[Open SoC Debug] CDM-OR1K implementation

Philipp Wagner philipp.wagner at tum.de
Tue May 22 09:31:16 CEST 2018

On 05/20/2018 04:25 PM, Stafford Horne wrote:
> On Sun, May 20, 2018 at 05:11:55PM +0530, SHIVAM AGGARWAL wrote:
>> I went through all the resources again. As per my understanding, CDM works
>> by storing breakpoint addresses in the debug registers.
> I don't think you need to think about it this way.  CDM just provides access
> to the CPU registers.  The debugger software decides what to write to which
> registers (i.e. this target driver in gdb/openocd).
>> If a  PC ever
>> matches a value in breakpoint registers, CPU raises an exception and
>> reports it to GDB. This is basically a hardware breakpoint.
>> As you mentioned above, by writing an l.trap instruction into memory we can
>> implement software breakpoint. For this part, we need access to memory.
>> (MAM)
>> Things to do now:
>> 1. To expand memory map to access a the other registers.
>> 2. To see how the existing system interface can be used to read/write
>> memory.
>> 3. To support software breakpoints, we need access to memory. This can be
>> achieved either by using MAM or extending CDM module.
> It should be MAM (for 2 and 3). Unless anyone objects.
> To support any debugging we need read/write access to memory.

Just as Stafford said: the MAM enables access to the (main) memory. So 
part 2 and 3 are already there. So the main thing to do is (1).

The main challenge on that one will be how to cleverly map the register 
address space of the mor1kx CPU to the limited register address space 
available through OSD. The question is essentially: can we come up with 
a core-agnostic, generic mapping, or do we need to pick certain address 
ranges out of the or1k register map and place it somewhere in the OSD 
register address space.

To be able to answer this question, we first need a more detailed 
overview of the registers which are accessed through GDB.

Shivam, could you provide this overview, and run it by Stafford to make 
sure you got everything covered? We can then in a second step discuss 
how to add this information to the CDM spec.


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