How does Linux determine the order of module init calls?

Question

I have a device with SPI flash storage I'd like to use an UBIFS filesystem on that flash device as my rootfs. The problem I'm facing is that the UBI module initializes before the SPI module initializes. Because of this, when UBI loads, it cannot attach to the UBI device that I've told it to (via the kernel command line), so there is no rootfs. The console output below illustrates this.

I've been diving into the source enough to see that init/main.c has a do_initcalls() function that simply calls a list of function pointers. Those function pointers point to the all the module_init() functions of the modules that are built-in to the kernel. Those function pointers are placed in a special section in the kernel binary, so this order is chosen at compile-time. However, I haven't yet figured out how that order is determined.

    [    0.482500] UBI error: ubi_init: UBI error: cannot initialize UBI, error -19
    [    0.492500] atmel_spi atmel_spi.0: Using dma0chan0 (tx) and  dma0chan1 (rx) for DMA transfers
    [    0.500000] atmel_spi atmel_spi.0: Atmel SPI Controller at 0xf0000000 (irq 13)
    [    0.507500] m25p80 spi0.1: mx25l25635e (32768 Kbytes)
    [    0.512500] Creating 7 MTD partitions on "jedec_flash":
    [    0.520000] 0x000000000000-0x000000020000 : "loader"
    [    0.527500] 0x000000020000-0x000000060000 : "u-boot"
    [    0.537500] 0x000000060000-0x000000080000 : "u-boot-env"
    [    0.547500] 0x000000080000-0x000000280000 : "kernel0"
    [    0.557500] 0x000000280000-0x000000480000 : "kernel1"
    [    0.567500] 0x000000480000-0x000001240000 : "fs"
    [    0.575000] 0x000001240000-0x000002000000 : "play"
    [    0.590000] AT91SAM9 Watchdog enabled (heartbeat=15 sec, nowayout=0)
    [    0.607500] TCP cubic registered
    [    0.615000] VFS: Cannot open root device "ubi0:root0" or unknown-block(0,0)
    [    0.622500] Please append a correct "root=" boot option; here are the available partitions:
    [    0.630000] 1f00             128 mtdblock0  (driver?)
    [    0.635000] 1f01             256 mtdblock1  (driver?)
    [    0.640000] 1f02             128 mtdblock2  (driver?)
    [    0.645000] 1f03            2048 mtdblock3  (driver?)
    [    0.650000] 1f04            2048 mtdblock4  (driver?)
    [    0.655000] 1f05           14080 mtdblock5  (driver?)
    [    0.660000] 1f06           14080 mtdblock6  (driver?)
    [    0.665000] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Answer 1

The init routines for a module that is initialized by the kernel (when they are statically linked into the kernel) are wrapped in an initcall() macro that indicates when in the startup sequence they should be run.

See the include file: include/linux/init.h for a list of the macros and their ordering.

The order specified there is:

• early_initcall
• pure_initcall
• core_initcall
• postcore_initcall
• arch_initcall
• subsys_initcall
• fs_initcall
• rootfs_initcall
• device_initcall
• late_initcall

Most of these have an "initcall_sync() phase, which is used to wait for completion of all module initialization routines within that phase. The macros are used to build a table of function pointers for each phase, which are called in sequence by do_initcalls().

If "module_init()" is used to wrap the initialization function, then by default initcall() puts the call in the "device" phase of initialization. Within that phase, the items are ordered by link order. This means that the table is created by the order of the functions as they are encountered by the linker.

You may be able to move an initialization to an earlier phase, by changing which initcall macro wraps the modules initialization function, but be careful because there are order dependencies between the various modules. Another method of changing the initialization order (within a phase) would be to adjust the link order for the module in the kernel.