linux-kernel - Re: [PATCH] ARM: OMAP5: Switch to THUMB mode if needed on secondary CPU

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Date:	Mon, 28 Apr 2014 12:21:49 -0500
From:	Joel Fernandes <joelf@...com>
To:	Dave Martin <Dave.Martin@....com>
CC:	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Nishanth Menon <nm@...com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH] ARM: OMAP5: Switch to THUMB mode if needed on secondary
 CPU

On 04/28/2014 12:20 PM, Joel Fernandes wrote:
> On 04/28/2014 11:43 AM, Dave Martin wrote:
>> On Tue, Apr 22, 2014 at 01:31:46PM -0500, Joel Fernandes wrote:
>>> On my DRA7 system, when the kernel is built in THUMB mode, the secondary CPU
>>> (Cortex A15) fails to come up causing SMP boot on second CPU to timeout. This
>>> seems to be because the CPU is in ARM mode once the ROM hands over control to
>>> the kernel.  Switch to THUMB mode if required once the kernel is control of
>>> secondary CPU. On OMAP4 on the other hand, it appears to be in THUMB mode on
>>> entry so this is not required and SMP boot works as is.
>>>
>>> Cc: Santosh Shilimkar <santosh.shilimkar@...com>
>>> Cc: Russell King <linux@....linux.org.uk>
>>> Cc: Nishanth Menon <nm@...com>
>>> Cc: Tony Lindgren <tony@...mide.com>
>>> Signed-off-by: Joel Fernandes <joelf@...com>
>>> ---
>>>  arch/arm/mach-omap2/omap-headsmp.S |    8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm/mach-omap2/omap-headsmp.S b/arch/arm/mach-omap2/omap-headsmp.S
>>> index 75e9295..1809dce 100644
>>> --- a/arch/arm/mach-omap2/omap-headsmp.S
>>> +++ b/arch/arm/mach-omap2/omap-headsmp.S
>>> @@ -1,7 +1,7 @@
>>>  /*
>>>   * Secondary CPU startup routine source file.
>>>   *
>>> - * Copyright (C) 2009 Texas Instruments, Inc.
>>> + * Copyright (C) 2014 Texas Instruments, Inc.
>>>   *
>>>   * Author:
>>>   *      Santosh Shilimkar <santosh.shilimkar@...com>
>>> @@ -28,9 +28,13 @@
>>>   * code.  This routine also provides a holding flag into which
>>>   * secondary core is held until we're ready for it to initialise.
>>>   * The primary core will update this flag using a hardware
>>> -+ * register AuxCoreBoot0.
>>> + * register AuxCoreBoot0.
>>>   */
>>>  ENTRY(omap5_secondary_startup)
>>
>> Are you sure this problem is not caused by the missing ENDPROC() for
>> omap5_secondary_startup?
>>
>> You have END() instead (which may have been accidental).
>>
>> Without ENDPROC(), the symbol is not marked as a function and so
>> the Thumb bit won't be set when taking a pointer -- so the kernel
>> is actually telling the firmware to enter in ARM state.
>>
>>
>> Try changing END() to ENDPROC() without this patch, and see if it
>> makes a difference.
>>
>> If it still doesn't work, then the firmware either doesn't support
>> entering in ARM, or is buggy.
> 
> Thanks for the suggestion. I'm guessing what you mean is with ENDPROC,
> interworking code uses bx instead of bl to set thumb mode.
> 
> But ROM/firmware doesn't have access to symbol table, how would it know
> the type of the symbol to be ARM or THUMB before it branches?
> 
Sorry what I meant is, say its of Type function. What tells the firmware
to switch to THUMB?

What's typically done is a boot address register is written by the
kernel, and the firmware jumps to it after WFE.

thanks,
-Joel

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