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

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Date:	Tue, 29 Apr 2014 21:55:38 -0500
From:	Joel Fernandes <joelf@...com>
To:	Dave Martin <Dave.Martin@....com>,
	Joel Fernandes <agnel.joel@...il.com>
CC:	Nishanth Menon <nm@...com>, Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: OMAP5: Switch to THUMB mode if needed on secondary
 CPU

On 04/29/2014 01:31 PM, Dave Martin wrote:
> On Tue, Apr 29, 2014 at 05:36:30PM +0100, Joel Fernandes wrote:
> 
> [...]
> 
>>>> 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.
> 
> ARM/thumb interworking making perfect sense?  I'll have to frame that
> and put it on the wall :)
> 
> objdump and nm conveniently mask off the Thumb bit from all function
> addresses they print out, but if you show the symbols using readelf
> instead you'll see addresses with bit 0 set for Thumb functions.
> 
> It's possible that the firmware still doesn't handle branching to Thumb
> correctly, but if it does it would be nice to remove the requirement to
> build an odd piece of a Thumb-2 kernel in ARM.

Sure, it works great! thanks, I sent out a patch that applies on top of
the first one. Thanks for spotting it.

Regards,
-Joel

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