Re: [PATCH] m68k: allow ColdFire m5441x parts to run with MMU enabled

[Angelo Dureghello <angelo@sysam.it>]

Hi Greg,

On 10/08/2017 09:06, Greg Ungerer wrote:
> Hi Angelo,
> 
> On 10/08/17 01:32, Angelo Dureghello wrote:
> [snip]
>> sure, on this board  http://sysam.it/cff_stmark2.html
>> there are 128MB of ddr2.
>>
>> External SDRAM is accessible, at least without any mmc support enabled,
>> from 0x40000000.
>>
>> I have following test config:
>>
>>    GNU nano 2.8.6                                                                              File: arch/m68k/configs/stmark2_defconfig
>>
>> CONFIG_LOCALVERSION="stmark2-001"
> [snip]
>>
>>
>> I tried still yesterday a bit, but seems there is no much support for
>> earlyprintk / low level debug for this architecture.
>>
>> In case i can try with a gpio toggling routine, at least to find
>> where kernel stops.
> 
> The attached patch, is a quick and dirty early console output method.
> It works for me on the m5475, should work for you "as is" on the 5441x too.
> 
> It is kind of an early printk. Of course it still needs the early
> kernel boot to have succeeded before you will get anything much coming out.
> But it is worth trying.

Ok many thanks. Btw i used a __square(); function written in asm, so i am
sure i see the gpio toggling in very early stages.

> 
> I am wondering if the non-0 base RAM may be a problem. I have only run
> the MMU enabled code on platforms with 0 based RAM so far. But lets see if
> the early console trace attached gives us anything before digging into that.
> 

This MCU has sdram area physically mapped at 0x4000 0000 so U-Boot, to be
able to execute the kernel must load it to that location/area anyway.

But i have seen that it is not a problem, after MMU is enabled in head.S
the jump
         
         movel   #_vstart,%a0      /* jump to "virtual" space */
         jmp     %a0@

works fine. Since that range is not hitting anything that is maintained
physical, it can be translated into virtual without any issue.

After some hard debug, i see the execution stops at:

asmlinkage __visible void __init start_kernel(void)
    ...
    setup_arch(&command_line);      setup_mm.c
       ...
       paging_init();               mm/mcfmmu.c
          ...
          empty_zero_page = (void *) alloc_bootmem_pages(PAGE_SIZE);
          ^line 47 mcfmmu.c

Inside alloc_bootmem_pages(), execution seems to end up finally to
mm/bootmem.c and likely to alloc_bootmem_bdata().
In case i can still proceed to find the exact place where execution stops,
but i suspect in the while(1), line 545.

As a curious thing, i find in a different cf CPU code "m54xx.c"
the following:

void __init config_BSP(char *commandp, int size)
{
#ifdef CONFIG_MMU
	cf_bootmem_alloc();
	mmu_context_init();
#endif
	
Do also m5441x.c maybe need this calls ?

Would be very nice to have MMU working. Strangely, i don't see any
board_config with it enabled. Was it ever tested on some Coldfire ?
Waiting your suggestion on how to proceed.

Regards,
Angelo Dureghello
  

> Regards
> Greg
>