how to debug Linux kernel boot process after turn MMU on stage [ I tried linux-arm-kernel mailing list arch workaround but issue]

[Alexandre Rusev]

On 09/25/2012 10:47 AM, Tryout check wrote:
> Hello,
> Thanks for your response.
>
> But can you suggest me a way to debug the start_kernel function 
> because i am not getting any console output or printk present in 
> start_kernel function. Because of this i debug the kernel startup 
> procedure through serial port. And here i am getting logs till mmu on 
> stage.
> Can you please suggest any method to debug this, except JTAG.

    Try  early_printk(), if your serial driver has hooks for usage with 
this function you'll be able to see something.
     Put early_printk call at the beginning of start_kernel().

    Also look for fun putc(), this one can print one character only. Yet 
it's even more reliable and I guess it writes directly to the serial 
port register or something likes that.
    After all you can clone putc() to your own my_putc() function, where 
you can use UART data register physical address befor MMU is on and
    the virtual address it mapped to after turning MMU on.
    There must be some code arount the putc() functions which verifieds 
if MMU is on, so that putc() may use the correct address (physical or 
virtual),
      depending on MMU state.

     The putc(char c) may be also called from assembly whcih run just 
before turning on MMU, there must  be examples of such call in head.S 
and tarmpolino;) file, ones likes this:

       mov r0, ='c'
       bl   putc

     the argument, a single character, is put into register r0 with some 
special syntax I just mmicing when write out th efirst line of assmbly above


>
> Thank & Regards
> Girish
>
> On Monday, September 24, 2012, Russell King - ARM Linux 
> <linux at arm.linux.org.uk <mailto:linux at arm.linux.org.uk>> wrote:
> > On Mon, Sep 24, 2012 at 02:32:49PM +0530, Tryout check wrote:
> >> Hi,
> >>
> >>
> >> I am trying to Boot Linux kernel 3.0.1 on a custom hardware.
> >>
> >> I am able to debug the Linux Kernel 3.0.1 boot process till
> >> __enable_mmufunction defined in head.S, using serial port.
> >>
> >> But as soon as __turn_mmu_on function is performed I am not able to 
> debug
> >> the boot process.
> >>
> >> I have gone through linux-arm-kernel mailing list archives & i have 
> tried
> >> their printascii() work around. But still I am not able to see any boot
> >> logs on serial console after __turn_mmu_on.
> >>
> >> Here I am able to get logs before __turn_mmu_on so my serial 
> console port
> >> is working fine.
> >>
> >> And one more thing - I don't have JTAG.
> >>
> >> Can anyone provide the solution how to debug Linux kernel Boot process
> >> after turn MMU on?
> >>
> >> Thank you for the support.
> >
> > Please post to the linux-arm-kernel list.
> >
> > Debugging in this area is extremely fraught to the extreme.  Most 
> peoples
> > attempts at debugging this result in total failure (as yours is) because
> > their debugging code creates more problems than it solves.
> >
> > Moreover, using debuggers in this area also seems to cause more 
> problems.
> >
> > The advice I've always given people is: remove all your debugging, and
> > check whether you get into the C code - start_kernel().  Most people who
> > do that are surprised, because they find that they are getting there.
> >
> > That's because the early assembly code has been well tested over the
> > years and is almost never a problem.  To reiterate the point - most
> > problems in this code come down to people modifying it through their
> > own "debugging" attempts.
> >
>
>
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