RE: [RFC PATCH] ARM: Debug kernel copy by printing

[Nicolas Pitre]

On Fri, 13 Jul 2018, Fabrizio Castro wrote:

> Dear All,
> 
> Has anybody had the chance to look into this? Does it make sense? Any feedback at all?
> Thanks!

Looks fine to me.



> 
> Fab
> 
> > Subject: [RFC PATCH] ARM: Debug kernel copy by printing
> >
> > It may happen that when we relocate the kernel we corrupt other
> > sensible memory (e.g. the memory needed by U-Boot for dealing
> > with bootm command) while copying the kernel. If we overwrite
> > the content of the memory area used by U-Boot's command bootm
> > (described by U-Boot's parameters bootm_low and bootm_size),
> > the kernel won't be able to boot. Troubleshooting the problem
> > then is not straightforward.
> >
> > This commit allows the user to easily print information on
> > where the kernel gets copied from/to in order to help with the
> > design of the system memory map (e.g. bootm_low and bootm_size)
> > at boot up.
> >
> > Signed-off-by: Fabrizio Castro <fabrizio.castro@xxxxxxxxxxxxxx>
> > Reviewed-by: Chris Paterson <Chris.Paterson2@xxxxxxxxxxx>
> > Acked-by: Biju Das <biju.das@xxxxxxxxxxxxxx>
> > ---
> > Dear All,
> >
> > shmobile_defconfig doesn't use kernel modules, everything gets
> > built-in. iwg20d and iwg22d platforms from iWave use uImage
> > to boot, DRAM starts at address 0x40000000, the kernel gets
> > loaded up in memory at address 0x40007fc0, bootm_low is
> > 0x40e00000, and bootm_size is 0x100000.
> >
> > The kernel is getting larger and larger, so much so that during
> > the relocation the kernel is copying itself right where the
> > bootm memory area lives, preventing Linux from booting.
> > Here is what this patch prints when applied on top of tag
> > next-20180625 and running on the iwg22d:
> >
> > C:0x400080C0-0x404922A0->0x40E90800-0x4131A9E0
> >
> > The designer then has to pick up a suitable memory range for
> > bootm memory area to fix this, but the only way to successfully
> > achieve this is by knowing where the kernel is going to copy
> > itself in memory, so that he can stay clear of it.
> >
> > Other platforms that use the same defconfig suffer from the same
> > issue (e.g. Koelsch et al.) as they have been designed some time
> > ago or the original BSP was based on an LTS kernel.
> >
> > Debugging this basically requires a JTAG debugger at this stage.
> >
> > Do you think this patch could be considered acceptable? If not,
> > what would be the best way to get useful/sensible/debug
> > information out of the kernel when the problem occours?
> >
> > Comments welcome!
> >
> > Thanks,
> > Fab
> >
> >  arch/arm/boot/compressed/head.S | 43 +++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 43 insertions(+)
> >
> > diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> > index 517e0e1..6c7ccb4 100644
> > --- a/arch/arm/boot/compressed/head.S
> > +++ b/arch/arm/boot/compressed/head.S
> > @@ -114,6 +114,35 @@
> >  #endif
> >  .endm
> >
> > +/*
> > + * Debug kernel copy by printing the memory addresses involved
> > + */
> > +.macro dbgkc, begin, end, cbegin, cend
> > +#ifdef DEBUG
> > +kputc   #'\n'
> > +kputc   #'C'
> > +kputc   #':'
> > +kputc   #'0'
> > +kputc   #'x'
> > +kphex   \begin, 8/* Start of compressed kernel */
> > +kputc#'-'
> > +kputc#'0'
> > +kputc#'x'
> > +kphex\end, 8/* End of compressed kernel */
> > +kputc#'-'
> > +kputc#'>'
> > +kputc   #'0'
> > +kputc   #'x'
> > +kphex   \cbegin, 8/* Start of kernel copy */
> > +kputc#'-'
> > +kputc#'0'
> > +kputc#'x'
> > +kphex\cend, 8/* End of kernel copy */
> > +kputc#'\n'
> > +kputc#'\r'
> > +#endif
> > +.endm
> > +
> >  .section ".start", #alloc, #execinstr
> >  /*
> >   * sort out different calling conventions
> > @@ -450,6 +479,20 @@ dtb_check_done:
> >  addr6, r9, r5
> >  addr9, r9, r10
> >
> > +#ifdef DEBUG
> > +sub     r10, r6, r5
> > +sub     r10, r9, r10
> > +/*
> > + * We are about to copy the kernel to a new memory area.
> > + * The boundaries of the new memory area can be found in
> > + * r10 and r9, whilst r5 and r6 contain the boundaries
> > + * of the memory we are going to copy.
> > + * Calling dbgkc will help with the printing of this
> > + * information.
> > + */
> > +dbgkcr5, r6, r10, r9
> > +#endif
> > +
> >  1:ldmdbr6!, {r0 - r3, r10 - r12, lr}
> >  cmpr6, r5
> >  stmdbr9!, {r0 - r3, r10 - r12, lr}
> > --
> > 2.7.4
> 
> 
> 
> 
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