wd at denx.de
Mon Oct 15 01:44:48 EST 2001
in message <15304.9109.488577.697206 at cargo.ozlabs.ibm.com> you wrote:
> Am I right in thinking that you are the author of PPCBoot?
No exactly: PPCBoot is an Open Source project and has many roots and
authors; but I'm coordinating all the efforts.
> The thing about the wrappers is that they are all maintained in the
> one tree along with the kernel. If we want to make a change to the
> interface between the wrappers and the kernel we can easily do that in
> a coordinated fashion, without creating any compatibility problems -
> old kernels naturally use the old wrapper and new kernels use the new
> wrapper. The wrapper can provide any translation necessary between
> what the firmware passes in and what the kernel expects.
That's fine. But there other things to keep in mind, too:
* Especially with embedded systems the memory footprint is often
critical. Adding an extra bootstrap loader which includes it's own
versions of functions like serial console driver and de-compression
code is just wasted memory.
In systems where a reliable remote software upgrade must be guaran-
teed you might find even more than one kernel image in the system -
then the effect of this waste of memory multiplies.
* When porting Linux to new hardware the wrapper adds an additional
layer which must be adapted and ported. If you have PPCBoot running
on a board, all that's needed to "port" Linux to that board is
copying things like ethernet intialization from the PPCBoot sources
- since PPCBoot is based on Linux device driver code, this is a
simple copy & paste.
* With PPCBoot, you can use the same kernel image on different boards
without need to recompile it just because you want differend default
* With PPCBoot, you can use the same kernel image in combination with
different ramdisk images without need to recompile it.
Etc. etc. It was one of our design goals with PPCBoot to get rid of
all the (IMHO) unneccessary maintenance of the bootstap loader code.
I can understand that the wrappers make sense for boards that come
with some firmware that cannot or shall not be changed; but this is
the only reason I see for it's existence.
Your argumentation sounds as if you expect many frequent changes of
the kernel boot interface. Is that true? In sucha case I'd suggest we
spend some more thoughts on that interface before changing it.
> That's great, but I wonder what happens when someone upgrades their
> PPCBoot to your new version and then wants to boot the old 2.4.2
> kernel that they have lying around to check whether it shows a
> particular problem they're chasing.
This is not the first incompatible change we've seen: for instance
the switch from passing the clock frequencies in HZ (instead of MHz
as it was before Patch 188.8.131.52) is such an incompatible change of the
We will deal with it.
> If we absolutely *have* to boot a vmlinux directly without a wrapper,
> then probably the best thing is to use bi_recs and pass in a pointer
> to them, since that is reasonably flexible and can be made to be
> forwards and backwards compatible without too much pain. Changing to
It was my assumption from the previous discussions that this was
going to happen. I like it because you have the option to chose the
implementation that fits best: if a wrapper seems appropriate it can
be used easily; if another solution seems simpler this can be used,
too. Enforcing want to be forced to use of a wrapper in all cases.
> this scheme is going to cause considerable pain in the short term,
I don't see much difference to the status quo: f or all the systems
using a wrapper it is a transparent change like any other; for the
other systems it's a new, incompatible kernel interface just like
what happened with the Mhz / Hz change.
Software Engineering: Embedded and Realtime Systems, Embedded Linux
Phone: (+49)-8142-4596-87 Fax: (+49)-8142-4596-88 Email: wd at denx.de
"We don't care. We don't have to. We're the Phone Company."
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