Re: Regression: boot failure on AMD Elan TS-5500

[Jeremy Fitzhardinge]

Sean Young wrote:
> On Mon, Jun 16, 2008 at 04:19:44PM +0000, Sean Young wrote:
>   
> > On Mon, Jun 16, 2008 at 06:37:12AM -0700, Jeremy Fitzhardinge wrote:
> >     
> > > Rusty Russell wrote:
> > >       
> > > > On Monday 16 June 2008 22:11:39 Sean Young wrote:
> > > >  
> > > >         
> > > > > The symptons are either a crash or reboot on booting the kernel. No
> > > > > printk's have occurred yet -- even with early printk on.
> > > > >
> > > > > 2.6.15 worked on this board however current does not. I've bisected it to:
> > > > >
> > > > > 	commit a24e785111a32ccb7cebafd24b1b1cb474ea8e5d
> > > > > 	Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> > > > > 	Date:   Sun Oct 21 16:41:35 2007 -0700
> > > > >
> > > > > 	    i386: paravirt boot sequence
> > > > >    
> > > > >           
> > > > Hi Sean,
> > > >
> > > >   Thanks for tracking this down.  Can we try reverting this in pieces to 
> > > >   see exactly what the cause was?
> > > >
> > > > 1) Revert arch/x86/boot/compressed/head_32.S
> > > > 2) If that doesn't fix it, Try removing the 8 lines which were added to 
> > > > arch/x86/kernel/head_32.S
> > > >  
> > > >         
> > > And the arch/x86/boot/compressed/misc_32.c change too, just in case...
> > >       
> > Reverting only head_32.S makes the problem go away. I didn't try any other
> > changes.
> >     
>
> Looking at the beginning of startup_32, it seems ds is used before it is set:
>
> startup_32:
>         cld
>         /* test KEEP_SEGMENTS flag to see if the bootloader is asking
>          * us to not reload segments */
>         testb $(1<<6), BP_loadflags(%esi)
>         jnz 1f
>
>         cli
>         movl $(__BOOT_DS),%eax
>         movl %eax,%ds
>         movl %eax,%es
>         movl %eax,%fs
>         movl %eax,%gs
>         movl %eax,%ss
> 1:
>
> Since the testb instruction is a dereference, ds is implicitly used. If
> I move the testb to after "movl %eax,%ds" it seems to work (not that it
> would make any sense there, but just to prove the point).
>
> 1) Am I barking up the wrong tree?
>
> 2) If I'm right I have no idea what the correct solution is; it seems that
>    a chicken & egg issue is introduced.
>
> Please advise. I am very new to all of this.

It's a bit odd that the boot loader neglected to set up ds properly, but 
changing the testb line to

	testb $(1<<6), %cs:BP_loadflags(%esi)

should work.  (Or perhaps a %ss: override would be better?)

I'm assuming that the GDT setup isn't completely mad and that the 
segments have the same base at least.

   J
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