From mboxrd@z Thu Jan 1 00:00:00 1970 From: mturquette@linaro.org (Mike Turquette) Date: Tue, 02 Apr 2013 17:38:30 -0700 Subject: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST In-Reply-To: <4b6b9df8-e221-4e52-82d3-efdcd5de9bd1@AM1EHSMHS021.ehs.local> References: <20130320001609.8663.21043@quantum> <20130320185051.GA28349@pengutronix.de> <20130321091531.GN20530@pengutronix.de> <20130327013703.4014.45615@quantum> <4b6b9df8-e221-4e52-82d3-efdcd5de9bd1@AM1EHSMHS021.ehs.local> Message-ID: <20130403003830.8177.40547@quantum> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Quoting S?ren Brinkmann (2013-04-01 16:24:12) > On Tue, Mar 26, 2013 at 06:37:03PM -0700, Mike Turquette wrote: > > Quoting S?ren Brinkmann (2013-03-26 15:45:22) > > > On Thu, Mar 21, 2013 at 10:15:31AM +0100, Uwe Kleine-K?nig wrote: > > > > Hello, > > > > > > > > On Wed, Mar 20, 2013 at 07:50:51PM +0100, Sascha Hauer wrote: > > > > > On Wed, Mar 20, 2013 at 09:32:51AM -0700, S?ren Brinkmann wrote: > > > > > > If the caller > > > > > > doesn't like the returned frequency he can request a different one. > > > > > > And he's eventually happy with the return value he calls > > > > > > clk_set_rate() requesting the frequency clk_round_rate() returned. > > > > > > Always rounding down seems a bit odd to me. > > > > > > > > > > > > Another issue with the current implmentation: > > > > > > clk_divider_round_rate() calls clk_divider_bestdiv(), which uses the ROUND_UP macro, returning a rather low frequency. > > > > > > > > > > And that is correct. clk_divider_bestdiv is used to calculate the > > > > > maximum parent frequency for which a given divider value does not > > > > > exceed the desired rate. > > > > The reason for that is that the (more?) usual constraint is like: This > > > > mmc card can handle up to 100 MHz. Or this i2c device can handle up to > > > > this and that frequency. Of course there are different constraints, e.g. > > > > for a UART if the target baud speed is 38400 you better run at 38402 > > > > than at 19201. > > > > > > > > I wonder if it depends on the clock if you want "best approximation <= > > > > requested value" or "best approximation" or on the caller. In the former > > > > case a flag for the clock would be the right thing (as suggested in this > > > > thread). If however it's the caller of round_rate who knows better which > > > > rounding is preferred than better extend the clk API. > > > > > > > > Extending the API could just be a convenience function that doesn't > > > > affect the implementations of the clk API. E.g.: > > > > > > > > long clk_round_rate_nearest(struct clk *clk, unsigned long rate) > > > > { > > > > long lower_limit = clk_round_rate(clk, rate); > > > > long upper_limit = clk_round_rate(clk, rate + (rate - lower_limit)); > > > > > > > > if (rate - lower_limit < upper_limit - rate) > > > > return lower_limit; > > > > else > > > > return upper_limit; > > > > } > > > > > > > I guess both approaches may work. Anybody has a preference? > > > > > > > A dedicated function like the one Uwe defined is better than adding > > subtlety to the existing clk_round_rate via a flag in a clock driver. > I looked at my problem again. > > A new API function is probably fine for UART, ethernet drivers and > similar. Although, compared to a flag it would add some redundant > rounding, since clk_set_rate() implicitly also rounds the rate. > clk_set_rate() > clk_calc_new_rates() > clk_round_rate() > But that is true for every driver which doesn't blindly call > clk_set_rate() and checks upfront through clk_round_rate() what > the actual frequency would look like. > > So, do we agree to add this additional clk_round_rate_nearest() > function? Cc'ing Russell if he has any comments on adding clk_round_rate_nearest, since he is author of the original clk api. Regards, Mike > And if, should I just make Uwe's proposal another patch, additionally to > the other clk-divider change I'm working on? > Or Uwe, do you prefer to submit it yourself? > > > For my original problem, though, this is only part of a solution. It > appeared to be a rounding issue, but the actual root cause is the loss > of resolution when OPPs are converted to a frequency table for cpufreq. > I'm not sure how this can be resolved, yet. > > > S?ren