[Buildroot] [PATCH] linux-headers: Account for LINUX_OVERRIDE_SRCDIR

[Andrey Smirnov]

On Thu, Jan 12, 2017 at 12:17 AM, Arnout Vandecappelle <arnout at mind.be> wrote:
>
>
> On 11-01-17 20:56, Andrey Smirnov wrote:
>> Use the value of LINUX_OVERRIDE_SRCDIR as override directory when
>> BR2_KERNEL_HEADERS_AS_KERNEL is specified.
>
>  Good catch! However, I'm not entirely convinced that we really want this.
>
>  A typical workflow would be to start from an upstream kernel, build a toolchain
> and userspace with it, and then begin with hacking on the kernel to e.g. add a
> driver. In that case it's rather unlikely that we really want to rebuild the
> toolchain, because the headers don't change.
>

I agree that typical "good day" workflow would be just as you
described. However, IMHO, there's also a "bad day" variant of that
workflow where "hacking on the kernel" implies "hacking on kernel
headers"(*cough* custom ioctls *cough*), which is also followed up by
a user-space tool that uses those changes and as such has
'linux-headers' as a dependency.

>  That said, another possible scenario is that you don't configure any kernel
> version but just use the OVERRIDE_SRCDIR to point to a local directory. Since we
> removed the BR2_LINUX_KERNEL_CUSTOM_LOCAL, it's the only way to support that
> scenario. In addition, the principle of least surprise would suggest that
> BR2_KERNEL_HEADERS_AS_KERNEL would really work all the time, even if
> LINUX_OVERRIDE_SRCDIR is set.
>
>  In short, I'm not really sure what is the best thing to do here. On thing though...
>
>
>> Signed-off-by: Andrey Smirnov <andrew.smirnov at gmail.com>
>> ---
>>  package/linux-headers/linux-headers.mk | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/package/linux-headers/linux-headers.mk b/package/linux-headers/linux-headers.mk
>> index 0900778..197efb4 100644
>> --- a/package/linux-headers/linux-headers.mk
>> +++ b/package/linux-headers/linux-headers.mk
>> @@ -10,6 +10,7 @@
>>  ifeq ($(BR2_KERNEL_HEADERS_AS_KERNEL),y)
>>
>>  LINUX_HEADERS_VERSION = $(call qstrip,$(BR2_LINUX_KERNEL_VERSION))
>> +LINUX_HEADERS_OVERRIDE_SRCDIR = $(LINUX_OVERRIDE_SRCDIR)
>
>  ... At least we should use ?= here so it is still possible to override each one
> separately.
>

I am more than happy to replace '=' with '?=', but I have to ask, why
would you ever want to do that? Why would you configure your build
system such that kernel headers are "Same as kernel being build" and
then set you kernel source and kernel header source to point at
different locations?

Thanks,
Andrey