Initialize DBCR0 for PPC440 targets

Initialize DBCR0 for PPC440 targets

[srikanth krishnakar]

Hi,

kernel- 2.6.29
Debug technique: KGDB

The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
prevents the use of software breakpoints in case of internal debug
mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
used by boot-loaders.
It seems head_44x.S lacks this step of DBCR0 register initialization.
So fixing this with initializing the DBCR0 register as shown below :

Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
bootloader

The kernel does not initialize the PPC440 DBCR0 register.
This prevents (among other things) the use of software
breakpoints with GDB. The boot loaders probably do initialize
this but few targets run without a boot loader
---
 arch/powerpc/kernel/head_44x.S |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/head_44x.S b/arch/powerpc/kernel/head_44x.S
index a4a890a..b413bc4 100644
--- a/arch/powerpc/kernel/head_44x.S
+++ b/arch/powerpc/kernel/head_44x.S
@@ -240,6 +240,18 @@ skpinv:	addi	r4,r4,1				/* Increment */
 	lis	r4,interrupt_base@h	/* IVPR only uses the high 16-bits */
 	mtspr	SPRN_IVPR,r4

+#if !defined(CONFIG_BDI_SWITCH)
+        /*
+         * The Abatron BDI JTAG debugger does not tolerate others
+         * mucking with the debug registers.
+         */
+        lis     r2,DBCR0_IDM@h
+        mtspr   SPRN_DBCR0,r2
+        isync
+        /* clear any residual debug events */
+        li      r2,-1
+        mtspr   SPRN_DBSR,r2
+#endif
 	/*
 	 * This is where the main kernel code starts.
 	 */

Any suggestions or comments on this ?

Thanks,
-Srikant

Re: Initialize DBCR0 for PPC440 targets

[David Gibson]

On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
> Hi,
> 
> kernel- 2.6.29
> Debug technique: KGDB
> 
> The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
> prevents the use of software breakpoints in case of internal debug
> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
> used by boot-loaders.
> It seems head_44x.S lacks this step of DBCR0 register initialization.
> So fixing this with initializing the DBCR0 register as shown below :
> 
> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
> bootloader
> 
> The kernel does not initialize the PPC440 DBCR0 register.
> This prevents (among other things) the use of software
> breakpoints with GDB. The boot loaders probably do initialize
> this but few targets run without a boot loader

Um.. how does this prevent the use of software breakpoints with gdb?
The trap instructions still work with IDM==0.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

Re: Initialize DBCR0 for PPC440 targets

[srikanth krishnakar]

Hi David,

I am not sure how the IDM behaves on few of PPC440 targets which don't
have boot loaders. I have a reference for your question:

http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td2304904=
4.html

Without this fix (given patch) I am facing problems with GDB, and
further target hangs while running gdbserver !

Thanks,
Srikanth

On Wed, May 20, 2009 at 5:23 AM, David Gibson
<david@gibson.dropbear.id.au> wrote:
> On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
>> Hi,
>>
>> kernel- 2.6.29
>> Debug technique: KGDB
>>
>> The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
>> prevents the use of software breakpoints in case of internal debug
>> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
>> used by boot-loaders.
>> It seems head_44x.S lacks this step of DBCR0 register initialization.
>> So fixing this with initializing the DBCR0 register as shown below :
>>
>> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
>> bootloader
>>
>> The kernel does not initialize the PPC440 DBCR0 register.
>> This prevents (among other things) the use of software
>> breakpoints with GDB. The boot loaders probably do initialize
>> this but few targets run without a boot loader
>
> Um.. how does this prevent the use of software breakpoints with gdb?
> The trap instructions still work with IDM=3D=3D0.
>
> --
> David Gibson =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| I'll have my music =
baroque, and my code
> david AT gibson.dropbear.id.au =A0| minimalist, thank you. =A0NOT _the_ _=
other_
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| _way_ _a=
round_!
> http://www.ozlabs.org/~dgibson
>



--=20
"The Good You Do, The Best You GET"

Regards
Srikanth Krishnakar
**********************

Re: Initialize DBCR0 for PPC440 targets

[David Gibson]

On Wed, May 20, 2009 at 11:20:46AM +0530, srikanth krishnakar wrote:
> Hi David,
> 
> I am not sure how the IDM behaves on few of PPC440 targets which don't
> have boot loaders. I have a reference for your question:
> 
> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049044.html
> 
> Without this fix (given patch) I am facing problems with GDB, and
> further target hangs while running gdbserver !

That doesn't answer my question.  It's not enough to say "this fixes a
problem" you need to explain *how* it fixes the problem.

And I don't see why IDM would have any effect on *software*
breakpoints.

> On Wed, May 20, 2009 at 5:23 AM, David Gibson
> <david@gibson.dropbear.id.au> wrote:
> > On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
> >> Hi,
> >>
> >> kernel- 2.6.29
> >> Debug technique: KGDB
> >>
> >> The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
> >> prevents the use of software breakpoints in case of internal debug
> >> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
> >> used by boot-loaders.
> >> It seems head_44x.S lacks this step of DBCR0 register initialization.
> >> So fixing this with initializing the DBCR0 register as shown below :
> >>
> >> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
> >> bootloader
> >>
> >> The kernel does not initialize the PPC440 DBCR0 register.
> >> This prevents (among other things) the use of software
> >> breakpoints with GDB. The boot loaders probably do initialize
> >> this but few targets run without a boot loader
> >
> > Um.. how does this prevent the use of software breakpoints with gdb?
> > The trap instructions still work with IDM==0.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

Re: Initialize DBCR0 for PPC440 targets

[srikanth krishnakar]

Hi David,

Just to correct the statements : How the DBCR0 is initialized on
targets that don't use boot loaders ?

-Srikant

On Wed, May 20, 2009 at 11:30 AM, David Gibson
<david@gibson.dropbear.id.au> wrote:
> On Wed, May 20, 2009 at 11:20:46AM +0530, srikanth krishnakar wrote:
>> Hi David,
>>
>> I am not sure how the IDM behaves on few of PPC440 targets which don't
>> have boot loaders. I have a reference for your question:
>>
>> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td2304=
9044.html
>>
>> Without this fix (given patch) I am facing problems with GDB, and
>> further target hangs while running gdbserver !
>
> That doesn't answer my question. =A0It's not enough to say "this fixes a
> problem" you need to explain *how* it fixes the problem.
>
> And I don't see why IDM would have any effect on *software*
> breakpoints.
>
>> On Wed, May 20, 2009 at 5:23 AM, David Gibson
>> <david@gibson.dropbear.id.au> wrote:
>> > On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
>> >> Hi,
>> >>
>> >> kernel- 2.6.29
>> >> Debug technique: KGDB
>> >>
>> >> The PowerPC kernel does not initialize the PPC440 DBCR0 register. Thi=
s
>> >> prevents the use of software breakpoints in case of internal debug
>> >> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
>> >> used by boot-loaders.
>> >> It seems head_44x.S lacks this step of DBCR0 register initialization.
>> >> So fixing this with initializing the DBCR0 register as shown below :
>> >>
>> >> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not havin=
g
>> >> bootloader
>> >>
>> >> The kernel does not initialize the PPC440 DBCR0 register.
>> >> This prevents (among other things) the use of software
>> >> breakpoints with GDB. The boot loaders probably do initialize
>> >> this but few targets run without a boot loader
>> >
>> > Um.. how does this prevent the use of software breakpoints with gdb?
>> > The trap instructions still work with IDM=3D=3D0.
>
> --
> David Gibson =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| I'll have my music =
baroque, and my code
> david AT gibson.dropbear.id.au =A0| minimalist, thank you. =A0NOT _the_ _=
other_
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| _way_ _a=
round_!
> http://www.ozlabs.org/~dgibson
>



--=20
"The Good You Do, The Best You GET"

Regards
Srikanth Krishnakar
**********************

Re: Initialize DBCR0 for PPC440 targets

[srikanth krishnakar]

Hello Grant,

Is there any conclusion of the below discussion:

http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td2304904=
4.html

Xilinx target (virtex5) hangs (while running GDBServer & KGDB) without
the DBCR0 initialization.

Can you please comment on this ?


Thanks,
-Srikanth

On Wed, May 20, 2009 at 11:20 AM, srikanth krishnakar
<skrishnakar@gmail.com> wrote:
> Hi David,
>
> I am not sure how the IDM behaves on few of PPC440 targets which don't
> have boot loaders. I have a reference for your question:
>
> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049=
044.html
>
> Without this fix (given patch) I am facing problems with GDB, and
> further target hangs while running gdbserver !
>
> Thanks,
> Srikanth
>
> On Wed, May 20, 2009 at 5:23 AM, David Gibson
> <david@gibson.dropbear.id.au> wrote:
>> On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
>>> Hi,
>>>
>>> kernel- 2.6.29
>>> Debug technique: KGDB
>>>
>>> The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
>>> prevents the use of software breakpoints in case of internal debug
>>> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
>>> used by boot-loaders.
>>> It seems head_44x.S lacks this step of DBCR0 register initialization.
>>> So fixing this with initializing the DBCR0 register as shown below :
>>>
>>> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
>>> bootloader
>>>
>>> The kernel does not initialize the PPC440 DBCR0 register.
>>> This prevents (among other things) the use of software
>>> breakpoints with GDB. The boot loaders probably do initialize
>>> this but few targets run without a boot loader
>>
>> Um.. how does this prevent the use of software breakpoints with gdb?
>> The trap instructions still work with IDM=3D=3D0.
>>
>> --
>> David Gibson =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| I'll have my music=
 baroque, and my code
>> david AT gibson.dropbear.id.au =A0| minimalist, thank you. =A0NOT _the_ =
_other_
>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| _way_ _=
around_!
>> http://www.ozlabs.org/~dgibson
>>
>
>
>
> --
> "The Good You Do, The Best You GET"
>
> Regards
> Srikanth Krishnakar
> **********************
>



--=20
"The Good You Do, The Best You GET"

Regards
Srikanth Krishnakar
**********************

Re: Initialize DBCR0 for PPC440 targets

[Grant Likely]

On Mon, May 25, 2009 at 12:30 AM, srikanth krishnakar
<skrishnakar@gmail.com> wrote:
> Hello Grant,
>
> Is there any conclusion of the below discussion:
>
> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049044.html
>
> Xilinx target (virtex5) hangs (while running GDBServer & KGDB) without
> the DBCR0 initialization.
>
> Can you please comment on this ?

IIRC, John Linn was hacking on a patch.  Search the mailing list
archives for DBCR0.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.

Re: Initialize DBCR0 for PPC440 targets

[srikanth krishnakar]

Hi John,

I am not finding any conclusion of the plan to add DBCR0
initialization code to head_44x.S after this discussion :

http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td2304904=
4.html#a23049044

Can you please comment ?

Thanks,
-Srikanth Krishnakar

On Mon, May 25, 2009 at 12:09 PM, Grant Likely
<grant.likely@secretlab.ca> wrote:
> On Mon, May 25, 2009 at 12:30 AM, srikanth krishnakar
> <skrishnakar@gmail.com> wrote:
>> Hello Grant,
>>
>> Is there any conclusion of the below discussion:
>>
>> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td2304=
9044.html
>>
>> Xilinx target (virtex5) hangs (while running GDBServer & KGDB) without
>> the DBCR0 initialization.
>>
>> Can you please comment on this ?
>
> IIRC, John Linn was hacking on a patch. =A0Search the mailing list
> archives for DBCR0.
>
> g.
>
> --
> Grant Likely, B.Sc., P.Eng.
> Secret Lab Technologies Ltd.
>



--=20
"The Good You Do, The Best You GET"

Regards
Srikanth Krishnakar
**********************

Re: Initialize DBCR0 for PPC440 targets

[Benjamin Herrenschmidt]

On Wed, 2009-05-20 at 16:00 +1000, David Gibson wrote:
> On Wed, May 20, 2009 at 11:20:46AM +0530, srikanth krishnakar wrote:
> > Hi David,
> > 
> > I am not sure how the IDM behaves on few of PPC440 targets which don't
> > have boot loaders. I have a reference for your question:
> > 
> > http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049044.html
> > 
> > Without this fix (given patch) I am facing problems with GDB, and
> > further target hangs while running gdbserver !
> 
> That doesn't answer my question.  It's not enough to say "this fixes a
> problem" you need to explain *how* it fixes the problem.
> 
> And I don't see why IDM would have any effect on *software*
> breakpoints.

No but gdb can use the DABR emulation and single step would be busted
too. GDB internally heavily uses single step in places you wouldn't
expect it to :-) Like I think when breaking on main, it sometimes single
step the whole init process of the executable until it hits it. At least
I've seen it do that when I was debugging the debug support for 64-bit
Book3E.

Cheers,
Ben.

> > On Wed, May 20, 2009 at 5:23 AM, David Gibson
> > <david@gibson.dropbear.id.au> wrote:
> > > On Tue, May 19, 2009 at 06:38:53PM +0530, srikanth krishnakar wrote:
> > >> Hi,
> > >>
> > >> kernel- 2.6.29
> > >> Debug technique: KGDB
> > >>
> > >> The PowerPC kernel does not initialize the PPC440 DBCR0 register. This
> > >> prevents the use of software breakpoints in case of internal debug
> > >> mode. Looking into head_fsl_booke.S for initialization of DBCR0 is
> > >> used by boot-loaders.
> > >> It seems head_44x.S lacks this step of DBCR0 register initialization.
> > >> So fixing this with initializing the DBCR0 register as shown below :
> > >>
> > >> Subject: [PATCH] powerpc: 44x: Initialize DBCR0 for targets not having
> > >> bootloader
> > >>
> > >> The kernel does not initialize the PPC440 DBCR0 register.
> > >> This prevents (among other things) the use of software
> > >> breakpoints with GDB. The boot loaders probably do initialize
> > >> this but few targets run without a boot loader
> > >
> > > Um.. how does this prevent the use of software breakpoints with gdb?
> > > The trap instructions still work with IDM==0.
> 

Re: Initialize DBCR0 for PPC440 targets

[David Gibson]

On Mon, May 25, 2009 at 05:04:35PM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2009-05-20 at 16:00 +1000, David Gibson wrote:
> > On Wed, May 20, 2009 at 11:20:46AM +0530, srikanth krishnakar wrote:
> > > Hi David,
> > > 
> > > I am not sure how the IDM behaves on few of PPC440 targets which don't
> > > have boot loaders. I have a reference for your question:
> > > 
> > > http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049044.html
> > > 
> > > Without this fix (given patch) I am facing problems with GDB, and
> > > further target hangs while running gdbserver !
> > 
> > That doesn't answer my question.  It's not enough to say "this fixes a
> > problem" you need to explain *how* it fixes the problem.
> > 
> > And I don't see why IDM would have any effect on *software*
> > breakpoints.
> 
> No but gdb can use the DABR emulation and single step would be busted
> too. GDB internally heavily uses single step in places you wouldn't
> expect it to :-) Like I think when breaking on main, it sometimes single
> step the whole init process of the executable until it hits it. At least
> I've seen it do that when I was debugging the debug support for 64-bit
> Book3E.

Right, but I believe the kernel does turn on IDM when it turns on
single step, or the DAC event bit.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

RE: Initialize DBCR0 for PPC440 targets

[John Linn]

> -----Original Message-----
> From: srikanth krishnakar [mailto:skrishnakar@gmail.com]
> Sent: Monday, May 25, 2009 12:51 AM
> To: John Linn
> Cc: Linuxppc-dev@ozlabs.org
> Subject: Re: Initialize DBCR0 for PPC440 targets
> =

> Hi John,
> =

> I am not finding any conclusion of the plan to add DBCR0
> initialization code to head_44x.S after this discussion :

There was no conclusion and I haven't had time yet to chase it further.

> =

> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23049=
044.html#a23049044
> =

> Can you please comment ?
> =

> Thanks,
> -Srikanth Krishnakar
> =

> On Mon, May 25, 2009 at 12:09 PM, Grant Likely
> <grant.likely@secretlab.ca> wrote:
> > On Mon, May 25, 2009 at 12:30 AM, srikanth krishnakar
> > <skrishnakar@gmail.com> wrote:
> >> Hello Grant,
> >>
> >> Is there any conclusion of the below discussion:
> >>
> >> http://www.nabble.com/Question-about-DBCR0-initialization-for-440-td23=
049044.html
> >>
> >> Xilinx target (virtex5) hangs (while running GDBServer & KGDB) without=

> >> the DBCR0 initialization.
> >>
> >> Can you please comment on this ?
> >
> > IIRC, John Linn was hacking on a patch. =A0Search the mailing list
> > archives for DBCR0.
> >
> > g.
> >
> > --
> > Grant Likely, B.Sc., P.Eng.
> > Secret Lab Technologies Ltd.
> >
> =

> =

> =

> --
> "The Good You Do, The Best You GET"
> =

> Regards
> Srikanth Krishnakar
> **********************


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