From mboxrd@z Thu Jan 1 00:00:00 1970 From: mitchelh@codeaurora.org (Mitchel Humpherys) Date: Fri, 10 Oct 2014 13:24:18 -0700 Subject: [PATCH v4 1/2] iopoll: Introduce memory-mapped IO polling macros In-Reply-To: <1922221.DKAGMVXrIn@wuerfel> (Arnd Bergmann's message of "Fri, 10 Oct 2014 21:50:23 +0200") References: <1412126893-15796-1-git-send-email-mitchelh@codeaurora.org> <2434048.k6JtGScJfe@wuerfel> <1922221.DKAGMVXrIn@wuerfel> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Oct 10 2014 at 12:50:23 PM, Arnd Bergmann wrote: > On Friday 10 October 2014 12:44:45 Mitchel Humpherys wrote: >> >> Regarding the division, for the overwhelmingly common case where the >> >> user of the API passes in a constant for sleep_us the compiler optimizes >> >> out this calculation altogether and just sticks the final result in (I >> >> verified this with gcc 4.9 and the kernel build system's built-in >> >> support for generating .s files). Conveying semantic meaning by using >> >> `DIV_ROUND_UP' is nice but if you feel strongly about it we can make >> >> this a shift instead. >> > >> > The more important question is probably if you want to keep the _ROUND_UP >> > part. If that's not significant, I think a shift would be better. >> >> If we drop the _ROUND_UP then passing a sleep_us <= 4 would result in a >> minimum sleep time of 0, so we'd be polling a lot faster than the user >> had expected. > > How about changing the semantics to sleep at least the sleep_us time, > and at most four times that? This would turn the expensive division into > a multiplication and avoid the need for rounding. We already have a bunch of code using this and I'm not sure what the effect would be on the system by changing this. It would probably be negligible but saving a couple of instructions hardly seems like justification for a change... More importantly, I think users would rather poll their register a little quicker than they asked, rather than slower. > If there are important reasons to keep doing the division, you could > instead use '(sleep_us >> 4) + 1', which is also very cheap to compute > and avoids the problem you mention. But I think you meant `(sleep_us >> 2) + 1', right? Incidentally, that illustrates the benefit of the semantic clarity provided by explicitly dividing :). In any case, I'm happy to go with a shift here for v6. -Mitch -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project