Joakim Tjernlund wrote:
[1] Sync is a big hammer, eieio is a medium size hammer. Don't use both, PPC people will know you don't know what you are doing. ;-)
Yet the in_bex()/out_bex() functions in PowerPC linux uses sync and all SOC drivers are encouraged to use them. What a waste :(
Jocke
Well, I was a little terse because I was cross-applying PPC instructions in a ARM discussion. Personally, I prefer to use sync vs. eieio. The size of the hammer isn't that different, I don't believe. The advantage of sync is that it flushes the read/write operation out on the bus *now*. When I'm writing to hardware to control the hardware, *now* is what I want.
The eieio merely guarantees the preceding operations will go out on the bus before following bus operations. The preceding operations could be hung up in the bus interface unit *indefinitely* if no "following" bus operations occur. This is an unlikely occurrence, but could be the result of running out of cache in a tight loop.
For instance, if you do a write to a hardware register, a eieio, and then wait in a tight loop until time goes by (reading the decrementer register) followed by another write, the write1/delay/write2 sequence could actually be delay/write1/write2. Note that the order of the writes on the bus is correct per the eieio, but it isn't what the hardware *needed*.
Illustration - what you did in code and intended to occur: write1 (eieio) delay write2 (eieio)
What actually may happen on the bus: delay write1 (eieio) write2 (eieio)
By using a sync, you guarantee the write isn't delayed: write1 (sync) delay write2 (sync)
Disclaimer: the above explanation is from my fertile imagination. It may or may not happen and *will* happen differently on different PPC processors. For instance, the eieio instruction is actually a NOP in the 603e core because it doesn't reorder bus operations, but it *does* have a BIU that can buffer and delay the bus operations, causing the above timing problem.
I contend using the "sync" instruction will always work correctly and the "eieio" instruction is at best a false economy and at worst a lot of very difficult, mysterious bugs to find, so I'm in agreement with the linux in_bex/out_bex recommendation.
Side note: I don't know if I communicated it properly, but when you see "eieio ; sync" or "sync ; eieio", you know the author of that code doesn't understand sync and eieio. "isync ; sync" is occasionally a valid combination, but I don't believe it is necessary other than when called for by the Users Manual in conjunction with writing to special purpose registers.
Best regards, gvb