On Mon, 2 Jul 2012 09:19:24 +0800, Daniel Kurtz wrote: > On Mon, Jul 2, 2012 at 5:20 AM, Jean Delvare <khali@xxxxxxxxxxxx> wrote: > > I've done some performance measurements, and it turns out that, with the > > version of this patch modified by me, all short transactions are twice > > as slow as before. This is because i801_transaction waits twice now: > > once for BUSY to be clear, and then again once for INTR to be set. > > Does a fast sequence of such transactions actually take any longer? Or > just a single short transaction? Both are affected, this is the problem. See: Original driver: # time i2cdump -y 8 0x2f b >/dev/null real 0m0.157s user 0m0.000s sys 0m0.010s After this patch: # time i2cdump -y 8 0x2f b >/dev/null real 0m0.279s user 0m0.003s sys 0m0.011s This is on a fast machine with recent kernel. On my ICH3-M laptop with kernel 2.6.32, the slowdown for a full register dump is from 2 seconds to 4 seconds. Big performance regression. > My understanding is that the INTR wait is really waiting for the > entire transaction to complete (ie., including i2c STOP condition), > not just the byte transfer phase. This is my understanding as well, but I'm fairly certain that this is the case of the BUSY flag as well. I think BUSY gets cleared at the same time INTR (or any of the error status bits) gets set. Which is why I think checking BUSY is redundant. As a matter of fact, we ignore BUSY completely in i801_block_transaction_byte_by_byte(), so I see no reason why we couldn't do the same in i2c_transaction(). > By waiting here at the end of a transaction, we make sure the status > bits are actually clear before starting the next transaction. I have no objection to clearing the status bits, simply I think the sequence is wrong. I'll write and post a RFC patch later today illustrating what I think should be done. -- Jean Delvare -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
