linux-kernel - Re: frv build failure in mainline kernel

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Message-ID: <20170516182207.GA10837@roeck-us.net>
Date:   Tue, 16 May 2017 11:22:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     David Howells <dhowells@...hat.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: frv build failure in mainline kernel

On Tue, May 16, 2017 at 10:58:20AM -0700, Matthias Kaehlcke wrote:
> El Tue, May 16, 2017 at 10:21:30AM -0700 Guenter Roeck ha dit:
> 
> > On Tue, May 16, 2017 at 05:04:32PM +0100, David Howells wrote:
> > > Guenter Roeck <linux@...ck-us.net> wrote:
> > > 
> > > > Turns out not here because ____cacheline_aligned_in_smp includes
> > > > __page_aligned_data which also declares the section, at least on x86.
> > > 
> > > If there's any sort of section specification, that should suffice, I think.
> > > The problem might come that jiffies and jiffies_64 don't coincide at the same
> > > address because FRV is BE not LE.
> > > 
> > > It ought to be possible to make jiffies 64-bit on FRV since it has double-word
> > > instructions that can load/store aligned 64-bit values atomically to/from a
> > > register pair.  That might require some compiler magic, though.  I'll have to
> > > try and work out if that's possible.
> > > 
> > I tried:
> > 
> > diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> > index 36872fbb815d..26d0655c4422 100644
> > --- a/include/linux/jiffies.h
> > +++ b/include/linux/jiffies.h
> > @@ -64,13 +64,15 @@ extern int register_refined_jiffies(long clock_tick_rate);
> >  /* TICK_USEC is the time between ticks in usec assuming fake USER_HZ */
> >  #define TICK_USEC ((1000000UL + USER_HZ/2) / USER_HZ)
> >  
> > +#define __jiffy_data  __attribute__((section(".data")))
> > +
> >  /*
> >   * The 64-bit value is not atomic - you MUST NOT read it
> >   * without sampling the sequence number in jiffies_lock.
> >   * get_jiffies_64() will do this for you as appropriate.
> >   */
> > -extern u64 __cacheline_aligned_in_smp jiffies_64;
> > -extern unsigned long volatile __cacheline_aligned_in_smp jiffies;
> > +extern u64 __cacheline_aligned_in_smp __jiffy_data jiffies_64;
> > +extern unsigned long volatile __cacheline_aligned_in_smp __jiffy_data jiffies;
> > 
> > This is what I get when building defconfig:
> > 
> >   CC      arch/x86/kernel/asm-offsets.s
> > In file included from ./include/linux/ktime.h:25:0,
> >                  from ./include/linux/rcupdate.h:46,
> >                  from ./include/linux/srcu.h:33,
> >                  from ./include/linux/notifier.h:15,
> >                  from ./include/linux/memory_hotplug.h:6,
> >                  from ./include/linux/mmzone.h:757,
> >                  from ./include/linux/gfp.h:5,
> >                  from ./include/linux/slab.h:14,
> >                  from ./include/linux/crypto.h:24,
> >                  from arch/x86/kernel/asm-offsets.c:8:
> > ./include/linux/jiffies.h:74:52: error: section of ‘jiffies_64’ conflicts with previous declaration
> >  extern u64 __cacheline_aligned_in_smp __jiffy_data jiffies_64;
> >                                                     ^
> > ./include/linux/jiffies.h:75:71: error: section of ‘jiffies’ conflicts with previous declaration
> >  extern unsigned long volatile __cacheline_aligned_in_smp __jiffy_data jiffies;
> >                                                                        ^
> > Kbuild:56: recipe for target 'arch/x86/kernel/asm-offsets.s' failed
> > make[1]: *** [arch/x86/kernel/asm-offsets.s] Error 1
> > Makefile:1061: recipe for target 'prepare0' failed
> > make: *** [prepare0] Error 2
> 
> Thanks for giving it a try!
> 
> What would work is this:
> 
> diff --git a/arch/frv/include/asm/cache.h b/arch/frv/include/asm/cache.h
> index 2797163b8f4f..25bf4e85f582 100644
> --- a/arch/frv/include/asm/cache.h
> +++ b/arch/frv/include/asm/cache.h
> @@ -20,4 +20,6 @@
>  #define __cacheline_aligned    __attribute__((aligned(L1_CACHE_BYTES)))
>  #define ____cacheline_aligned  __attribute__((aligned(L1_CACHE_BYTES)))
>  
> +#define __jiffies_arch_xyz     __attribute__((__section__(".data")))
> +
>  #endif
> diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> index 36872fbb815d..8904eb52928d 100644
> --- a/include/linux/jiffies.h
> +++ b/include/linux/jiffies.h
> @@ -70,7 +70,7 @@ extern int register_refined_jiffies(long clock_tick_rate);
>   * get_jiffies_64() will do this for you as appropriate.
>   */
>  extern u64 __cacheline_aligned_in_smp jiffies_64;
> -extern unsigned long volatile __cacheline_aligned_in_smp jiffies;
> +extern unsigned long volatile __cacheline_aligned_in_smp __jiffies_arch_xyz jiffies;
>  
>  #if (BITS_PER_LONG < 64)
>  u64 get_jiffies_64(void);
> 
> 
> We'd only have to find a halfway decent name for __jiffies_arch_xyz.

>From __jiffy_data: __jiffy_arch_data or __jiffies_arch_data.

You'd also need

#ifndef __jiffy_arch_data
#define __jiffy_arch_data
#endif

That assumes that __cacheline_aligned_in_smp is not defined to be part of 
a different section for the architecture, which I guess should be acceptable
here (but maybe asks for a respective comment).

Guenter