What is the Linux built-in driver load order?

Question

我们如何定制内置驱动程序加载顺序（首先加载一些内置驱动程序模块，然后加载依赖模块）？

Answer 1

Built-in drivers wont be loaded , hence built-in. Their initialization functions are called and the drivers are activated when kernel sets up itself. These init functions are called in init/main.c::do_initcalls() . All init calls are classified in levels, which are defined in initcall_levels and include/linux/init.h

These levels are actuall symbols defined in linker script ( arch/*/kernel/vmlinux.lds.* ). At kernel compile time, the linker collects all function marked module_init() or other *_initcall() , classify in levels, put all functions in the same level together in the same place, and create like an array of function pointers.

What do_initcall_level() does in the run-time is to call each function pointed by the pointers in the array. There is no calling policy, except levels, in do_initcall_level, but the order in the array is decided in the link time.

So, now you can see that the driver's initiation order is fixed at the link time, but what can you do?

1. put your init function in the higher level, or
2. put your device driver at the higher position in Makefile

The first one is clear if you've read the above. ie) use early_initcall() instead if it is appropriate.

The second one needs a bit more explanation. The reason why the order in a Makefile matter is how the current kernel build system works and how the linkers works. To make a long story short, the build system takes all object files in obj-y and link them together. It is highly environment dependent but there is high probability that the linker place first object file in the obj-y in lower address, thus, called earlier.

If you just want your driver to be called earlier than other drivers in the same directory, this is simplest way to do it.

Answer 2

depmod examines the symbols exported and required by each module and does a topological sort on them that modprobe can later use to load modules in the proper order. Requiring the symbols from modules you wish to be dependent on is enough for it to do the right thing.

Answer 3

正确的模块顺序和依赖关系由modprobe处理，即使在initrd中也是如此 。

Answer 4

Recently i got this problem my charger driver is having dependency on ADC driver so before loading ADC driver charger driver has loaded and checking for adc phandle which is defined in DTS file and has to intialize by ADC driver. its got resolved by changing the order of the module in drivers/Makefile