On Thu, 2014-03-20 at 11:59 +0000, David Laight wrote: > I tried to work out what the 'twi, isync' instructions were for (in > in_le32()). > The best I could come up with was to ensure a synchronous bus-fault. > But bus faults are probably only expected during device probing - not > normal operation, and the instructions will have a significant cost. > > Additionally in_le32() and out_le32() both start with a 'sync' instruction. > In many cases that isn't needed either - an explicit iosync() can be > used after groups of instructions.
The idea is that it's better to be maximally safe by default, and let performance critical sections be optimized using raw accessors and explicit synchronization if needed, than to have hard-to-debug bugs due to missing/wrong sync. A lot of I/O is slow enough that the performance impact doesn't really matter, but the brain-time cost of getting the sync right is still there. -Scott _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
