The way to use Kconfig in U-Boot

[Bin Meng]

Hi,

Kconfig is a flexible language and there are different ways to set a
value for a specific platform.

We can either:

- Use Kconfig overriding functionality
- Use Kconfig conditional set syntax like "default xxx if FOO"

Based on current Kconfig files hierarchy, in the root directory we
have the following come at the very beginning:

# Allow defaults in arch-specific code to override any given here
source "arch/Kconfig"

Based on this I thought our original design was to use the overriding.

But it seems not everyone is consistent on doing such. For example, we
have a bunch of unmaintainable (IMO) Kconfig options like this:

config SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR
hex "Address on the MMC to load U-Boot from"
depends on SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR
default 0x50 if ARCH_SUNXI
default 0x75 if ARCH_DAVINCI
default 0x8a if ARCH_MX6 || ARCH_MX7
default 0x100 if ARCH_UNIPHIER
default 0x140 if ARCH_MVEBU
default 0x200 if ARCH_SOCFPGA || ARCH_AT91
default 0x300 if ARCH_ZYNQ || ARCH_KEYSTONE || OMAP34XX || OMAP44XX || \
         OMAP54XX || AM33XX || AM43XX || ARCH_K3
default 0x4000 if ARCH_ROCKCHIP
help
  Address on the MMC to load U-Boot from, when the MMC is being used
  in raw mode. Units: MMC sectors (1 sector = 512 bytes).

The "default xxx if FOO" list is crazy!

I think we need to discuss and come up with a unified way of doing this.

I personally am in favor of the overriding mechanism, which is how
current x86 architecture Kconfig is organized. In the x86 arch
Kconfig, we have:

# board-specific options below
source "board/advantech/Kconfig"
...

# platform-specific options below
source "arch/x86/cpu/apollolake/Kconfig"
...

# architecture-specific options below

So that board behavior overrides platform/SoC behavior over
architecture behavior, and over the U-Boot common Kconfig options.
This to me is very clear.

Regards,
Bin