On 2014/1/22 19:42, Russell King - ARM Linux wrote: > On Wed, Jan 22, 2014 at 07:25:14PM +0800, Wang Nan wrote: >> This patch relaxes the restriction set by commit 309caa9cc, which >> prohibit ioremap() on all kernel managed pages. >> >> Other architectures, such as x86 and (some specific platforms of) powerpc, >> allow such mapping. >> >> ioremap() pages is an efficient way to avoid arm's mysterious cache control. >> This feature will be used for arm kexec support to ensure copied data goes into >> RAM even without cache flushing, because we found that flush_cache_xxx can't >> reliably flush code to memory. > > Yes, let's bypass the check and allow this in violation of the > architecture specification by allowing mapping the same memory with > different types, which leads to unpredictable behaviour. Yes, that's > a very good idea, because what we want to do is far more important than > following the requirements of the architecture. > > So... NAK. > > Yes, flush_cache_xxx() doesn't flush back to physical RAM, that's not > what it's defined to do - it's defined that it flushes enough of the > cache to ensure that page table updates are safe (such as when tearing > down a page mapping.) So it's hardly surprising that doesn't work. > > If you want to be able to have DMA access to memory, then you need to > use an API which has been designed for that purpose, and if there isn't > one, then you need to discuss your requirements, rather than trying to > hack around the problem. So what is correct API which is designed for this propose? > > The issue here will be that the APIs we currently have for DMA become > extremely expensive when you want to deal with (eg) all system RAM. > Or, there's flush_cache_all() which should flush all levels of cache > in the system, and thus push all data back to RAM. > > Now, why are you copying your patches to the stable people? That makes > no sense - they haven't been reviewed and they haven't been integrated > into an existing kernel. So, they don't meet the basic requirements > for stable tree submission... >
