2015-02-19 23:22 GMT+09:00 Simon Glass sjg@chromium.org:
Hi Masahiro,
On 19 February 2015 at 00:47, Masahiro Yamada yamada.m@jp.panasonic.com wrote:
Since commit b724bd7d6349 (dm: Kconfig: Move CONFIG_SYS_MALLOC_F_LEN to Kconfig), the ".config" created by the configuration has been wrong.
For example, the following is a snippet of the ".config" generated by "make beaver_defconfig":
--------------->8----------------- CONFIG_CC_OPTIMIZE_FOR_SIZE=y # CONFIG_SYS_MALLOC_F is not set CONFIG_SYS_MALLOC_F_LEN=0x1800 # CONFIG_EXPERT is not set ---------------8<-----------------
CONFIG_SYS_MALLOC_F_LEN is supposed to depend on CONFIG_SYS_MALLOC_F (see the top level Kconfig), but the ".config" above is not actually following that dependency.
This is caused by two mistakes of commit b724bd7d6349.
[1] Wrong default value of CONFIG_SYS_MALLOC_F CONFIG_SYS_MALLOC_F is a boolean option, but its default value is set to 0x400.
I sent a patch for this.
OK. But this change alone could still generate a strange .config file.
[2] Missing "if SYS_MALLOC_F" in the default setting in each Kconfig For example, arch/arm/cpu/armv7/tegra-common/Kconfig has the line "default 0x1800" for SYS_MALLOC_F_LEN. It must be described as "default 0x1800 if SYS_MALLOC_F" to follow the dependency.
Does this actually matter? What does it break?
I left a comment in your patch: http://patchwork.ozlabs.org/patch/441669/
Those two bugs together create such a broken ".config".
Unfortunately, even if we correct both [1] and [2], the value of CONFIG_SYS_MALLOC_F_LEN is not set as we expect. The "default 0x1800 if SYS_MALLOC_F" would be simply ignored because the "default 0x400" in the top level Kconfig is parsed first.
Notice that if multiple default lines appear for the same CONFIG, the first one takes precedence.
I sent a patch for this also as I don't really think the current ordering is useful.
I think this is a good idea.
So, this commit correct [1] and [2], also leaves some comments in arch/arm/cpu/armv7/tegra-common/Kconfig and arch/x86/Kconfig to notify not-working default values.
If you want to change the default value of CONFIG_SYS_MALLOC_F_LEN, the easiest way would be to specify it in each *_defconfig.
It is true that describing SoC-common default values in each Kconfig seems handy, but it often introduces nasty problems. If you do not understand well how Kconfig works, as you see above, you could easily create a broken .config file.
It is actually quite painful to not support this. If you add a new board you need to work out what the settings should be for that board. As we add more and more settings this is going to get even harder. It seems much better for Tegra (for example) to be able to specify sensible defaults that will work on most boards.
Otherwise we have no place to record that (for example) Tegra124 needs this feature on, or this value set. Before Kconfig we have tegra-common.h and tegra124-common.h. We discussed this problem before and I thought it was agreed that defaults in Kconfig were the best way forward.
If we can't use Kconfig then I think we will need to figure out something else - perhaps includes in the defconfig files as previously discussed. But that would be a new kconfig feature so I don't think anyone is thrilled with the idea.
I would not go as far as say it must be banned.
Please be careful when you add default values in Kconfig.