[U-Boot-Users] Pull request: u-boot-freebsd

[Marcel Moolenaar]

On Dec 17, 2007, at 9:17 AM, Rafal Jaworowski wrote:

>>  * How is this really different from the existing jumptable stuff? It
>>    looks like it's just a different set of exported functions. Will
>>    the crufty old jumptable interface be removed at some point?
>>    Presumably, the new interface is superior, so it should be. Right?

Let me chime in for this, quickly, because we did try to use the
jump table and it proved to be useless. As Rafal pointed out, the
problem with using the jump table is that:
1) getting to the jump table requires that you know the layout of
    the global_data structure, because it's at the end. The layout
    of the structure is highly variable due to the optional fields.
    As such, in order to use the jump table, you need to configure
    the "client" program in the same way as U-Boot, just to make
    it work. This is especially frustrating if the client program
    itself has, inherently, no configurable options and you need it
    to run on a couple of flavors of boards that need slightly
    different U-Boot configurations.
2) The jump table itself is not fixed in that there are optional
    entries. On top of that, the jump table has no way to account
    for extensions or growth, because there's no function you can
    call that gives you a version or some sorts.

The first point can easily be fixed by putting the jump table first
or at least at the head of the global_data structure. The second
point can partially be fixed by making all entries non-optional
and provide stubs in case a function has no implementation in a
certain configuration. This however would be against the basic
design philosophy of U-Boot, so would by itself raise concerns.

More importantly, the crux of the second point could not be
addressed and that's typically where the problems arise going
forward. In fact, we knew that we were going to run into problems,
because the current jumptable did not have the interfaces we'd
like to see and there were even ones we didn't need right from the
start, but knew we'd want within a year. And, once this has become
a known interface, who knows what people want to do with it?

The bottom line was that while the existing jump table would get
us 80% of the way, getting that additional 20% would be impossible
without doing exactly something along the lines of what Rafal did.

I fully realize that for developers who's focus is U-Boot, anything
that runs on top of U-Boot and that wants something from U-Boot is
a burden at times, but it is important to realize that U-Boot is
most of the time nothing more than the first step towards a useful
machine and that there can be more than 2 steps in getting on OS
to run. The ability to make use of the (hardware) knowledge that
U-Boot has, and at the same time its ability to abstract it for
the next steps, is in my opinion just as important as having its
own prompt.

Just my $0.02

-- 
Marcel Moolenaar
xcllnt at mac.com