[U-Boot] Kernel XIP

[Wolfgang Denk]

Dear Camilo Andres Roca Duarte,

In message <CAMdPjjUHad-UuJjkM8MR6bb6Vic64zzW7WopmZMnbE_qXczSUQ at mail.gmail.com> you wrote:
>
> - I would like to know if the bootm command is actually loading the
> complete kernel to RAM or if it is loading only the required pages from
> flash.

"bootm" has no knowledge about things like what a kernel is or what
required pages might be,  From the image header it gets only
information about start address and size of a memory range, and this
is what it will process.  I guess this matches what you call "complete
kernel".

But please be careful: when talking about the "loading" that takes
place, then keep in mind that this is a copy to system memory, or a
decompress and store to system memory, but in both cases the origin
of the data is also somewhwere in the memory address range, i. e. in
RAM, NOR flash or similar.

Actually the 'm' in "bootm" means "boot an image that is located
somewhere in *M*emory".

> - Also, please correct me if I'm wrong but from the description of the
> bootm command, it seemed to me as if the bootm command should be enough to
> actually start the kernel, nevertheless I encountered myself with needing
> to previously read the kernel from the flash, to later be able to boot it,
> why is that so?

With "flash", you mean NAND flash.  NAND flash is not memory, but a
storage device.  So you need other means to read the data from the
storage device into memory.  Only then you can use "bootm".

> - I have also tried an xipImage for the kernel, nevertheless once I use
> this binary format, uboot promts an error: can't get kernel image!

Using XIP requires a really detailed understand of all the steps of
the boot process, both in U-Boot and in the kernel.

> I checked the tutorial available at your website
> <http://www.denx.de/wiki/bin/view/DULG/ConfigureLinuxForXIP>, and I

Please have a look at the age of this page - the information there
cannot be used directly on recent code; you will need to adjust it
carefully.  [And if you do and get it working, it would be magnificent
if you could also update the wiki page so it is up to date again.]

> confirmed that my linux configuration creates an xipImage (not a uImage as
> stated in the tutorial). Does u-boot supports kernel execution in place?

In principle it does, but you really have to kow exactly what you are
doing.

> and if so, does it only loads the necesary pages or does it fully loads the
> kernel from the nand flash.

You asked the same before - see above.

Best regards,

Wolfgang Denk

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