Linux – the difference between “select” vs “depends” in the Linux kernel Kconfig

A kernel module is a bit of compiled code that can be inserted into the kernel at run-time, such as with insmod or modprobe.

A driver is a bit of code that runs in the kernel to talk to some hardware device. It "drives" the hardware. Most every bit of hardware in your computer has an associated driver.¹ A large part of a running kernel is driver code.²

A driver may be built statically into the kernel file on disk.³ A driver may also be built as a kernel module so that it can be dynamically loaded later. (And then maybe unloaded.)

Standard practice is to build drivers as kernel modules where possible, rather than link them statically to the kernel, since that gives more flexibility. There are good reasons not to, however:

• Sometimes a given driver is absolutely necessary to help the system boot up. That doesn't happen as often as you might imagine, due to the initrd feature.

• Statically built drivers may be exactly what you want in a system that is statically scoped, such as an embedded system. That is to say, if you know in advance exactly which drivers will always be needed and that this will never change, you have a good reason not to bother with dynamic kernel modules.

• If you build your kernel statically and disable Linux's dynamic module loading feature, you prevent run-time modification of the kernel code. This provides additional security and stability at the expense of flexibility.

Not all kernel modules are drivers. For example, a relatively recent feature in the Linux kernel is that you can load a different process scheduler. Another example is that the more complex types of hardware often have multiple generic layers that sit between the low-level hardware driver and userland, such as the USB HID driver, which implements a particular element of the USB stack, independent of the underlying hardware.

Asides:

1. One exception to this broad statement is the CPU chip, which has no "driver" per se. Your computer may also contain hardware for which you have no driver.

2. The rest of the code in an OS kernel provides generic services like memory management, IPC, scheduling, etc. These services may primarily serve userland applications, as with the examples linked previously, or they may be internal services used by drivers or other intra-kernel infrastructure.

3. The one in /boot, loaded into RAM at boot time by the boot loader early in the boot process.

make oldconfig is used to apply your old .config file to the newer kernel.

For exapmle, you have .config file of your current kernel and you downloaded new kernel and want to build your new kernel. Since very likely new kernel will have some new configuration options, you will need to update your config. The easiest way to do this is to run make oldconfig, which will prompt you questions about the new configuration options. (that is the ones your current .config file doesn't have)