[LEDE-DEV] Handling the image generation process on ARC770

[Felix Fietkau]

On 2017-07-21 18:53, Alexandru Gagniuc wrote:
> Hello,
> 
> I am looking to write some rules to generate images on an SOC we're 
> trying to port over.
> 
> I need a vmlinux-initramfs ELF file with DWARF debugging information. 
> This is useful because it goes straight to gdb and can be loaded over 
> JTAG. I can write the following rule:
> 
> define Device/adaptrum-device
> 	KERNEL_SUFFIX := .elf
> 	KERNEL := kernel-bin | patch-dtb
> 	KERNEL_INITRAMFS_NAME = vmlinux-initramfs.debug
> 	KERNEL_SUFFIX := .elf
> 	DEVICE_PROFILE := adaptrum
> 	DEVICE_DTS := adaptrum_device_dts
> endef
> 
> Now, I need a uImage with initramfs that can be loaded from u-boot. This 
> is useful, since we don't yet have any working drivers for the 
> underlying storage media. At this point, I can write the following rule:
> 
> define Device/adaptrum-device
> 	KERNEL_SUFFIX := -uImage
> 	KERNEL := kernel-bin | patch-dtb | uImage none
> 	KERNEL_NAME := vmlinux
> 	KERNEL_ENTRY := $(call kernel_ep,$(KDIR)/$$(KERNEL_INITRAMFS_NAME).elf)
> 	KERNEL_INITRAMFS_NAME := vmlinux-initramfs
> 	KERNEL_INITRAMFS := kernel-bin | patch-dtb | uImage none
> 	DEVICE_DTS := adaptrum_device_fpga
> 	DEVICE_PROFILE := adaptrum
> endef
> 
> Because the two rules now need a different KERNEL_INITRAMFS_NAME, we 
> can't really put them in one rule. It seems illogical to define two 
> devices to generate different images for the same device.
> 
> But wait! there's more!! Now I also need a uImage without an initramfs. 
> Because arc decided to put .init.ramfs before the entry point, we need 
> to change the entry point too:
> 
> 	KERNEL_ENTRY := $(call kernel_ep,$(KDIR)/$$(KERNEL_NAME).elf)
> 
> That's fine, but now the initramfs-uImage no longer works. As a result, 
> if I want a kernel ELF, kernel initramfs elf, uImage, and initramfs 
> uImage, I really need three different "define Device/"
> 
> I'm assuming that's not how the buildsystem was designed. Is there any 
> sensible way of putting these rules together into a single 'define Device/'?
I think the most sensible way would be to change the kernel to support
load_addr == entry_point. This could be achieved by adding a small
assembly stub that gets linked to the beginning of the kernel image
which jumps to the real entry entry point.
Some other architectures, e.g. MIPS, have been doing this for a long
time. Is it feasible on ARC as well?

- Felix