WRT1900AC - Kernel image is too big

For Developers
1

Hi.
I'm trying to build my own OpenWRT firmware from latest git/master branch and "testing kernel" option enabled.
I get this error while packing the images:

mkimage -A arm -O linux -T kernel -C none -a 0x00008000 -e 0x00008000 -n 'ARM OpenWrt Linux-5.10.18'   -d /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux           -mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin.new
Image Name:   ARM OpenWrt Linux-5.10.18
Created:      Thu Mar  4 02:57:19 2021
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    3764599 Bytes = 3676.37 KiB = 3.59 MiB
Load Address: 00008000
Entry Point:  00008000
mv /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin.new /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin
WARNING: Image file /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin is too big: 3764663 > 3145728
[ -f /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linksys_wrt1900ac-v1-kernel.bin -a -f /build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/root.squashfs ]
Makefile:138: recipe for target '/build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/tmp/openwrt-snapshot-mvebu-cortexa9-linksys_wrt1900ac-v1-squashfs-sysupgrade.bin' failed
make[5]: *** [/build/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/tmp/openwrt-snapshot-mvebu-cortexa9-linksys_wrt1900ac-v1-squashfs-sysupgrade.bin] Error 1

.config file contains this:
CONFIG_TARGET_KERNEL_PARTSIZE=16
CONFIG_TARGET_ROOTFS_PARTSIZE=256

cat /proc/mtd
dev: size erasesize name
mtd0: 00100000 00020000 "u-boot"
mtd1: 00040000 00020000 "u_env"
mtd2: 00040000 00020000 "s_env"
mtd3: 00100000 00020000 "devinfo"
mtd4: 02800000 00020000 "kernel1"
mtd5: 02500000 00020000 "ubi"
mtd6: 02800000 00020000 "kernel2"
mtd7: 02500000 00020000 "rootfs2"
mtd8: 02600000 00020000 "syscfg"
mtd9: 00780000 00020000 "unused_area"
mtd10: 00008000 00008000 "spi0.0"

Why am I limited to 3MB kernel image size?

2

The kernel partition of WRT1900AC v1 overwraps with the rootfs partition, only 3MiB can be used for the kernel.

And the limitation of size for kernel is defined here:

2 Likes
3

Can I simply change that limit, or must I modify bootloader configuration too?

4

Sorry, I can't comment because I don't know where this limitation came from...
And there is no information about it in OpenWrt Wiki.

5

Might be of interest:

6 Likes
6

Yes, that patch works nice.

7

a series of builds that incorporate the (4MB) kernel size patch is maintained at
https://divested.dev/unofficial-openwrt-builds/mvebu-linksys/
with forum thread at

8

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