[Suggestion] PowerPC: kernel: cross compiling issue with allmodconfig

[Chen Gang]

Hello All:

summary:
  the root cause is no enough room in exception area (0x5500 -- 0x7000).

  it is caused by the patches "for saving/restre PPR":
    they consumed much space of this area (0x5500 -- 0x7000).
    for pseries_defconfig and ppc64_defconfig, it is still ok.
    but for allmodconfig and "some additional config", it will cause issue.

  the solving patch "Make room in exception vector area" can make room larger.
    it can let "some additional config" ok.
    but for allmodconfig, it is still not enough.


details
  reason:
    it is caused by:
       commit number: 13e7a8e846c2ea38a552b986ea49332f965bbb7a
       commit number: 44e9309f1f357794b7ae93d5f3e3e6f11d2b8a7f
    they are "for saving/restore PPR"
    by Haren Myneni <haren at linux.vnet.ibm.com> Thu, 6 Dec 2012
    compiling result:
      pseries_defconfig: pass   (cpu for POWER7)
      ppc64_defconfig:   pass   (cpu for POWER7)
      allmodconfig:      failed (cpu for POWER7)

  analysing:
    solving patch:
      ------------------------------------------------------------------
      commit number: 61383407677aef05928541a00678591abea2d84c
      Author: Benjamin Herrenschmidt <benh at kernel.crashing.org>
      Date:   Thu Jan 10 17:44:19 2013 +1100

        powerpc: Make room in exception vector area
    
        The FWNMI region is fixed at 0x7000 and the vector are now
        overflowing that with some configurations. Fix that by moving
        some hash management code out of that region as it doesn't need
        to be that close to the call sites (isn't accessed using
        conditional branches).
      ------------------------------------------------------------------

      but for allmodconfig (not only for "some configurations"):
        it really can reduce much overflow bytes,
          (maybe from hundreds bytes to dozens bytes)
        but still not enough (still content overflow bytes)

    additional trying:
	after del CONFIG_VSX and CONFIG_PPC_970_NAP in allmodconfig,
          (will reduce dozens bytes in the region .0x5500 -- .0x7000)
        it can pass compiling (not overflow).


next:
  I am sorry:
      I am not quite familiar with the detail features of powerpc.
      it seems I am not the suitable member to continue trying.

  I prefer Benjamin to continue trying (just like what he has done).

  if Benjamin will not do it (e.g. maybe no time to do)
    I should continue: "make additional room in exception vector area".
      (if get no reply within a week: before 2013-03-28, I should continue)



  welcome any members' (especially Benjamin) suggestions or completions.

  thanks.

  :-)


On 2013年03月15日 13:14, Chen Gang wrote:
> 于 2013年03月15日 12:52, Michael Neuling 写道:
>> Yep it's a known problem but no one has bothered to fix it since it
>> doesn't happen in a config that anyone cares about like
>> pseries_defconfig and ppc64_defconfig.  We've been moving code around in
>> this area a lot recently hence the breakage.
>>
>> It should be fixed though.  Patches welcome. :-)
> 
>   thanks, and I should try, and very glad to try.
> 
>   :-)  :-)
> 
>   excuse me, I try to provide related patch within this month (2013-03-31), is it ok ?
>   the reason is:
>     I am not familiar with ppc assembly code, neither ppc kernel,
>     so need additional time resource.
>       (originally, I worked for x86(_64) core dump analysing for kernel and user programs)
> 
>   thanks.
> 


-- 
Chen Gang

Asianux Corporation