[U-Boot] [PATCH] 0/12 Multiadapter/multibus I2C

[Wolfgang Denk]

Dear ksi at koi8.net,

In message <Pine.LNX.4.64ksi.0902152236430.17769 at home-gw.koi8.net> you wrote:
> 
> Yep. But nobody's perfect and you can have a situation when you need to
> access several busses before relocation. It is not hardware for U-Boot, it
> is U-Boot for hardware. When hardware designers design their hardware they
> don't make their decisions based on U-Boot limitation. That is us who should
> accomodate what they designed.

We don't have to make the mainline U-Boot implementation  unnecessary
complex  just  for  the  small  chance  that there is sombeody stupid
enough to design a broken system.

It is our task as software engineers to tell the  hardware  designers
which  designs  are  easy to support, nd which will cause problems in
the software.

> There is also another consideration -- when having several adapters which
> one should be initialized at boot time, before relocation? Another problem

The one that is needed there, if any at all.

> is init() function that can be unique for each adapter. To make the lower
> layer transparent I'm reprogramming muxes if any when switching busses. It
> is necessary to make I2C API simple and uniform between muxed and non-muxed
> busses. That essentially means that we can NOT do i2c_set_bus_num() to
> execute init() for a particular adapter -- adapter MUST be initialized for
> i2c_set_bus_num() to succeed.
> 
> Your suggestion requires total LOGIC change.

Maybe. But that's why we're discussing this here.

> > Is this needed? If so, you must before call a i2c_set_bus_num(), and after
> > you finished call it again with the old busnumber. So it is done for example
> > in do_date () common/cmd_date.c
> 
> You can not do it before all adapters are initialized. And you WON'T be able
> to initialize adapters because you will not be able to switch busses.

This sounds like a design problem to me, then.

Please keep in  mind  that  according  to  U-Boot  philosophy  it  is
forbidden to always initialize all adapters. Only those actually used
by U-Boot may be initialized, and shall be de-initialized after use.

> > Yes, thats a point. But do we need this before running from ram (except
> > one hardwareadapter)?
> 
> Yes, see above.

Um... Maybe I missed something - did you give an example (except for
broken designs) where this really might be needed?

> > Yes, I know. But again, do we need this?
> 
> We do. Otherwise we can essentially throw everything to trash and start
> over. This requires changing the logical design, architecture. And this is
> that logic that is most difficult and takes most thinking. Coding is easy.

You say: "we do [need that]". I ask: why? what for?

> Another reason why macros are used is speed. Not everyone is running U-Boot
> on 10 GHz Pentium-9 with gigabyte of cache. In bitbanged I2C every
> instruction counts if you want to run a bus at a decent speed (I won't even
> start with regular 100kHz less for 400kHz; 50kHz would be very good.) Your

Are you sure? If I remember correctly soft-I2C can even run 400 KHz on
a slow 50 MHz MPC8xx system.

Do you have other numbers?

> must be executed. And remember, there is probably no instruction cache and
> you're running off of flash so every instruction fetch is something like

I tend to say that an U-Boot port where instruction cache is disabled
is misconfigured and should be fixed :-)

Best regards,

Wolfgang Denk

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