Re: 7447A DFS support for PowerBook G4

[Michael <macallan%netbsd.org@localhost>]

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Hello,

On Oct 9, 2010, at 2:32 PM, Frank Wille wrote:

> here is a small patch for macppc/dev/obio.c, which enables cpu speed  
> control
> for 7447A PowerBooks. In contrast to other Intrepid-based  
> PowerBooks, which
> can change the cpu speed with two GPIO registers (frequency-gpio and
> voltage-gpio), this one uses the DFS2 bit of the 7447A's HID1  
> register to
> halve the frequency.
>
> This is a special feature of the 7447A called Dynamic Frequency  
> Switching.
> The 7448 even has an additional DFS4 bit to divide the frequency by  
> 4 (are
> there any popular systems equipped with 7448?).
>
> For the user it is completely transparent. There is no difference to  
> other
> PowerBooks. Just compile a kernel with option OBIO_SPEED_CONTROL and  
> use
> the machdep.cpu_speed sysctl to configure the cpu into half or full  
> speed.
>
> Problems to be resolved until I commit the patch:
>
> 1. What I don't like is reading/writing to HID1 within obio.c. This  
> should
> probably go into powerpc/oea/cpu_subr.c? I'm open to any suggestion  
> about a
> cleaner implementation.

This should go into cpu-specific code since it has nothing whatsoever  
to do with obio. Both should provide an interface that powerd can  
understand ( yes, I need to do that for obio ;) )
And just to make things worse there is nothing to prevent a given  
machine to have both.

> 2. What about multiple cpus? Do systems with two 7447A exist? The  
> DFS2 bit
> would have to be set for both. I don't know how do to it yet.

Probably, at least there should be SMP-capable CPUs which support DFS.

> 3. The PowerBook starts with DFS2 set (i.e. half speed). So the cpu- 
> speed is
> calculated wrong. Should that be fixed somehow? For example by  
> taking the
> status of DFS2 into account and double the measured frequency when  
> set?

Is it always halved or is the divider somehow machine- or CPU model  
dependent?

have fun
Michael

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