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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6AAF9C433E0 for ; Fri, 12 Jun 2020 11:57:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5535A20801 for ; Fri, 12 Jun 2020 11:57:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726101AbgFLL5f (ORCPT ); Fri, 12 Jun 2020 07:57:35 -0400 Received: from mga14.intel.com ([192.55.52.115]:8570 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726024AbgFLL5e (ORCPT ); Fri, 12 Jun 2020 07:57:34 -0400 IronPort-SDR: JPCMpBjJsIC4SUFCRaBZXw10CM/OpSo2aT+sWsmOTtWOvwfBgSMefVKMPx6lo4YGC/dLKAMjIk aTeByTOk6Wcw== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Jun 2020 04:57:33 -0700 IronPort-SDR: ABSPo+JBvBU79vQhZigciL7kR9UOM6gudQ79a9E/eot+fTbjWJoQC0Uc00I99Okq50ZXR+JZHs +RV39hC8Hnhg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,503,1583222400"; d="scan'208";a="296922866" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by fmsmga004.fm.intel.com with ESMTP; 12 Jun 2020 04:57:30 -0700 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1jjiJQ-00CYBT-2r; Fri, 12 Jun 2020 14:57:32 +0300 Date: Fri, 12 Jun 2020 14:57:32 +0300 From: Andy Shevchenko To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: Hans de Goede , Thierry Reding , Jani Nikula , Joonas Lahtinen , Ville =?iso-8859-1?Q?Syrj=E4l=E4?= , "Rafael J . Wysocki" , Len Brown , linux-pwm@vger.kernel.org, intel-gfx , dri-devel@lists.freedesktop.org, Mika Westerberg , linux-acpi@vger.kernel.org Subject: Re: [PATCH v2 03/15] pwm: lpss: Add range limit check for the base_unit register value Message-ID: <20200612115732.GC2428291@smile.fi.intel.com> References: <20200607181840.13536-1-hdegoede@redhat.com> <20200607181840.13536-4-hdegoede@redhat.com> <20200608035023.GZ2428291@smile.fi.intel.com> <90769dc0-3174-195b-34e0-ef4bb9d9b982@redhat.com> <20200611221242.3bjqvnhcwwxaocxy@taurus.defre.kleine-koenig.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200611221242.3bjqvnhcwwxaocxy@taurus.defre.kleine-koenig.org> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org On Fri, Jun 12, 2020 at 12:12:42AM +0200, Uwe Kleine-König wrote: > On Mon, Jun 08, 2020 at 01:07:12PM +0200, Hans de Goede wrote: > > On 6/8/20 5:50 AM, Andy Shevchenko wrote: > > > On Sun, Jun 07, 2020 at 08:18:28PM +0200, Hans de Goede wrote: > > > > When the user requests a high enough period ns value, then the > > > > calculations in pwm_lpss_prepare() might result in a base_unit value of 0. > > > > > > > > But according to the data-sheet the way the PWM controller works is that > > > > each input clock-cycle the base_unit gets added to a N bit counter and > > > > that counter overflowing determines the PWM output frequency. Adding 0 > > > > to the counter is a no-op. The data-sheet even explicitly states that > > > > writing 0 to the base_unit bits will result in the PWM outputting a > > > > continuous 0 signal. > > > > > > So, and why it's a problem? > > > > Lets sya the user requests a PWM output frequency of 100Hz on Cherry Trail > > which has a 19200000 Hz clock this will result in 100 * 65536 / 19200000 = > > 0.3 -> 0 as base-unit value. So instead of getting 100 Hz the user will > > now get a pin which is always outputting low. > > I didn't follow the complete discussion but note that the general rule > is: > > round period down to the next possible implementable period > round duty_cycle down to the next possible implementable duty_cycle > > so if a small enough period (and so a small duty_cycle) is requested it > is expected that duty_cycle will be zero. ...which brings me an idea that PWM framework should expose API to get a capabilities, like DMA Engine has. In such capabilities, in particular, caller can get ranges of the correct frequencies of the underneath hardware. -- With Best Regards, Andy Shevchenko