Re: [GIT PULL] omap changes for v2.6.39 merge window

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On Wed, 30 Mar 2011, Nicolas Pitre wrote:

On Wed, 30 Mar 2011, david@xxxxxxx wrote:

On Wed, 30 Mar 2011, Nicolas Pitre wrote:


this means that you need to have some group doing the equivalent of assigning
device numbers for the different devices (and in this case going just a little
further to define what setup parameters will be needed), initially this may be
a little rough, but after a very short time I would expect the people doing
this work to start recognising that even though vendor A who first proposes
this device has some things hard-wired, the definition format should support
these things as variables instead of being assumed.

Ideally, yes.  but if every vendor has a different set of peripherals,
and from one SOC revision to the next from the same vendor you still
have different hardware knobs, then you still have to add yet more code
to the kernel.  And that doesn't solve the issue of dynamic clock and
power management at runtime either for which custom code is still
required.

As long as SOC vendors keep producing wildly different architectures
besides the core CPU we'll have this problem.  Denying the reality won't
make that problem go away either.  And device tree won't stop those
vendor from still trying to do things differently (better?) because they
are not constrained by having to ensure this single proprietary software
stack still boot.

the thing that you are not convincing us of is that all these different SoCs are so wildly different architectures.

back in the early days of the PCs, different systems from different vendors had different bus types, peripherals at different addresses, etc. that didn't make all of those vendors systems different architectures, instead those things were varients of the x86 architecture.

with ARM you do have a couple different architectures (arm5 vs arm7 for example), but what you are hearing people say is that

arm7+IPblock1+IPblock2
arm7+IPblock1+IPblock3>
arm7+IPblock2+IPblock3>

are not three different architectures, they are one architecture with different devices attached.

what's more, you seem to be saying that

arm7+IPblock1

and

arm7+IPblock1

are different architectures if the wiring between the arm core and IPblock1 are different (they are different 'boards' or different chip models, possibly from different manufacturers)

I see the variations here as a good thing, just like having a huge number of pluggable cards in a PC was a good thing (even though it made it hard to have an OS that supports every card out there)

in the case of the PC, systems that were too different died off, systems that could have their differences abstracted into different drivers prospered.

I am _not_ saying that all arm systems need to standardize on one interrupt controller, I am saying that the kernel support for ARM needs to be able to _easily_ be told that this chip has interrupt chip type 24 connected this way, and interrupt chip type 87 connected that way, without needing to create a new architecture. If the kernel is compiled with the appropriate drivers, it should even be able to be done without needing to recompile the kernel.

Now I understand that this isn't how things are done today in ARM, but that's not how things were done 10 years ago in x86 either. back then you had kernels compiled specifically for each system. nowdays that is still done where space or performance is critical, but a huge number of systems sacrafice a few % of speed, and some storage and ram for the flexibility and supportability of using a one-size-fits-many kernel (along with a large number of loadable modules)

why can't ARM do this?


look at serial port support on x86.

while serial ports are becoming rare nowdays, the kernel has support for _many_ different types of ports, and all of the port types support being wired up in different ways (to different addresses and interrupts)

the kernel could have gone the route that ARM did of having a master config that listed every system known and where it's serial ports were wired, but thanks to the fact that many of these were plug-in options, that would have been painful enough that it drove them to do it the right way.

take ARM down a similar path, treat the on-chip devices in a similar way to off-chip devices.

define the different types of GPIO behavior in arch-level drivers and make the chip-level (or board level) definitions say "I have a type 6 GPIO device wired this way" instead of including an entire GPIO driver in that definition.


what is happening in ARM is being driven by the short-term ease of the chip manufacturers, they do things any way they want, and their engineers cut-n-paste their way to make things work as a new architecture.

in the long run, making things more flexible and easier for the device designers and the people modifying the designs down the road will grow the pie by making it much easier for people to drop an ARM onto a new design, add a smattering of random (to the chip manufacturer) chips to do various things, and get linux running on the result.

get it down to where a new board can be designed by a guy in his garage, with a linux ARM distro able to run out of the box on it (with the appropriate definitions of how the guy wired it), and the variations will proliferate to the point where the current variation in ARM will look trivial by comparison, and the result will be even more use (and therefor sales) of ARM chips in all sorts of things that nobody can imagine today.




I'm working on projects that are small embedded systems, right now it's a big-boy's game. If you want to do a small project you better copy the reference board pretty closely or have a lot of kernel hacking ability (along with the ability to convince the management types that you can do this). In my project I lost the argument and instead of using ARM and linux the people funding things opted to go with a propriatary OS from the chip vendor instead. this maze is costing users and developers.

Turn things around and make it easier to have more variations and you will get more support from all directions.

David Lang
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