Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753291AbbETNDr (ORCPT ); Wed, 20 May 2015 09:03:47 -0400 Received: from mailgw02.mediatek.com ([210.61.82.184]:37084 "EHLO mailgw02.mediatek.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1752210AbbETNDo (ORCPT ); Wed, 20 May 2015 09:03:44 -0400 X-Listener-Flag: 11101 Message-ID: <1432127020.20394.15.camel@mtksdaap41> Subject: Re: [PATCH v8 2/3] I2C: mediatek: Add driver for MediaTek I2C controller From: Yingjoe Chen To: Uwe =?ISO-8859-1?Q?Kleine-K=F6nig?= CC: Eddie Huang , Mark Rutland , Xudong Chen , , Pawel Moll , Ian Campbell , Wolfram Sang , "Liguo Zhang" , , , Rob Herring , , , Sascha Hauer , Kumar Gala , "Matthias Brugger" , Date: Wed, 20 May 2015 21:03:40 +0800 In-Reply-To: <20150520085715.GA17078@pengutronix.de> References: <1431967209-5261-1-git-send-email-eddie.huang@mediatek.com> <1431967209-5261-3-git-send-email-eddie.huang@mediatek.com> <20150520085715.GA17078@pengutronix.de> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 8bit MIME-Version: 1.0 X-MTK: N Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3461 Lines: 107 Hi Uwe, On Wed, 2015-05-20 at 10:57 +0200, Uwe Kleine-König wrote: > Hello, > > now that I understood the formula some more comments to the calculation. > > On Tue, May 19, 2015 at 12:40:08AM +0800, Eddie Huang wrote: > > +#define I2C_DEFAUT_SPEED 100000 /* hz */ > DEFAULT? > > > +#define MAX_FS_MODE_SPEED 400000 > > +#define MAX_HS_MODE_SPEED 3400000 > > +#define MAX_SAMPLE_CNT_DIV 8 > > +#define MAX_STEP_CNT_DIV 64 > > +#define MAX_HS_STEP_CNT_DIV 8 > > [...] > > +/* calculate i2c port speed */ > > +static int mtk_i2c_set_speed(struct mtk_i2c *i2c, unsigned int clk_src_in_hz) > > +{ > add a comment here, that clk_src_in_hz is the parent clock already > divided by clock-div. > > > + unsigned int khz; > > + unsigned int step_cnt; > > + unsigned int sample_cnt; > > + unsigned int sclk; > > + unsigned int hclk; > > + unsigned int max_step_cnt; > > + unsigned int sample_div = MAX_SAMPLE_CNT_DIV; > > + unsigned int step_div; > > + unsigned int min_div; > > + unsigned int best_mul; > > + unsigned int cnt_mul; > > + > > + if (i2c->speed_hz > MAX_HS_MODE_SPEED) > > + return -EINVAL; > According to the plan to tune for the highest possible rate <= > i2c->speed_hz, you should handle the case (i2c->speed_hz > > MAX_HS_MODE_SPEED) like i2c->speed_hz == MAX_HS_MODE_SPEED. > Well, you might want to prevent an overflow in the calculation below > however. The check here means we don't support speed > MAX_HS_MODE_SPEED. This is different then slightly slower bus speed due to rounding error. > > + else if (i2c->speed_hz > MAX_FS_MODE_SPEED) > > + max_step_cnt = MAX_HS_STEP_CNT_DIV; > > + else > > + max_step_cnt = MAX_STEP_CNT_DIV; > So I assume this is the hardware limit on the step_cnt value. For > FS_MODE and below you have 6 bits and writing X corresponds to > step_cnt = X + 1. For HS_MODE there are only 3 bits. right? Yes, correct. > > + step_div = max_step_cnt; > > + /* Find the best combination */ > > + khz = i2c->speed_hz / 1000; > > + hclk = clk_src_in_hz / 1000; > Why are you dividing here? There shouldn't be an overflow problem and > you're loosing precision. Agreed, they should be removed. > > + min_div = ((hclk >> 1) + khz - 1) / khz; > The shift accounts for the fixed divider 2 in > > i2c_bus_freq = parent_clk / (clock-div * 2 * sample_cnt * step_cnt > > ? Maybe better call this opt_div instead of min_div? So now we're > searching for the best pair (sample_cnt, step_cnt) with: > > * 0 < sample_cnt < MAX_SAMPLE_CNT_DIV > * 0 < step_cnt < max_step_cnt > * sample_cnt * step_cnt >= min_div > * optimizing for sample_cnt * step_cnt being minimal > > Right? Yes. > > + best_mul = MAX_SAMPLE_CNT_DIV * max_step_cnt; > > + > > + for (sample_cnt = 1; sample_cnt <= MAX_SAMPLE_CNT_DIV; sample_cnt++) { > > + step_cnt = (min_div + sample_cnt - 1) / sample_cnt; > DIV_ROUND_UP > > > + cnt_mul = step_cnt * sample_cnt; > > + if (step_cnt > max_step_cnt) > > + continue; > I think it can happen that you have step_cnt > max_step_cnt here, but > that (sample_cnt, max_step_cnt) still is a good pair to consider. So: If step_cnt > max_step_cnt, then sample_cnt * max_step_cnt < min_div. This means (sample_cnt, max_step_cnt) is not a valid. Joe.C -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/