[U-Boot-Users] disabling d-cache in 'bootelf' for QNX

[Matthias Fuchs <matthias.fuchs@esd-electronics.com>]

Hi,

there is a simple approach to disable dcaching on 440 used for the 440 usb code (cpu/ppc4xx/usb.c).
The code is used to disable dcaching before starting the USB subsystem. It updates the tlb entry.
But it is not complete working. Disbling works, but reenabling (via 'usb stop') does not work.
But with CONFIG_4xx_DCACHE enabled this at least allows you to use USB but with disabled 
dcache from that moment on.

Perhaps it is possible to move the change_tlb() call into the dcache_disable when it is 
working sometime. 

Matthias

On Sunday 06 January 2008 15:11, Rafal Jaworowski wrote:
> Josh Boyer wrote:
> >>>> Yeah, after a second thought I tend to agree. Maybe the way to go is doing
> >>>> data cache flush from within dcache_disable() properly i.e. bring it in for
> >>>> arch variations that don't do it currently like 85xx... Actually to confirm
> >>>> my observations I tested a working patch that flushes d-cache at
> >>>> cache_disable() just like 86xx and it works for me, this is: my problems
> >>>> disappear. Do you think this is a better option?
> >>> Yes, I think this is the way to go. Please provide a patch and send it to the 
> >>> 85xx maintainer. Best would be if you could check the other ARCH's (at least 
> >>> PPC) for this dcache_disable() behaviour too.
> >>>
> >> I checked, and only 7xx/74xx, 86xx and 4xx do it properly. For all other PPC
> >> variants cache_disable() does not flush d-cache before disabling it... So the
> >> problem is quite widespread. I'll try to come up with something generic, but
> >> am not sure if it'll fit every other variant.
> > 
> > Out of curiosity, how do you disable dcache on 44x?  The only way I
> > know how to turn the cache off on 440 is to set the Guarded bit in the
> > TLB entries.  There is no cache disable bit.
> > 
> 
> I believe this is how "disabling" is achieved currently in U-Boot: the DRAM
> region is TLB-mapped with the G bit set. But I was rather referring to
> introducing a d-cache flush as a first step of dcache_disable() in cases it is
> not there, and not re-working the disable sequence itself. 440 is special
> about this and we probably have to live with d-cache being either always
> present or "disabled" via G bit.
> 
> > (And from what I see, dcache_disable, icache_disable, and icache_enable
> > all just simply return without doing anything on 440).
> > 
> 
> I mistyped: it is 40x that does flushing before disabling d-cache; on 440, as
> you say, there are stubs for these ops.
> 
> kind regards,
> Rafal