linux-kernel - Re: samples/kdbus/kdbus-workers.c and cross compiling MIPS

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Message-ID: <CANq1E4TgWK-8JkUtOYfTOL9Dx=jWeVpA-h881TXSA3BNjp+MPw@mail.gmail.com>
Date:	Wed, 29 Jul 2015 18:31:02 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	David Herrmann <dh.herrmann@...glemail.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>, linux-mips@...ux-mips.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: samples/kdbus/kdbus-workers.c and cross compiling MIPS

Hi

On Wed, Jul 29, 2015 at 6:19 PM, Paul Gortmaker
<paul.gortmaker@...driver.com> wrote:
> Hi David,
>
> Does it make sense to build this sample when cross compiling?
>
> The reason I ask is that it has been breaking the linux-next build of
> allmodconfig for a while now, with:
>
>   HOSTCC  samples/kdbus/kdbus-workers
> samples/kdbus/kdbus-workers.c: In function ‘prime_new’:
> samples/kdbus/kdbus-workers.c:934:18: error: ‘__NR_memfd_create’ undeclared (first use in this function)
>   p->fd = syscall(__NR_memfd_create, "prime-area", MFD_CLOEXEC);
>                   ^
> samples/kdbus/kdbus-workers.c:934:18: note: each undeclared identifier is reported only once for each function it appears in
> scripts/Makefile.host:91: recipe for target 'samples/kdbus/kdbus-workers' failed
> make[2]: *** [samples/kdbus/kdbus-workers] Error 1

mips does have this syscall, so I assume the problem is out-of-date
kernel headers. You can fix this by running:

    $ make headers_install

This will put the sanitized headers in your local kernel tree
"./usr/". This is preferred over "/usr" as include path for the kernel
examples, hence, everything should work fine then.

The kernel samples/ directory is explicitly used for example programs
for the kernel. Hence, I think it is quite fine to use new kernel
features. Same applies to the selftests.

I'd be fine making kdbus-workers a no-op program if __NR_memfd_create
is not defined. But I'm not really sure that fixes real problems. I
mean, new samples and selftests will be added by other subsystems and
those might as well require new kernel headers.

Thanks
David
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