Re: Question on mpc52xx_common.c

[Robert Schwebel <r.schwebel@pengutronix.de>]

Hi Grant,

On Tue, Apr 08, 2008 at 03:26:11PM -0600, Grant Likely wrote:
> I disagree and that is not my point.

Well, having something like a device tree that describes the hardware is
definitely a good thing, in general.

> Now, if out-of-tree ports continue to break then we've got a problem
> that needs to be fixed.

Right, that's the point. Don't get me wrong, I'm deeply against
maintaining large code bases out-of-tree. But it shows in this case that
we have a maintenance problem with the current oftree concept.

When a simple BSP that supports almost nothing than CPU, mem, serial
console and flash needs > 1 week of hacking from an experienced kernel
hacker, just in order to get all the string changes right, something is
wrong.

> Once a binding is established (which usually takes a few kernel
> releases) it should be very stable and even if the definition of ideal
> is changed, backwards compatibility must be maintained.

That's theory, but in practise we see it changing every second day.

> >  The ARM method of using just a device number is so much easier ...
>
> Only if the assumption is made that very little data needs to be
> shared between the kernel and the firmware. The moment you try to do
> something more complex you either have the nightmare of bd_info or you
> use a structured data format (like the device tree)

I agree that the usual ARM hardwares are much less complicated and
configurable than what's available elsewhere. We usually need the
information "this is board FOO_BAR", maybe, if it is a module, on a
"BAZ" baseboard, and this is everything we need to register the platform
devices in some arch/arm/mach-*/my_cpu.c and arch/arm/mach-*/my_board.c
file.

> On another node, there are platforms where a device number is
> unworkable. For example, for Linux on an FPGA like the Xilinx Virtex,
> there would need to be a new platform number every time the FPGA
> bitstream was updated because it is effectively an entirely different
> platform.

Well, we have done FPGA based boards with ARM in the past, and we've
just added hardware auto-detection to the IP cores.

> Finally, using a device number means you need to encode into the
> kernel the exact layout of every platform it supports.  That adds up
> to a lot of code in a real hurry; even if most of it is just
> boilerplate instantiations.

You are right in general. However, it doesn't change the fact that we
are living in maintenance-nightmare land right now ...

Robert
-- 
 Dipl.-Ing. Robert Schwebel | http://www.pengutronix.de
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