[PATCH v3] ARMv7M: Fix exception handler for supporting FPU

[Gedare Bloom]

Martin's ticket works, and his commit can  #close it.

On Mon, Aug 31, 2015 at 3:22 PM, sudarshan.rajagopalan
<sudarshan.rajagopalan at vecna.com> wrote:
> On 2015-08-31 13:39, Daniel Gutson wrote:
>>
>> On Mon, Aug 31, 2015 at 2:34 PM, Gedare Bloom <gedare at rtems.org> wrote:
>>>
>>> I'd approve 2 patches in case you want to give credit. First patch
>>> with Sudarshan's fix, and Martin's improvement second.
>>
>>
>> +1
>>
>
> Sounds fair enough! :) Thanks for the support, Daniel. Should I make a
> separate ticket for committing the initial patch or Martin's ticket works
> out well for this?
>
> Thanks and Regards,
> Sudarshan
>
>
>>>
>>> On Mon, Aug 31, 2015 at 1:28 PM, Daniel Gutson
>>> <daniel.gutson at tallertechnologies.com> wrote:
>>>>
>>>> Side question to Joel and others: is this enough to credit Sudarshan?
>>>> He did an amazing job for finding the bug and proposing an initial
>>>> solution.
>>>>
>>>> On Mon, Aug 31, 2015 at 11:37 AM, Martin Galvan
>>>> <martin.galvan at tallertechnologies.com> wrote:
>>>>>
>>>>> On exception entry, _ARMV7M_Exception_default stores the previous Stack
>>>>> Pointer
>>>>> in a CPU_Exception_frame. The SP can be MSP or PSP, depending on the
>>>>> mode
>>>>> in which the exception was taken. To know this, we must check the value
>>>>> of LR.
>>>>>
>>>>> Right now the code checks whether it should store MSP or PSP by
>>>>> comparing LR to
>>>>> -3. However, this doesn't work if we're using an FPU. This patch fixes
>>>>> that
>>>>> by checking bit 2 of LR, which indicates which SP we were using.
>>>>>
>>>>> Thanks to Sudarshan Rajagopalan for finding the bug and proposing a
>>>>> first
>>>>> version of the fix.
>>>>>
>>>>> Closes #2401.
>>>>>
>>>>> ---
>>>>>  cpukit/score/cpu/arm/armv7m-exception-default.c | 27
>>>>> +++++++++++++++++--------
>>>>>  1 file changed, 19 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/cpukit/score/cpu/arm/armv7m-exception-default.c
>>>>> b/cpukit/score/cpu/arm/armv7m-exception-default.c
>>>>> index e890cdf..d04dbea 100644
>>>>> --- a/cpukit/score/cpu/arm/armv7m-exception-default.c
>>>>> +++ b/cpukit/score/cpu/arm/armv7m-exception-default.c
>>>>> @@ -22,16 +22,27 @@
>>>>>
>>>>>  void __attribute__((naked)) _ARMV7M_Exception_default( void )
>>>>>  {
>>>>> +    /* On exception entry, ARMv7M saves context state onto a stack
>>>>> pointed to
>>>>> +     * by either MSP or PSP. The value stored in LR indicates whether
>>>>> we were
>>>>> +     * in Thread or Handler mode, whether we were using the FPU (if
>>>>> any),
>>>>> +     * and which stack pointer we were using.
>>>>> +     * In particular, bit 2 of LR will be 0 if we were using MSP.
>>>>> +     *
>>>>> +     * For a more detailed explanation, see the Exception Entry
>>>>> Behavior
>>>>> +     * section of the ARMv7M Architecture Reference Manual.
>>>>> +     */
>>>>> +
>>>>> +    /* As we're in Handler mode here, we'll always operate on MSP.
>>>>> +     * However, we need to store the right SP in our
>>>>> CPU_Exception_frame.
>>>>> +     */
>>>>>    __asm__ volatile (
>>>>> -    "sub sp, %[cpufsz]\n"
>>>>> +    "sub sp, %[cpufsz]\n" /* Allocate space for a CPU_Exception_frame.
>>>>> */
>>>>>      "stm sp, {r0-r12}\n"
>>>>> -    "mov r2, lr\n"
>>>>> -    "mrs r1, msp\n"
>>>>> -    "mrs r0, psp\n"
>>>>> -    "cmn r2, #3\n"
>>>>> -    "itt ne\n"
>>>>> -    "movne r0, r1\n"
>>>>> -    "addne r0, %[cpufsz]\n"
>>>>> +    "tst lr, #4\n"       /* Check if bit 2 of LR is zero. (If so,
>>>>> PSR.Z = 1) */
>>>>> +    "itte eq\n"          /* IF bit 2 of LR is zero... (PSR.Z == 1) */
>>>>> +    "mrseq r0, msp\n"    /* THEN we were using MSP. */
>>>>> +    "addeq r0, %[cpufsz]\n" /* THEN, set r0 = old MSP. */
>>>>> +    "mrsne r0, psp\n"    /* ELSE it's not zero; we were using PSP. */
>>>>>      "add r2, r0, %[v7mlroff]\n"
>>>>>      "add r1, sp, %[cpulroff]\n"
>>>>>      "ldm r2, {r3-r5}\n"
>>>>> --
>>>>> 1.9.1
>>>>
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Daniel F. Gutson
>>>> Chief Engineering Officer, SPD
>>>>
>>>> San Lorenzo 47, 3rd Floor, Office 5
>>>> Córdoba, Argentina
>>>>
>>>> Phone:   +54 351 4217888 / +54 351 4218211
>>>> Skype:    dgutson
>>>> LinkedIn: http://ar.linkedin.com/in/danielgutson
>
>