Re: [ECOS] ARM vector.s -- suspicious code in return_from_exception

["Michael Anburaj" <embeddedeng at hotmail dot com>]

Hi,

Well, I don?t know enough about ecos either. But after having looked at the 
code, I agree with you.

I am also curious about yet other piece of code (if 
CYGOPT_HAL_ARM_PRESERVE_SVC_SPSR is defined the follow is the final piece of 
code executed by return_from_exception).

// we are returning to svc mode thus we must restore the
       // pre-exception cpsr before returning to interrupted code
       msr     cpsr, r0
       ldmfd   sp, {r0-r14, pc}

Can this handle both ARM & THUMB pre-exception code?

What will happen if the T bit is set (return to THUMB pre-exception code)? 
Quote from ARM manuals : ? Note that the software must never change the 
state of the T bit in the CPSR. If this happens, the processor will enter an 
unpredictable state.?. Because ?ldmfd   sp, {r0-r14, pc}? was compiled as 
ARM code & T bit is enabled before executing this.

May be this code was not intended to work with THUMB mode pre-exception code 
(i.e., if CYGOPT_HAL_ARM_PRESERVE_SVC_SPSR is defined, then no THUM mode 
execution), so that T bit is never on, when ?msr     cpsr, r0? is executed. 
Or may be that you all are aware of it.

Cheers,
-Mike



> From: "Eric de Jong" <list_ericdejong_10@gmx.net>
> To: "Ecos-Discuss" <ecos-discuss@sources.redhat.com>
> Subject: [ECOS] ARM vector.s   --  suspicious code in return_from_exception
> Date: Mon, 23 Jun 2003 11:45:10 +0200
>
> Hello,
>
> In the file hal/arm/arch/current/src/vector.s (recent snapshot) I found the 
> code
> below.
> Please note the line "msr     spsr,r0". While it is used in an if/else 
> case, r0
> it is alway written. Unfortunate I am not familiar enough with ecos, but 
> would
> anyone take a look at it? I would guess !1! is not intended but then again 
> this
> might be a quick fix which has to be cleaned.
>
> Eric.
>
> vector.s line 545
>         .code   32
>         //
>         // Return from exception
>         //
> return_from_exception:
>
>         ldr     r0,[sp,#armreg_cpsr]
> !1!      msr     spsr,r0
>
>         // return to supervisor mode is simple
>         and     r1,r0,#CPSR_MODE_BITS
>         cmp     r1,#CPSR_SUPERVISOR_MODE
>
> #ifndef CYGOPT_HAL_ARM_PRESERVE_SVC_SPSR
> !2!      msr     spsr,r0
>         ldmeqfd sp,{r0-r14,pc}^
> #else
>         // we must take care of not corrupting the current (svc)
>         // spsr which happens to be also the pre-exception spsr
>         bne     1f
>         // we are returning to svc mode thus we must restore the
>         // pre-exception cpsr before returning to interrupted code
> !3a!      msr     cpsr, r0
>         ldmfd   sp, {r0-r14, pc}
> 1:
>         // we are not returning to svc mode thus we can safely restore
>         // svc spsr
> !3b!      msr     spsr, r0
> #endif
>
>
> ----- Original Message -----
> From: "QiangHuang" <jameshq@liverpool.ac.uk>
> To: "Ecos-Discuss" <ecos-discuss@sources.redhat.com>
> Sent: Friday, June 20, 2003 12:24 AM
> Subject: [ECOS] ARM vectors.S question. Thank you.
>
>
> Hi :
>
>    Can anyone give me some suggestion about the following code? thanks a
> lot.
>
> memory map:
>     SECTION_fixed_vectors (ram, 0x20, LMA_EQ_VMA)
>     SECTION_rom_vectors (ram, 0x40000, LMA_EQ_VMA)
>     SECTION_text (ram, ALIGN (0x4), LMA_EQ_VMA)
>
> ARM vectors.S
> ................
>         ldr     r1,=__exception_handlers
>         // **Question 1** Is r1 = 0x40000 here ???
> ..........
>
> #ifdef CYGHWR_HAL_ARM_ICE_THREAD_SUPPORT
>
>         ldr     r2,=ice_thread_vector
>         // **Question 2** by here what is value of r2 by now, is
> r2=0x20+offset(ice_thread_vector to fixed_vectors) ???? Is this Correct???
>
>         sub     r2,r2,r1        // compute fixed (low memory) address
>         // **Question 3** if the above correct why this is needed??? what 
> is
> value of r2=r2-r1 ??  r2 = r2-0x40000??? what is going here, how the 
> address
> is defined??
>
>         ldr     r3,=0x4D494345  // 'MICE'
>         str     r3,[r2],#4
>         ldr     r3,=hal_arm_ice_thread_handler
>         str     r3,[r2],#4
>         mov     r3,#1
>         str     r3,[r2],#4
>         ldr     r3,=0x47444220  // 'GDB '
>         str     r3,[r2],#4
> #endif // CYGHWR_HAL_ARM_ICE_THREAD_SUPPORT
>
> Thanks a lot.
>
>
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>
>
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