Hello Wolfgang Am 03.12.2010 16:33, schrieb Wolfgang Denk: > Dear =?ISO-8859-1?Q?Matthias_Wei=DFer?=, > > In message <4cf90819.7040...@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.
I understand this behavior. But it is only true if the memory area in question is marked as cacheable. >> 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? Please see arch/arm/lib/cache-cp15.c The code there creates 4096 page table entries (1MB each) for the whole 4GB address space and initializes each entry in a way that it is not cacheable (mmu_setup():71). It then changes the page table entries which are pointing to a RAM area to make these, and only these, cacheable (dram_bank_mmu_setup():57). If the whole address space would be cached I would expect that even writing to NOR flash fails as the write accesses wouldn't reach the flash chip. But that works perfect on both of my systems. Thanks Matthias Weißer _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
