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

[Joel Fernandes <agnel.joel@gmail.com>]

> On Apr 29, 2014, at 2:17 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> 
>> On Mon, Apr 28, 2014 at 06:21:49PM +0100, Joel Fernandes wrote:
>>> 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@ti.com>
>>>>> Cc: Russell King <linux@arm.linux.org.uk>
>>>>> Cc: Nishanth Menon <nm@ti.com>
>>>>> Cc: Tony Lindgren <tony@atomide.com>
>>>>> Signed-off-by: Joel Fernandes <joelf@ti.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@ti.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.
> 
> Using ENTRY(x) ... ENDPROC(x) causes the symbol seen by the linker
> for x to have the Thumb bit set if the code is Thumb.
> 
> This means that any reference the linker fixes up for that symbol
> will have the Thumb bit set appropriately.  This applies to any kind
> of reference, so code in another file that takes the address of the
> symbol and then passes that address to the firmware should result in the
> firmware getting an address with the Thumb bit.
> 
> From the firmware's point of view it just gets a raw address, but
> the Thumb bit will now be set.  The firmware still needs to handle
> this correctly when jumping, but from the look of the code this may
> already work on omap3/4.  It would be interesting to know whether it
> works on omap5.

Thanks a lot for the explanation. That makes perfect sense. I will try it and let you know if it works on OMAP5.

Regards,
-Joel


> 
> Cheers
> ---Dave
> 
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