors. We could change this loading order to favor the
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stored sectors. But when secure boot is enabled, these sectors are used for
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signature header and needs to be loaded before the FIT image. So it is important
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to understand the device tree in FIT image should be the one actually used, or
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leave it absent to favor the stored sectors. It is easier to deploy the FIT
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image with embedded static device tree to multiple boards.
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Macro CONFIG_SYS_SPL_ARGS_ADDR serves two purposes. One is the pointer to load
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the stored sectors to. Normally this is the static device tree. The second
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purpose is the memory location of signature header for secure boot. After the
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FIT image is loaded into memory, it is validated against the signature header
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before individual components are extracted (and optionally decompressed) into
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their final memory locations, respectively. After the validation, the header
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is no longer used. The static device tree is copied into this location. So
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this macro is passed as the location of device tree when booting Linux.
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Steps to prepare static device tree
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-----------------------------------
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To prepare the static device tree for Layerscape boards, it is important to
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understand the fixups in U-Boot. Memory size and location, as well as reserved
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memory blocks are added/updated. Ethernet MAC addressed are updated. FMan
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microcode (if used) is embedded in the device tree. Kernel command line and
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initrd information are embedded. Others including CPU status, boot method,
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Ethernet port status, etc. are also updated.
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Following normal booting process, all variables are set, all images are loaded
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before "bootm" command would be issued to boot, run command
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spl export fdt <address>
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where the address is the location of FIT image. U-Boot goes through the booting
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process as if "bootm start", "bootm loados", "bootm ramdisk"... commands but
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stops before "bootm go". There we have the fixed-up device tree in memory.
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We can check the device tree header by these commands
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fdt addr <fdt address>
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fdt header
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Where the fdt address is the device tree in memory. It is printed by U-Boot.
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It is useful to know the exact size. One way to extract this static device
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tree is to save it to eMMC/SD using command in U-Boot, and extract under Linux
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with these commands, repectively
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mmc write <address> <sector> <sectors>
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dd if=/dev/mmcblk0 of=<filename> bs=512 skip=<sector> count=<sectors>
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Note, U-Boot takes values as hexadecimals while Linux takes them as decimals by
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default. If using NAND or other storage, the commands are slightly different.
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When we have the static device tree image, we can re-make the FIT image with
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it. It is important to specify the load addresses in FIT image for every
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components. Otherwise U-Boot cannot load them correctly.
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Generate FIT image with static device tree
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------------------------------------------
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Example:
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/dts-v1/;
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/ {
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description = "Image file for the LS1043A Linux Kernel";
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#address-cells = <1>;
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images {
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kernel {
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description = "ARM64 Linux kernel";
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data = /incbin/("./arch/arm64/boot/Image.gz");
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type = "kernel";
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arch = "arm64";
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os = "linux";
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compression = "gzip";
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load = <0x80080000>;
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entry = <0x80080000>;
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};
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fdt-1 {
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description = "Flattened Device Tree blob";
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data = /incbin/("./fsl-ls1043ardb-static.dtb");
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type = "flat_dt";
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arch = "arm64";
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compression = "none";
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load = <0x90000000>;
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};
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ramdisk {
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description = "LS1043 Ramdisk";
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data = /incbin/("./rootfs.cpio.gz");
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type = "ramdisk";
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arch = "arm64";
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os = "linux";
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compression = "none";
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load = <0xa0000000>;
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};
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};
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configurations {
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default = "config-1";
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config-1 {
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description = "Boot Linux kernel";
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kernel = "kernel";
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fdt = "fdt-1";
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ramdisk = "ramdisk";
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loadables = "fdt", "ramdisk";
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};
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};
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};
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The "loadables" is not optional. It tells SPL which images to load into memory.
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Falcon mode with QSPI boot
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--------------------------
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To use falcon mode with QSPI boot, SPL needs to be enabled. Similar to SD or
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NAND boot, a RAM version full feature U-Boot is needed. Unlike SD or NAND boot,
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SPL with QSPI doesn't need to combine SPL image with RAM version image. Two
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separated images are used, u-boot-spl.pbl and u-boot.img. The former is SPL
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image with RCW and PBI commands to load the SPL payload into On-Chip RAM. The
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latter is RAM version U-Boot in FIT format (or legacy format if FIT is not
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used).
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Other things to consider
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-----------------------
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Falcon boot skips a lot of initialization in U-Boot. If Linux expects the
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hardware to be initialized by U-Boot, the related code should be ported to SPL
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build. For example, if Linux expect Ethernet PHY to be initialized in U-Boot
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(which is not a common case), the PHY initialization has to be included in
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falcon boot. This increases the SPL image size and should be handled carefully.
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If Linux has PHY driver enabled, it still depends on the correct MDIO bus setup
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in U-Boot. Normal U-Boot sets the MDC ratio to generate a proper clock signal.
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