[PATCH v2 2/3] ARM: iwmmxt: Port problematic iwmmxt support code to v7/Thumb-2

[Eric Miao]

On Thu, Sep 8, 2011 at 10:13 AM, Jon Medhurst (Tixy)
<jon.medhurst at linaro.org> wrote:
> On Thu, 2011-09-08 at 10:03 -0700, Eric Miao wrote:
>> On Thu, Sep 8, 2011 at 9:45 AM, Arnd Bergmann <arnd at arndb.de> wrote:
>> > On Thursday 08 September 2011, Dave Martin wrote:
>> >> The iwmmxt code contains some code to implement a pseudo-ISB, but
>> >> this is not buildable for Thumb-2.
>> >>
>> >> This patch replaces the pseudo-ISB with a real one for Thumb-2
>> >> kernels.
>> >>
>> >> Signed-off-by: Dave Martin <dave.martin at linaro.org>
>> >> ---
>> >>  arch/arm/kernel/iwmmxt.S |    9 +++++++++
>> >>  1 files changed, 9 insertions(+), 0 deletions(-)
>> >
>> > Acked-by: Arnd Bergmann <arnd at arndb.de>
>> >
>>
>> Maybe it'll be much simpler to have something like below:
>>
>> diff --git a/arch/arm/kernel/iwmmxt.S b/arch/arm/kernel/iwmmxt.S
>> index a087838..5998f7d 100644
>> --- a/arch/arm/kernel/iwmmxt.S
>> +++ b/arch/arm/kernel/iwmmxt.S
>> @@ -319,8 +319,9 @@ ENTRY(iwmmxt_task_switch)
>>         PJ4(eor r1, r1, #0xf)
>>         PJ4(mcr p15, 0, r1, c1, c0, 2)
>>
>> -       mrc     p15, 0, r1, c2, c0, 0
>> -       sub     pc, lr, r1, lsr #32             @ cpwait and return
>> +       XSC(mrc p15, 0, r1, c2, c0, 0)
>> +       PJ4(isb)
>> +       mov     pc, lr                          @ cpwait and return
>
> Don't we still need "sub pc, lr, r1, lsr #32" in the XSC case?
> I had assumed this was to introduce a data dependency. I.e. PC depends
> on value of R1 which is loaded by the MRC, so it can't complete until
> that has.

Indeed, that's actually a good point.