linux-kernel - Re: [tip:x86/vdso] clocksource: Replace vread with generic arch data

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Date:	Thu, 21 Jul 2011 13:23:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	luto@....edu, johnstul@...ibm.com, tony.luck@...el.com,
	fenghua.yu@...el.com, tglx@...utronix.de, hpa@...ux.intel.com,
	clemens@...isch.de
CC:	linux-tip-commits@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Linux Arch Mailing List <linux-arch@...r.kernel.org>
Subject: Re: [tip:x86/vdso] clocksource: Replace vread with generic arch data

On 07/14/2011 09:24 PM, tip-bot for Andy Lutomirski wrote:
> 
> diff --git a/arch/x86/include/asm/clocksource.h b/arch/x86/include/asm/clocksource.h
> new file mode 100644
> index 0000000..a5df33f
> --- /dev/null
> +++ b/arch/x86/include/asm/clocksource.h
> @@ -0,0 +1,16 @@
> +/* x86-specific clocksource additions */
> +
> +#ifndef _ASM_X86_CLOCKSOURCE_H
> +#define _ASM_X86_CLOCKSOURCE_H
> +
> +#ifdef CONFIG_X86_64
> +
> +#define __ARCH_HAS_CLOCKSOURCE_DATA
> +
> +struct arch_clocksource_data {
> +	cycle_t (*vread)(void);
> +};
> +
> +#endif /* CONFIG_X86_64 */
> +
> +#endif /* _ASM_X86_CLOCKSOURCE_H */
> --- /dev/null
> +++ b/include/asm-generic/clocksource.h
> @@ -0,0 +1,4 @@
> +/*
> + * Architectures should override this file to add private userspace
> + * clock magic if needed.
> + */
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index d4646b4..0fb83c2 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -22,6 +22,8 @@
>  typedef u64 cycle_t;
>  struct clocksource;
>  
> +#include <asm/clocksource.h>
> +
>  /**

Hi Andy,

I should have spotted this sooner... Ingo pointed out to me that this
breaks building on any non-x86 architecture.

asm-generic doesn't work quite the way you think it does, here; it's a
library for architectures to include from, not something that gets
included on all architectures by default.

To make a file from asm-generic appear in asm/ it needs to at least
appear in a generic-y statement in a Makefile; however, that is kind of
pointless in the case of an empty file.

One could argue that it would be nice if we had such a fallback
director, of if asm-generic was such a fallback directory, but currently
it is not.

The easiest way to deal with this is probably to make
ARCH_HAS_CLOCKSOURCE_DATA here a Kconfig option (autoselected for
x86-64); the only other would be to add this as generic-y stubs for
every single architecture.

Cc: Arnd Bergmann who is the asm-generic maintainer for a
recommendation, and linux-arch.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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