From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5B808C4321E for ; Wed, 20 Apr 2022 12:13:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352746AbiDTMPs (ORCPT ); Wed, 20 Apr 2022 08:15:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55572 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1352807AbiDTMPl (ORCPT ); Wed, 20 Apr 2022 08:15:41 -0400 Received: from metis.ext.pengutronix.de (metis.ext.pengutronix.de [IPv6:2001:67c:670:201:290:27ff:fe1d:cc33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F1F7D393EE for ; Wed, 20 Apr 2022 05:12:54 -0700 (PDT) Received: from drehscheibe.grey.stw.pengutronix.de ([2a0a:edc0:0:c01:1d::a2]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1nh9CU-00022V-CW; Wed, 20 Apr 2022 14:12:50 +0200 Received: from [2a0a:edc0:0:900:1d::77] (helo=ptz.office.stw.pengutronix.de) by drehscheibe.grey.stw.pengutronix.de with esmtp (Exim 4.94.2) (envelope-from ) id 1nh9CV-0049dR-2D; Wed, 20 Apr 2022 14:12:49 +0200 Received: from ukl by ptz.office.stw.pengutronix.de with local (Exim 4.94.2) (envelope-from ) id 1nh9CS-004Tjg-Ap; Wed, 20 Apr 2022 14:12:48 +0200 From: =?UTF-8?q?Uwe=20Kleine-K=C3=B6nig?= To: Laurent Pinchart , Simon Horman , Geert Uytterhoeven , Magnus Damm , Thierry Reding , Lee Jones Cc: linux-renesas-soc@vger.kernel.org, linux-pwm@vger.kernel.org, kernel@pengutronix.de Subject: [PATCH v2 6/6] pwm: renesas-tpu: Improve precision of period and duty_cycle calculation Date: Wed, 20 Apr 2022 14:12:40 +0200 Message-Id: <20220420121240.67781-7-u.kleine-koenig@pengutronix.de> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220420121240.67781-1-u.kleine-koenig@pengutronix.de> References: <20220420121240.67781-1-u.kleine-koenig@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Developer-Signature: v=1; a=openpgp-sha256; l=3068; h=from:subject; bh=qhmj+/kClimdnaMQquEEOOHX5xZ+ZRCX9FU00iWnViM=; b=owEBbQGS/pANAwAKAcH8FHityuwJAcsmYgBiX/i09g8CTVqTyKus3foEapayKgO+ir799otZKwof S7D4f3qJATMEAAEKAB0WIQR+cioWkBis/z50pAvB/BR4rcrsCQUCYl/4tAAKCRDB/BR4rcrsCb1fCA CSWg4qs1CDKt47OWQYbajjaAIkiiTZiL6UiB7gD8Rk9sq+I4g3X3dyD+5LAtCYwVHm+p1lgQa1a0PS fAqLWMPfKBYRwPrsekaR4qa2FfQ42NOq3sN7JA2Vkj8wb7/kECp9j2n3dI1M0pGJrZAurXDDnUt/sn AK3lN79Q3Or/NRbsULoEAIj3HyvhyfoBarxzGg/o4gj/JbpKzzkq0+AWMBOKT66EtLIIKaeA4w76pr sifzthRq27fo+KsTH0w9qPrwrUekqHj/0xn3jvWVTvsKh4UBkF65jHOIzWIiThmXMGN9QGbi2wvSlM b5nuyxPpah01sQ+Zsw3bIkNB0C1XeH X-Developer-Key: i=u.kleine-koenig@pengutronix.de; a=openpgp; fpr=0D2511F322BFAB1C1580266BE2DCDD9132669BD6 Content-Transfer-Encoding: 8bit X-SA-Exim-Connect-IP: 2a0a:edc0:0:c01:1d::a2 X-SA-Exim-Mail-From: ukl@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-renesas-soc@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-renesas-soc@vger.kernel.org Dividing by the result of a division looses precision. Consider for example clk_rate = 33000000 and period_ns = 500001. Then clk_rate / (NSEC_PER_SEC / period_ns) has the exact value 16500.033, but in C this evaluates to 16508. It gets worse for even bigger values of period_ns, so with period_ns = 500000001, the exact result is 16500000.033 while in C we get 33000000. For that reason use clk_rate * period_ns / NSEC_PER_SEC instead which doesn't suffer from this problem. To ensure this doesn't overflow add a safeguard check for clk_rate. Note that duty > period can never happen, so the respective check can be dropped. Incidentally this fixes a division by zero if period_ns > NSEC_PER_SEC. Another side effect is that values bigger than INT_MAX for period and duty_cyle are not wrongly discarded any more. Fixes: 99b82abb0a35 ("pwm: Add Renesas TPU PWM driver") Signed-off-by: Uwe Kleine-König --- drivers/pwm/pwm-renesas-tpu.c | 28 +++++++++++++++++----------- 1 file changed, 17 insertions(+), 11 deletions(-) diff --git a/drivers/pwm/pwm-renesas-tpu.c b/drivers/pwm/pwm-renesas-tpu.c index 60ba7cf275c7..9e6978c39788 100644 --- a/drivers/pwm/pwm-renesas-tpu.c +++ b/drivers/pwm/pwm-renesas-tpu.c @@ -242,20 +242,29 @@ static void tpu_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm) } static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm, - int duty_ns, int period_ns, bool enabled) + u64 duty_ns, u64 period_ns, bool enabled) { struct tpu_pwm_device *tpd = pwm_get_chip_data(pwm); struct tpu_device *tpu = to_tpu_device(chip); unsigned int prescaler; bool duty_only = false; u32 clk_rate; - u32 period; + u64 period; u32 duty; int ret; clk_rate = clk_get_rate(tpu->clk); + if (unlikely(clk_rate > NSEC_PER_SEC)) { + /* + * This won't happen in the nearer future, so this is only a + * safeguard to prevent the following calculation from + * overflowing. With this clk_rate * period_ns / NSEC_PER_SEC is + * not greater than period_ns and so fits into an u64. + */ + return -EINVAL; + } - period = clk_rate / (NSEC_PER_SEC / period_ns); + period = mul_u64_u64_div_u64(clk_rate, period_ns, NSEC_PER_SEC); /* * Find the minimal prescaler in [0..3] such that @@ -292,18 +301,15 @@ static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm, period >>= 2 * prescaler; - if (duty_ns) { - duty = (clk_rate >> 2 * prescaler) - / (NSEC_PER_SEC / duty_ns); - if (duty > period) - return -EINVAL; - } else { + if (duty_ns) + duty = mul_u64_u64_div_u64(clk_rate, duty_ns, + (u64)NSEC_PER_SEC << (2 * prescaler)); + else duty = 0; - } dev_dbg(&tpu->pdev->dev, "rate %u, prescaler %u, period %u, duty %u\n", - clk_rate, 1 << (2 * prescaler), period, duty); + clk_rate, 1 << (2 * prescaler), (u32)period, duty); if (tpd->prescaler == prescaler && tpd->period == period) duty_only = true; -- 2.35.1