Re: [GIT PULL 1/2] Kbuild updates for v4.15

[Masahiro Yamada]

Hi Linus


2017-11-18 11:01 GMT+09:00 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>:
> Oh, and I forgot to ask..
>
> On Fri, Nov 17, 2017 at 9:22 AM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>>
>> One of the most remarkable improvements in this cycle is, Kbuild is
>> now able to cache the result of shell commands.
>
> I see the "limit it to 500 lines", but I don't see any real coherency.

The limit is 1000 lines.
If your cache file exceeds 1000 lines,
it will be cut down to 500 lines at the next invocation of build.

I used two values 1000 and 500
so that the cache shrink operation is not triggered every time.


> So I take it that if you upgrade your gcc version, you may need to
> blow this cache away manually?

Right.  This is a limitation of this feature.
But, this limitation has existed since before.

When you upgrade your gcc,
you need to do "make clean" anyway to blow all *.o files
so that all objects are re-compiled by the new gcc.

Kbuild stores build commands in .*.cmd files,
but it cannot notice the compiler upgrade.



> Or is there something subtle going on that I've missed?
>
> FWIW, I still think we should probably make the compiler versions etc
> available to the configuration management rather than necessarily
> cache them.


Do you mean something like this?

https://lkml.org/lkml/2016/12/9/577



At first, I thought it was allowed to use a different compiler
for external modules than the one compiled the kernel.
But, Greg said we do not support that case.

Then, the runtime test of compiler capabilities is pointless,
so I think it is a possible solution.
CONFIG_CC_STACKPROTECTOR_AUTO will even more mess up the top Makefile.
https://patchwork.kernel.org/patch/9981173/


-- 
Best Regards
Masahiro Yamada