From mboxrd@z Thu Jan 1 00:00:00 1970 From: Richard Cochran Subject: Re: [PATCH 7/9] net: ethernet: ti: cpts: calc mult and shift from refclk freq Date: Wed, 14 Sep 2016 22:26:19 +0200 Message-ID: <20160914202618.GC12195@netboy> References: <20160914130231.3035-1-grygorii.strashko@ti.com> <20160914130231.3035-8-grygorii.strashko@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20160914130231.3035-8-grygorii.strashko@ti.com> Sender: linux-kernel-owner@vger.kernel.org To: Grygorii Strashko Cc: "David S. Miller" , netdev@vger.kernel.org, Mugunthan V N , Sekhar Nori , linux-kernel@vger.kernel.org, linux-omap@vger.kernel.org, WingMan Kwok List-Id: linux-omap@vger.kernel.org On Wed, Sep 14, 2016 at 04:02:29PM +0300, Grygorii Strashko wrote: > +static void cpts_calc_mult_shift(struct cpts *cpts) > +{ > + u64 maxsec; > + u32 freq; > + u32 mult; > + u32 shift; > + u64 ns; > + u64 frac; > + > + if (cpts->cc_mult || cpts->cc.shift) > + return; > + > + freq = clk_get_rate(cpts->refclk); > + > + /* Calc the maximum number of seconds which we can run before > + * wrapping around. > + */ > + maxsec = cpts->cc.mask; > + do_div(maxsec, freq); > + if (!maxsec) > + maxsec = 1; This doesn't pass the smell test. If the max counter value is M, you are figuring M*1/F which is the time in seconds corresponding to M. We set M=2^32-1, and so 'freq' would have to be greater than 4 GHz in order for 'maxsec' to be zero. Do you really expect such high frequency input clocks? > + else if (maxsec > 600 && cpts->cc.mask > UINT_MAX) > + maxsec = 600; What is this all about? cc.mask is always 2^32 - 1. > + clocks_calc_mult_shift(&mult, &shift, freq, NSEC_PER_SEC, maxsec); > + > + cpts->cc_mult = mult; > + cpts->cc.mult = mult; In order to get good resolution on the frequency adjustment, we want to keep 'mult' as large as possible. I don't see your code doing this. We can rely on the watchdog reader (work queue) to prevent overflows. Thanks, Richard > + cpts->cc.shift = shift; > + /* Check calculations and inform if not precise */ > + frac = 0; > + ns = cyclecounter_cyc2ns(&cpts->cc, freq, cpts->cc.mask, &frac); > + > + dev_info(cpts->dev, > + "CPTS: ref_clk_freq:%u calc_mult:%u calc_shift:%u error:%lld nsec/sec\n", > + freq, cpts->cc_mult, cpts->cc.shift, (ns - NSEC_PER_SEC)); > +}