linux-kernel - Re: [PATCH] ARM: hw_breakpoint: Enable debug powerdown only if system supports 'has

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Date:	Thu, 28 Mar 2013 11:59:00 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Will Deacon <will.deacon@....com>
CC:	Santosh Shilimkar <santosh.shilimkar@...com>,
	Will Deacon <will@...ldeacon.co.uk>,
	Lokesh Vutla <lokeshvutla@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCH] ARM: hw_breakpoint: Enable debug powerdown only if system
 supports 'has_ossr'

On 19/03/13 10:28, Will Deacon wrote:
> On Tue, Mar 19, 2013 at 06:39:38AM +0000, Santosh Shilimkar wrote:
>> On Monday 18 March 2013 10:36 PM, Will Deacon wrote:
>>> Any chance you could follow up with your firmware/hardware guys about this
>>> please? I'd really like to understand how we end up in this state in case we
>>> can do something in the architecture to stop it from happening in future.
>>>
>> I will check on this part and update you when I hear from them.
>
> Ok, thanks.
>
> Dietmar -- I seem to remember that you thought GDB did actually work with
> hardware breakpoints on Pandaboard across low-power states. Can you
> confirm/deny this please?

Sorry for the late response.

I did some testing on my Pandaboard trying to set hwbp from gdb.

System is a vanilla Linaro 13.02.
gdb is (GNU gdb (GDB) 7.5.1), build from tag gdb_7_5_1-2012-11-29-release

(gdb) hb *0x00008440
Hardware assisted breakpoint 2 at 0x8440: file test.c, line 14.
(gdb) c
Continuing.
Unexpected error setting breakpoint address: No such device.

With additional kernel logs:

[<c0011597>] (unwind_backtrace+0x1/0x92) from [<c0012603>]
(arch_validate_hwbkpt_settings+0x23/0x1d4)
[<c0012603>] (arch_validate_hwbkpt_settings+0x23/0x1d4) from
[<c0088aa9>] (register_perf_hw_breakpoint+0x19/0x30)
[<c0088aa9>] (register_perf_hw_breakpoint+0x19/0x30) from [<c0088ae9>]
(hw_breakpoint_event_init+0x29/0x48)
[<c0088ae9>] (hw_breakpoint_event_init+0x29/0x48) from [<c0086c81>]
(perf_init_event+0x79/0xcc)
[<c0086c81>] (perf_init_event+0x79/0xcc) from [<c0086e97>]
(perf_event_alloc+0x1c3/0x32c)
[<c0086e97>] (perf_event_alloc+0x1c3/0x32c) from [<c00871eb>]
(perf_event_create_kernel_counter+0x19/0xc6)
[<c00871eb>] (perf_event_create_kernel_counter+0x19/0xc6) from
[<c00885e9>] (register_user_hw_breakpoint+0x2d/0x38)
[<c00885e9>] (register_user_hw_breakpoint+0x2d/0x38) from [<c03dd723>]
(ptrace_hbp_create+0x53/0x5c)
[<c03dd723>] (ptrace_hbp_create+0x53/0x5c) from [<c000e0d7>]
(ptrace_sethbpregs+0x95/0x1a6)
[<c000e0d7>] (ptrace_sethbpregs+0x95/0x1a6) from [<c000e777>]
(arch_ptrace+0x3bf/0x3ec)
[<c000e777>] (arch_ptrace+0x3bf/0x3ec) from [<c002f621>]
(sys_ptrace+0x251/0x278)
[<c002f621>] (sys_ptrace+0x251/0x278) from [<c000c781>]
(ret_fast_syscall+0x1/0x52)
perf_event_create_kernel_counter:6721 ret=-19
ptrace_sethbpregs:557 ret=-19

The call to monitor_mode_enabled() in arch_validate_hwbkpt_settings()
fails since DSCR.15 MDBGen is not set:

monitor_mode_enabled:239 dscr=0x1030002

I don't get the warning "Failed to enable monitor mode on CPU" during
system start-up though.

Doing the same on my TC2 gives me:

monitor_mode_enabled:239 dscr=0x2008002

So I guess that setting hwbp from self-hosted debugger on Pandaboard is
not possible at all.

There is no CPUidle enabled on Linaro 13.02 Pandaboard.

-- Dietmar


>
>>> Well, we could just add the warn_once prints but that doesn't stop debug
>>> from breaking after the first pm/suspend/hotplug operation.
>>>
>> Probably we should drop the $subject patch approach and use warn_once
>> at least for current rc. Ofcourse it doesn't help to get working
>> hw_breakpoint support. Patch end of the email with warn_once.
>
> Yeah, I'll merge that, but it's a real shame that this breaks hardware debug
> on one of the few platforms where it was reported to work.
>
> Will
>

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