Cortex A9 MMU

Hi Florian,

Many thanks for your detailed answer. I use the ARM DS-Ultimate toolchain, a linker script is not needed. I can only envoke the armlinker using:

armlink --cpu=Cortex-A9 --scatter="...\scatter_SDRAM.scat" --info=sizes --entry =alt_interrupt_vector

Some further context:

The reason I need to use the MMU, is so that I can make use of the cache. I am quite sure that the MMU is activated and populated correctly, and that the cache works. Because they work fine in the debugger.  Furthermore the above mentioned function alt_pt_init() is provided by Intel, I presume it is correct.

The problem with the unalligned access only occurs, when the application.bin-file started, instead of the axf-file. The bin file is created with the following command.

-fromelf --bincombined application.axf --output=application.bin

When the MMU is not used i.e. when the further above mentioned function alt_pt_init() is not called no allignment error occurs. 

Question:

Why is it possible that I can have an allignment error when the program is started directly on the machine - but no allignement error when the program is run in the Debugger?

I would be very grateful for further tipps. All the best

Deas

Hi Florian,

Many thanks for your detailed answer. I use the ARM DS-Ultimate toolchain, a linker script is not needed. I can only envoke the armlinker using:

armlink --cpu=Cortex-A9 --scatter="...\scatter_SDRAM.scat" --info=sizes --entry =alt_interrupt_vector

Some further context:

The reason I need to use the MMU, is so that I can make use of the cache. I am quite sure that the MMU is activated and populated correctly, and that the cache works. Because they work fine in the debugger.  Furthermore the above mentioned function alt_pt_init() is provided by Intel, I presume it is correct.

The problem with the unalligned access only occurs, when the application.bin-file started, instead of the axf-file. The bin file is created with the following command.

-fromelf --bincombined application.axf --output=application.bin

When the MMU is not used i.e. when the further above mentioned function alt_pt_init() is not called no allignment error occurs. 

Question:

Why is it possible that I can have an allignment error when the program is started directly on the machine - but no allignement error when the program is run in the Debugger?

I would be very grateful for further tipps. All the best

Deas

I recommed to set up your own vector table before anything else. It seems that still the bootloader is active.

Or again: Who outputs the error? Your code? Bootloader? If you also print the DFAR/IFAR registers you can come closer to the real cause.
And again: The registers seem to be weird. FP is in the lower memory, SP in the higher. LR in the lower, but PC in the higher.

You must pay attention to the fact that the debugger execution is different from the code execution without debugger.
You cannot assume that debugger execution is OK so that your code should be 100% OK under any conditions.

Debugger execution is a invasive debug method. Armv7-A Architecture reference manual (ARM.ARM) says:

<quote>

Invasive debug authentication controls whether an debug event:
• causes the processor to enter Debug state
• generates a debug exception
• is ignored
• becomes pending.

</quote>

So the debugger execution perhaps hides the unaligned access failure or the debug event adds the explicit barrier for you.
You still need to figure out where is the possible unaligned data access in your application.bin file.   Check the offending data abort address further.

I agree with Zhifei and Bastian, you should check the Data Abort detailed information:

- Data Fault Status Register (DFSR) gives you the type of exception. Cortex A9 exceptions are encoded with the short descriptor format. Refer to ARMv7 architecture reference manual for the codes of all data abort exceptions.

- Data Fault Address Register (DFAR) gives you the address of the memory access which generated the abort, if this is a sycnhronous abort type (refer to DFSR)

If it is an MMU fault, it is synchronous and DFAR is valid. If it is a cache parity or DRAM ECC error, it is asynchronous basically, and DFAR is not valid.

With these information, you may be able to find the root cause.

Dear Flongnos,

thanks for your tip. The value that is stored in the DFSR is: 0x0000_1C97 resulting in the fault status of 0x17. However, this value is not shown in the table of the short descriptor format encodings (arm armv7A table B3-23), according to the specifications, all encodings not shown in the table are reserved.

I checked the different registers in debug mode in the ARM-DS-5 Debugger.The DFAR does not seem to be valid

Question:

The fault status of the DFSR does not point to a specifc error (as defined in arm armv7A B3-23). Does this imply that no data fault occured?

Many thanks for your reply.

Deas

Hi,

I think you did not decode it properly. For the short descriptor format, we have the fault status as the concatenation of bit 10 and bits 3~0.

So we have FS = ((DFSR[10] & 0x400) >> 6) | (DFSR[3:0] & 0xF)

As a result I found that you have FS = 0x7, that is SDTFMT_MMU_L2_TRANSLATION_FAULT

And the DFAR should be valid

Florian

Hello Florian,

thanks for your answer! Perhaps I am missing something too obvious here ;), but as far as I can tell, I followed the exact same procedure as you. i.e.:

The content in DFSR is: 0x0000_1C97

*DFSR = 0x0000_1C97 = 0b0000..._0001_1100_1001_0111;

stringing together the bit numbers [10,3,2,1,0] leads to: 0b10111=0x17.

Could you give me hint how you got to the fault status of 0x7 instead of 0x17?

Many thanks again.

Deas

Sorry Deas, I meant 0x17 indeed. But that code does not make sense, except if Large Page Address Extension and Long Descriptor Format are used and supported.

Maybe check something related to privilege level... 

Hi Florian,

thanks for the clarification an the further input, I will look into it.

cheers

Deas