[U-Boot] mkimage and XIP vs entry point

[Wolfgang Denk]

Dear Mike Frysinger,

In message <201001181955.55856.vapier at gentoo.org> you wrote:
>
> the current mkimage requires XIP images to have their entry point set to the 
> load address + 64 bytes.  my question is simply, why ?  why cant the entry
> point be an arbitrary location ?  i cant seem to find any technical reason for 
> this restriction.

It's a quick & dirty hack which has only hysterical reasons.

The reason for the 64 byte offset is that the classic uImage header
takes 64 bytes, and that PowerPC always hat the entry point right at
the start of the image. When we implemented this some 8 years ago we
had only PowerPC (actually only MPC8xx) in mind, and AFAICT it has
not been much used on other platforms or even more recent kernels
than the 2.4.4 version we've been using by then (for details please
see http://www.denx.de/wiki/view/DULG/ConfigureLinuxForXIP)

We wanted to use uImages (to keep the checksum verification etc.), and
this old format did not provide any spare flags for information like
XIP etc. This is where the code is coming from.

I am aware that it is ugly and most likely does not even work wih
recent kernels, not even on PPC.

We lost interest in this feature after we had it running and were
able to measure actual results. The thing is, that with XIP you can
save a few MB of RAM (2...3 with the 2.4.4 kernel), but you need much
larger NOR flash (which is way more expensive than RAM), and boot
time will usually become longer - on the systems we tested we usually
found that it's faster to load a compressed image to RAM and
uncompress it (ideally on the fly, but it's still faster when done in
a second stage) and run it form RAM. Given the fact that processors
have become much faster (compared to 8 years ago when we did these
tests) but memory bandwidth is still about the same, the results today
will be probably even more in favour of not using XIP.

> so what am i missing ?  or should i submit a patch to delete this check from 
> the mkimage tool and let the entry point be arbitrary like non-xip ?

If you have any configuration that is actually working for you, then
please feel free to rework the code as needed. Don't be afraid of
breaking anything - I don't think the existing code has been tested at
all in the last N years, N > 5. [Any actual users of this please speak
up - here and now!]

Note that I would not attampt to support this feature today based on
the old plain uImage format. I would rather see this being based on a
FIT image.

Best regards,

Wolfgang Denk

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