[U-Boot] U-Boot's need for MMU

[U-Boot] U-Boot's need for MMU

[cmfairfa at rockwellcollins.com]

Hi,
I am wondering what the ramifications of not using the MMU in U-Boot are?
I have an MPC8360 (e300 core) where it is capable of disabling the MMU via 
the MSR[IR] and MSR[DR] bits.

Before disabling the MMU, I'm just wondering why U-Boot bothers to set up 
the MMU on CPU's that providing disabling of the MMU.

PS
I'm not disabling the MMU just for kicks, there are/may be some 
program-specific scenarios where I may need to use the option of disabling 
the MMU.

Thanks.



Christopher M. Fairfax
Sr. Software Engineer
Rockwell Collins
+1 540-428-3344
+1 540-428-3301
cmfairfa at rockwellcollins.com
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[U-Boot] U-Boot's need for MMU

[Scott Wood]

cmfairfa at rockwellcollins.com wrote:
> Hi,
> I am wondering what the ramifications of not using the MMU in U-Boot are?
> I have an MPC8360 (e300 core) where it is capable of disabling the MMU via 
> the MSR[IR] and MSR[DR] bits.
> 
> Before disabling the MMU, I'm just wondering why U-Boot bothers to set up 
> the MMU on CPU's that providing disabling of the MMU.

A big one is that you can't have certain regions be cached (memory) and 
others uncached (I/O) -- you'll need to turn the D-cache off altogether.

And even then I'm not sure what happens to the guarded bit when the MMU 
is off.

> I'm not disabling the MMU just for kicks, there are/may be some 
> program-specific scenarios where I may need to use the option of disabling 
> the MMU.

Such as?  One should generally lead with the reason they want to do 
something, and only afterwards ask "why not". :-)

-Scott