[linuxsh-dev] Re: XIP

[Jesper S. <js...@re...>]

[please write proper English. I'm getting too old to find the
short'n'cute form cool anymore - now it's just annoying]

>>>>> "Manoj" == Manoj Kumar <man...@so...> writes:

Manoj> Hello Jesper, Thanx for ur response regarding the XIP patch for
Manoj> SH-3 on the mailing list.

Manoj> I have a few doubts.

Manoj> 1. My understanding of XIP is that the kernel image (which
Manoj> includes the text + data) sits in the ROM and executes from
Manoj> there.  Can u plz explain this scheme a bit more in detail?

That is correct. Worth noting is that there is no MMU mapping
(page-wise) of that area: we rely on the direct mapping available in
kernel space.

Manoj> 2. From ur explanation in the mail,i see that 2Megs of RAM is
Manoj> used for kernel text+initrd and 5Megs for mapping.  The kernel
Manoj> text starts at CONFIG_XIP_ROM_START=0c100000,which is a RAM
Manoj> address.  Since in the XIP scheme,the kernel sits in the
Manoj> ROM,why r u placing it in the RAM area?  I could be wrong in my

For debugging purpose. Setting breakpoints in ROM is hard :)

For a proper XIP configuration, you'd select
CONFIG_XIP_ROM_START=00100000 (which is in ROM) and
CONFIG_MEMORY_START=0x0c100000.

Build the image and program it into the ROM at that address (or
rather, at 0x00101000). Start it by jumping to 0x80102000. It will
copy the data to 0x0c100000 and start executing...

Manoj> interpretation of the entire thing.  What is the
Manoj> CONFIG_MEMORY_START for?Is it the starting address of the area
Manoj> which will be used by the Linux kernel.

CONFIG_MEMORY_START defines RAM start - as in: the area where the
paged mapping of memory begins.

Jesper