Date:	Wed, 28 Oct 2009 14:58:22 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Claudio Scordino <claudio@...dence.eu.com>
Cc:	linux-kernel@...r.kernel.org, rmk+kernel@....linux.org.uk,
	akpm@...ux-foundation.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH][RE-SUBMIT] Default setting of the ARM_UNWIND option

On Wed, 2009-10-28 at 12:37 +0100, Claudio Scordino wrote:
> Catalin Marinas ha scritto:
> > On Mon, 2009-10-26 at 09:27 +0100, Claudio Scordino wrote:
> >   
> >> My ARM board hanged at the initial "Calibrating delay loop" message.
> >>
> >> After some inspection, I found out the problem to be with commit
> >> adf8b37bafc1495393201a2ae4235846371870d0. This commit introduces stack
> >> unwinding for ARM, and set it enabled by default. However, it seems to
> >> not work with buggy or not-EABI compilers.
> >>
> >> My suggestion is to keep the feature (which is fine) but change the
> >> default setting of the option (see the attached patch).
> >
> > The option still depends on EXPERIMENTAL, so you get ARM_UNWIND on when
> > enabling that.
> >
> > I'm more in favour of a #warning on #error in the unwind.c file based on
> > the compiler version rather than not having it on by default. The reason
> > is that people reported performance improvements when compiling the
> > kernel without frame pointers.
> 
> [Sorry, I sent the wrong patch... This is the new one.]

and the wrong ARM kernel list (I changed the Cc line).

> This solution is fine too (even if I still think that changing the
> default setting is better).
> 
> Please, consider the patch in attachment: is it like you would have it ?

You can just send it to Russell's patch system once there are no more
comments on it.

See below for my comments.

> BTW, do we have any list of buggy or not-EABI versions of the gcc
> compiler ?

No, but for non-EABI compilers I think the __ARM_EABI__ check is enough.

> From: Claudio Scordino <claudio@...dence.eu.com>
> Date: Mon, 26 Oct 2009 17:13:18 +0100
> Subject: [PATCH 1/1] Some warnings when compiling ARM unwind support with a buggy or not-EABI compiler.
> 
> ARM unwind support is known to build only with EABI and not-buggy compilers.
> Now we check the compiler and raise a #warning in case of wrong compiler.

Maybe you can clarify a bit that the problem is not unwinding
information but the -fno-frame-pointer option added as a result of 
!CONFIG_FRAME_POINTER.

> Signed-off-by: Claudio Scordino <claudio@...dence.eu.com>
> ---
>  arch/arm/kernel/unwind.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 39baf11..47345c1 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -26,6 +26,15 @@
>   * http://infocenter.arm.com/help/topic/com.arm.doc.subset.swdev.abi/index.html
>   */
>  
> +#if !defined (__ARM_EABI__)
> +#warning Your compiler does not have EABI support.
> +#warning    ARM unwind support it is known to compile only on EABI compilers.

Maybe "ARM unwind is known ... only with ..." (though English is not my
first language).

> +#warning    Change compiler or disable ARM_UNWIND option.
> +#elif (__GNUC__ == 4 && __GNUC_MINOR__ == 2)

Could we assume this for (__GNUC_MINOR__ <= 2)?

> +#warning Your compiler is too buggy; it is known to not compile ARM unwind support.
> +#warning    Change compiler or disable ARM_UNWIND option.
> +#endif
> +
>  #include <linux/kernel.h>
>  #include <linux/init.h>
>  #include <linux/module.h>

-- 
Catalin

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