On Fri, Sep 07, 2018 at 04:13:35PM +0200, Arnd Bergmann wrote: > On Fri, Sep 7, 2018 at 2:55 PM Guo Ren <ren_guo@xxxxxxxxx> wrote: > > > > On Fri, Sep 07, 2018 at 10:14:38AM +0200, Arnd Bergmann wrote: > > > On Fri, Sep 7, 2018 at 5:04 AM Guo Ren <ren_guo@xxxxxxxxx> wrote: > > > > On Thu, Sep 06, 2018 at 04:31:16PM +0200, Arnd Bergmann wrote: > > > Similarly, an MMIO read may be used to see if a DMA has completed > > > and the device register tells you that the DMA has left the device, > > > but without a barrier, the CPU may have prefetched the DMA > > > data while waiting for the MMIO-read to complete. The __io_ar() > > > barrier() in asm-generic/io.h prevents the compiler from reordering > > > the two reads, but if an weakly ordered read (in coherent DMA buffer) > > > can bypass a strongly ordered read (MMIO), then it's still still > > > broken. > > __io_ar() barrier()? not rmb() ?! I've defined the rmb in asm/barrier, So > > I got rmb() here not barrier(). > > > > Only __io_br() is barrier(). > > Ah right, I misremembered the defaults. It's probably ok then. Thx for the review and comments. These let me re-consider the mmio issues and help to improve the csky asm/io.h in future. > > > > > > - How does endianess work? Are there any buses that flip bytes around > > > > > when running big-endian, or do you always do that in software? > > > > Currently we only support little-endian and soc will follow it. > > > > > > Ok, that makes it easier. If you think that you won't even need big-endian > > > support in the long run, you could also remove your asm/byteorder.h > > > header. If you're not sure, it doesn't hurt to keep it of course. > > Em... I'm not sure, so let me keep it for a while. > > Ok. I think overall the trend is to be little-endian only for most > architectures: powerpc64 moved from big-endian only to little-endian > by default, ARM rarely uses big-endian (basically only for legacy > applications ported from BE MIPS or ppc), and all new architectures > we added in the last years are little-endian (OpenRISC being the > main exception). Good news, I really don't want to support big-endian and it makes CI double. Best Regards Guo Ren
