DTS files, 83xx, clock frequencies

DTS files, 83xx, clock frequencies

[Russell McGuire]

All,

I note that in most of the 83xx dts config files that the different
frequency fields are being populated with non-zero numbers. i.e. bus-freq,
clock-freq, core-freq, etc...

Isn't default behavior that these fields are kept from the <BIOS> U-boot
during boot, if a zero is present? Is there any advantage to resetting them
within the dts file during linux boot up? Or is it necessary to populate
these values for correct operations, specifically with in the QUICC Engines?

Over all my curiosity is that between versions of the board, i.e. different
CPU's that may be clocked faster or slower, I see no reason to change the
dts/blobs if I already have to change the BIOS to correctly boot up.


-Russ

Re: DTS files, 83xx, clock frequencies

[Scott Wood]

Russell McGuire wrote:
> I note that in most of the 83xx dts config files that the different
> frequency fields are being populated with non-zero numbers. i.e. bus-freq,
> clock-freq, core-freq, etc...
> 
> Isn't default behavior that these fields are kept from the <BIOS> U-boot
> during boot, if a zero is present?

U-boot overwrites those values regardless of whether they were zero in 
the dts.

> Is there any advantage to resetting them
> within the dts file during linux boot up?

No, and it doesn't.  If you're looking arch/powerpc/boot, then that's 
for compatibility with non-device-tree-aware u-boots.

-Scott

Re: DTS files, 83xx, clock frequencies

[Timur Tabi]

Russell McGuire wrote:

> Isn't default behavior that these fields are kept from the <BIOS> U-boot
> during boot, if a zero is present? Is there any advantage to resetting them
> within the dts file during linux boot up?

Depending on which version of U-Boot, a given property may or may not be 
initialized by U-Boot.  The problem is that as new properties are defined, 
U-Boot is not always updated to initialize that property, and sometimes it's 
only updated on some CPU families.

The qe/brg-frequency property is a good example.  Prior to U-Boot 1.3, it was 
not being initialized at all.  With U-Boot 1.3, it's only initialized on 83xx, 
even though some 85xx boards have a QE.  I have a QE UART driver that looks at 
qe/brg-frequency, and if it's non-zero, I use it.  If it is zero, then I take 
the qe/bus-frequency property and divide it in half.


-- 
Timur Tabi
Linux kernel developer at Freescale