[PATCH v2] ARM: PJ4: make coprocessor access sequences buildable in Thumb2 mode

Ard Biesheuvel ard.biesheuvel at linaro.org
Tue Dec 22 04:19:22 PST 2015

On 22 December 2015 at 13:09, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Tue, Dec 22, 2015 at 08:21:09AM +0100, Ard Biesheuvel wrote:
>> The PJ4 inline asm sequence to write to cp15 cannot be built in Thumb-2
>> mode, due to the way it performs arithmetic on the program counter, so it
>> is built in ARM mode instead. However, building C files in ARM mode under
>> CONFIG_THUMB2_KERNEL is problematic, since the instrumentation performed
>> by subsystems like ftrace does not expect having to deal with interworking
>> branches.
>> Since the sequence in question is simply a poor man's ISB instruction,
>> let's use a straight 'isb' instead when building in Thumb2 mode. Thumb2
>> implies V7, so 'isb' should always be supported in that case.
> I think, given how close Christmas is, and I suspect that the merge
> window will be opening shortly afterwards, it'll be better to drop
> the change and sort it out in the new year, rather than trying to
> rush this in, especially as we're already at the 3rd iteration of
> this supposedly simple change.  It obviously isn't quite as simple
> as it looks.

I agree. Please apply it whenever you feel it is appropriate to do so.

Apologies for the trouble

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