AT91: How copy kernel code to SRAM and execute?

[plagnioj@jcrosoft.com (Jean-Christophe PLAGNIOL-VILLARD)]

On 21:05 Sun 26 Feb     , Russell King - ARM Linux wrote:
> On Sun, Feb 26, 2012 at 09:48:04PM +0100, Ulf Samuelsson wrote:
> > On 2012-02-26 18:50, Russell King - ARM Linux wrote:
> >> On Sun, Feb 26, 2012 at 06:37:25PM +0100, ARM Linux wrote:
> >>> I am looking for some examples on how to copy a small piece of kernel code
> >>> to internal SRAM and then execute it.
> >> See fncpy.h
> >>
> > Thanks, this does the copy.
> > My problem is in making the SRAM executable.
> > I have already copied to SRAM (I think), but when I jump there
> > I get a trap and a stackdump.
> 
> 
> 
> > The "mmap" system call, seems to do what I want, but is this callable
> > from within the kernel, or is there a better way?
> >
> > I can see that is has been split upo into "old_mmap" (obsolete) and the  
> > newer
> > "sys_mmap2" which turns off the MAP_EXECUTABLE.
> > "mprotect" maybe can be used to fix that.
> >
> > Googling for how to call a system call from within the kernel gave:
> >
> >     mm_segment_t     oldfs = get_fs();
> >     set_fs(KERNEL_DS);
> >     /* Do SysCalls */
> >         sys_mprotect(sram_shutdown, PAGE_SIZE, ... );
> >     set_fs(old_fs);
> 
> Don't even think about that.  Google doesn't always answer the correct
> question.  In this case, it's completely the wrong thing because all
> the userspace APIs are _only_ designed to work with userspace addresses.
> 
> Moreover, this will fail because it won't find a VMA for the address
> (which is a good thing - as it will attempt to modify the pages in
> ways which could end up exposing them to userspace.)
> 
> In general, if you're coding inside the kernel, calling kernel syscalls
> is 99.999% of the time totally the wrong thing to do.
> 
> The way to do this is to use __arm_ioremap_exec() to map the SRAM.
Ulf try to fix something the wrong way, this probelem is known for longtime on
at91 and fixed at the mainline on all SoC

Best Regards,
J.