[U-Boot] arm: Activating dcache breaks 'usb start' and 'tftpboot' on jadecpu

[Wolfgang Denk <wd@denx.de>]

Dear =?ISO-8859-1?Q?Matthias_Wei=DFer?=,

In message <4CF90819.7040904@arcor.de> you wrote:
> 
> >> Has anyone an explanation for this behavior? Is anyone out there having
> >> dcache running on an ARM926 and working usb/tftpboot?
> >
> > Many drivers have not been written to work with enabled caches.
> 
> What is the reason that special handling is needed when dcache is 
> enabled? If a driver doesn't use any DMA there should be no need as the 
> dcache is only enabled for the RAM and not for any memory mapped IO if I 
> understand the code in arch/arm/lib/cache-cp15.c right.

On ARM, device write accesses are typically just store instructions
(in C: assignments to a volatile pointer). With caches on, these
accesses will be - guess what? cached, i. e. they are NOT written to
the device, at least not immediately.  And if you repeatedly read a
register (like when polling for some status bit to change) these
accesses will be cached, too.

You need to make sure that caches are properly flushed / invalidated
at the right points.

> > As far as USB is concerned, you might be lucky that your system usies
> > a EHCI controller, so setting CONFIG_EHCI_DCACHE should help.
> 
> No, only OHCI.

Bad luck.

> As the memory mapped network controller (SMSC9221) is not cached it 
> shouldn't be a problem or do I miss something here?

You said you had enabled the data cache, so why do you think these
accesses are not cached?

Best regards,

Wolfgang Denk

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