[U-Boot] why u-boot relocate it self to RAM from flash?

[Graeme Russ]

Hi,

On Tue, Dec 3, 2013 at 1:18 AM, FengHua <fenghua at phytium.com.cn> wrote:

>
> > Hi,
> >
> > I can not understand why u-boot relocate it self to RAM from flash? Why
> is
> > it not executing from flash?
> > Can anybody explain.
> >
>
> First, flash is slow.
> Second, there are many data structures or variables which should be
> writable.
> So, we should copy u-boot to RAM and relocate it.
>

I'll expand a bit...

In theory the writeable could be relocated into RAM, but this would require
the relocation address to be hard-coded. Optimally, U-Boot places itself in
a memory location to provide the maximum contiguous memory space to load
the operating system kernel and supporting data (RAM disks, FDTs, etc).

On most systems now, relocation is done by calculating the delta between
the ROM address (TEXT_BASE) and the relocated address in RAM. The
relocation code first copies the entire U-Boot image to RAM (apart from a
few structures that are used only during the relocation calculation). It
then scans the relocation entries (*.rel sections) to find where in the
U-Boot executable all the references into the data sections. At each
reference point in the executable, the relocation code modifies the
reference by the relocation offset. None of this can happen if the U-Boot
executable is in Flash.

The other advantage of relocating U-Boot to RAM is that it makes it
possible to flash a new U-Boot image from U-Boot. If you try to do this
while U-Boot is running from flash, it will crash as you are overwriting
the executable code while it's executing.

 Regards,

Graeme