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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 287A4C433F5 for ; Sun, 31 Oct 2021 10:39:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0961560F70 for ; Sun, 31 Oct 2021 10:39:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230160AbhJaKmL (ORCPT ); Sun, 31 Oct 2021 06:42:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57048 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229638AbhJaKmK (ORCPT ); Sun, 31 Oct 2021 06:42:10 -0400 Received: from gofer.mess.org (gofer.mess.org [IPv6:2a02:8011:d000:212::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 573EEC061570; Sun, 31 Oct 2021 03:39:38 -0700 (PDT) Received: by gofer.mess.org (Postfix, from userid 1000) id 30383C63FB; Sun, 31 Oct 2021 10:39:34 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mess.org; s=2020; t=1635676774; bh=uhJMdNnrBQp8vDJjBnlyia4UxjQNMMW0JmjLg7bCz7Y=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=e1YebnWFKr+SwciHOLCYzoOIlMGOwnzYkfNhJedo5+jvjmaKM9M4rVMkIYDL10imf XAtKAt3J48dij/4ZQIVp7zAYRDG1kp8yK4RW2QyOEHk1qXwyLAOkruhSliCCRKNw7c 9ZkzpY7l0iFoTVqN0p9B7ZZSWflUWJTzOHjJxsFhGUHf5Yx99nUy5CrcPJLb15kgnf /T/7yekoWZ56bNOF3qDJcJJ+/0dquOV9ZVxShg/uCQXPmGnsQ0UUcnW+R5wswekQVw 8nKUwlsbLN/UvChUOt9Hq9wP38xS9BTHKPHNcuL9VtmOfrlH9/Ic6biY2Ydtu0Kw8/ nbodbbyb5XsLA== Date: Sun, 31 Oct 2021 10:39:34 +0000 From: Sean Young To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: =?iso-8859-1?Q?Ma=EDra?= Canal , mchehab@kernel.org, thierry.reding@gmail.com, lee.jones@linaro.org, linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org Subject: Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API Message-ID: <20211031103933.GA28316@gofer.mess.org> References: <20211028064513.guziv6uaivzlk6ki@pengutronix.de> <20211028091442.GA16514@gofer.mess.org> <20211028111535.x7xgz7domx2lpyfh@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20211028111535.x7xgz7domx2lpyfh@pengutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Uwe, On Thu, Oct 28, 2021 at 01:15:35PM +0200, Uwe Kleine-König wrote: > On Thu, Oct 28, 2021 at 10:14:42AM +0100, Sean Young wrote: > > We still have the problem that the pwm drivers calculate the period > > incorrectly by rounding down (except pwm-bcm2835). So the period is not > > as good as it could be in most cases, but this driver can't do anything > > about that. > > Yeah, some time ago I started coding a round_state function > (wip at > https://git.pengutronix.de/cgit/ukl/linux/commit/?h=pwm-wip&id=ae348eb6a55d6526f30ef4a49819197d9616391e) > but this was pushed down on my todo-list by more important stuff. > > If you want to experiment with that ... I was thinking about this problem this morning. - The pwm-ir-tx driver gets a carrier set in Hz, which it has to convert to a period (1e9 / carrier). There is loss of accuracy there. - When it gets to the pwm driver, the period is converted into the format the pwm hardware expects. For example the pwm-bcm2835 driver converts it into clock cycles (1e9 / 8e8). Both calculations involve loss of accuracy because of integer representation. Would it make more sense for the pwm interface to use numer/denom rational numbers? struct rational { u64 numer; u64 denom; }; If pwm-ir-tx would like to set the carrier, it could it like so: struct rational period = { .numer = NUSEC_PER_SEC, .denom = carrier, }; pwm_set_period(&period); Now pwm-bcm2835 could do it like so: int bcm2835_set_period(struct rational *period) { struct rational rate = { .numer = NUSEC_PER_SEC, .denum = clk_get_rate(clk), }; rational_div(&rate, period); int step = rational_to_u64(&rate); } Alternatively, since most of the pwm hardware is doing scaling based on the clock (I think), would not make more sense for the pwm driver interface to take a frequency rather than a period? Then the integer calculations can be simpler: just divide the clock rate by the required frequency and you have the period. Just some thoughts. Sean