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Hi Daniel,
Thanks for the hints! On 12.03.2016 00:58, Daniel Glöckner wrote:
On Sun, Feb 07, 2016 at 09:57:37PM +0100, Mateusz Kulikowski wrote:
[...]
+.global _fastboot_header +_fastboot_header:
- b _start
- add x13, x18, #0x16
- /* Image load offset from start of RAM, little-endian */
- .quad CONFIG_SYS_TEXT_BASE-PHYS_SDRAM_1
- /* Effective size of kernel image, little-endian */
- .quad 0 /* 0x60000 */
- /* Informative flags, little-endian */
- .quad 0
- .quad 0 /* reserved */
- .quad 0 /* reserved */
- .quad 0 /* reserved */
- .byte 0x41 /* Magic number, "ARM\x64" */
- .byte 0x52
- .byte 0x4d
- .byte 0x64
- .word 0 /* reserved */
I don't think fastboot is the correct term to use here. The structure in head.S is the ARM64 Linux kernel header described in section 4 of this document: https://www.kernel.org/doc/Documentation/arm64/booting.txt Fastboot is AFAIK a USB protocol spoken by bootloaders used on Android devices. Little Kernel can do fastboot, but it doesn't do it to run this image.
Good point, in that case I'll just rename it to arm64_header :)
It is also confusing to have the "add" instruction in there without an explanation, especially because having it at offset 4 instead of 0 defeats its original purpose (MZ EXE signature for EFI).
My bad, I should have put it the other way around: add x13, x18, #0x16, b _start
Will do that for v3
+6) generate qualcomm device tree, use dtbTool to generate it +$ dtbTool -o dt.img arch/arm/dts
+7) generate image with mkbootimg: +$ mkbootimg --kernel=u-boot-dtb.bin --output=u-boot.img --dt=dt.img --pagesize 2048 --base 0x80000000 --ramdisk=rd --cmdline=""
I would have liked a bit more text about what is done with the device trees here. Little Kernel refuses to run the "kernel" unless it can find a device tree matching the hardware it is running on. It adds some information to the device tree and passes it to the kernel in the x0 register. U-Boot discards the contents of the x0 register and uses the device tree appended to its image. So there is no need to point dtbTool to the same dtb file used by U-Boot. A smaller one containing only the IDs checked by Little Kernel would be enough. And dtbTool does not generate a device tree, it generates the Qualcomm device tree table containing all dtb files in the directory.
I'll clarify readme, thanks for verbose explanation. I know (almost) empty dtb is enough (I use it personally), but I didn't wanted to pollute dts directory with fake device trees.
I know the goal is to eventually replace Little Kernel, but how about using the device tree passed by it? We could add some code to head.S that saves x0 in sp_el0 and abuse CONFIG_OF_HOSTFILE to retrieve it.
I prefer not to (at least for now) - It will not give us significant benefits (image will be a bit smaller and boot slightly faster, but it's not an issue as U-Boot replaces huge kernel image) and will just confuse people that already are using it. Is it fine with you?
Regards, Mateusz