From mboxrd@z Thu Jan 1 00:00:00 1970 From: arnd@arndb.de (Arnd Bergmann) Date: Fri, 10 Oct 2014 21:50:23 +0200 Subject: [PATCH v4 1/2] iopoll: Introduce memory-mapped IO polling macros In-Reply-To: References: <1412126893-15796-1-git-send-email-mitchelh@codeaurora.org> <2434048.k6JtGScJfe@wuerfel> Message-ID: <1922221.DKAGMVXrIn@wuerfel> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. 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. Arnd