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

[Albert ARIBAUD]

Le 03/12/2010 16:33, Wolfgang Denk a écrit :

>> 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.

In addition to making sure that register reads/write are not bitten by 
caching, note that some controllers have DMA capabilities which require 
proper cache handling for DMA memory buffers -- typically flushing them 
from cache before a DMA to the device, and invalidating their cache 
entries after a DMA from the device.

> Best regards,
>
> Wolfgang Denk

Amicalement,
-- 
Albert.