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Fri, 15 Jan 2021 04:07:23 +0000 Date: Thu, 14 Jan 2021 22:07:20 -0600 From: Jeff LaBundy To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: Thierry Reding , linux-pwm@vger.kernel.org, Lee Jones , kernel@pengutronix.de Subject: Re: [PATCH v3] pwm: iqs620a: Fix overflow and optimize calculations Message-ID: <20210115040720.GC27243@labundy.com> References: <20210114215026.163424-1-u.kleine-koenig@pengutronix.de> Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210114215026.163424-1-u.kleine-koenig@pengutronix.de> User-Agent: Mutt/1.5.24 (2015-08-30) X-Originating-IP: [136.49.173.128] X-ClientProxiedBy: SA0PR12CA0011.namprd12.prod.outlook.com (2603:10b6:806:6f::16) To SN6PR08MB5517.namprd08.prod.outlook.com (2603:10b6:805:fb::32) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from labundy.com (136.49.173.128) by SA0PR12CA0011.namprd12.prod.outlook.com (2603:10b6:806:6f::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3763.11 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: =?iso-8859-1?Q?iVdTZ+7+5xnMzVTBXfh4CaZjVZSOy0dDHARJJaqbgO/MdVi1HANe0TWu4Q?= =?iso-8859-1?Q?JSkqa9UsNgB7ET6eWjQOuhrR+iVxaGlyL5sB7jZOC3TiqHKyUX+TcTy6g1?= =?iso-8859-1?Q?JGO8Rjv7Nr8E1cBpV53jgJFBeerQrMeC1lD7nZpziR699dAxlqGQtM142V?= =?iso-8859-1?Q?7vgu4Cje84mIDh7s8eNzegN3ySi1hccR0VqFMKMUQ0kdeRwtSyJjGO0XHK?= =?iso-8859-1?Q?W9lC9SCu/5os0/QzVAEDllhItVl7xBgEf1O/73qj+imr3mXxVzqlkDEMvj?= =?iso-8859-1?Q?WtMbeYsGLNvtNuIMG7y8eZ9De1zgGqm6YLzpNHC9r85yMdEr4/g/TyZwZD?= =?iso-8859-1?Q?tBio8ZQTGZdEmhmUH0bi07RlAUuGHd4nSMgVa8d2fP30ESdi4OkhzaD2+k?= =?iso-8859-1?Q?3qEafMcmOZdB/ZOIS0bi2yG8wLihjHcWINcj9wCJiHyIY4n5Pi709Qlpr5?= =?iso-8859-1?Q?UuqkY6F4FN/6vsYUQkTkN94v+WO0nOrAcskDDQ3XJCvlSCuQjexd7FFx+C?= =?iso-8859-1?Q?nSIO7ylAIgiq3KU/jWkClLfdzDEdTtlBMiql281mZxylk8lZoiemBYD6pw?= =?iso-8859-1?Q?NCGN/4NupkV3LkoUDTFjc9P+hENisD3sjbmFOsIbDc0ToiP92ZGIRzIMXC?= =?iso-8859-1?Q?ZXCtlX9Da7OeSV0Lnig2ubJQaQ4PkBOiwJK/LRkG3fyykMDcimj+ZrgEkp?= =?iso-8859-1?Q?Kyzcc6p2IJeEJHAFLHmltPf07myIA6Z6QjjxvD3pW4eyRLeWV/scDVwukI?= =?iso-8859-1?Q?MNr4f8EyVxx0EhwoCyVgLqZCggIphPOoMq32DQdgOrKwO+yvoVcP7n2gnO?= =?iso-8859-1?Q?eRm6yp/q2l9Rcm7XnywvhC4QgC7kBW6Jpep6AtTItQR0YHx39N+qilxgXY?= =?iso-8859-1?Q?2sDTK5IBsld+8Of/34Zp6dplidBEDHomikXu6JyFx/0cZ1sI3L5lZeMDxK?= =?iso-8859-1?Q?tpYyCR3wzFBf01d9qgvpoMu+84zP+pnqsaP676SHNuNMqn5S4tsnVuAos0?= =?iso-8859-1?Q?WSiXMpS9vtTEXsBDP4wFqj5+kmEj55M5e+u6bX?= X-OriginatorOrg: labundy.com X-MS-Exchange-CrossTenant-Network-Message-Id: 9626554c-5a47-4bf9-ef82-08d8b90b0f84 X-MS-Exchange-CrossTenant-AuthSource: SN6PR08MB5517.namprd08.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Jan 2021 04:07:23.0831 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 00b69d09-acab-4585-aca7-8fb7c6323e6f X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: kV4O+Pk605ZIctT6nS11Cvy8NiQVZT2NQf/kkJSBa9vy1UCf8n4+hJDBrqBiJFOZ8KIBPLUd/0r88F+nQOAfMQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN2PR0801MB2335 Precedence: bulk List-ID: X-Mailing-List: linux-pwm@vger.kernel.org Hi Uwe, Thank you for your work here; it's coming together quite nicely. On Thu, Jan 14, 2021 at 10:50:26PM +0100, Uwe Kleine-König wrote: > If state->duty_cycle is 0x100000000000000, the previous calculation of > duty_scale overflows and yields a duty cycle ratio of 0% instead of > 100%. Fix this by clamping the requested duty cycle to the maximal > possible duty cycle first. This way it is possible to use a native > integer division instead of a (depending on the architecture) more > expensive 64bit division. > > With this change in place duty_scale cannot be bigger than 256 which > allows to simplify the calculation of duty_val. > > Fixes: 6f0841a8197b ("pwm: Add support for Azoteq IQS620A PWM generator") > Signed-off-by: Uwe Kleine-König > --- > drivers/pwm/pwm-iqs620a.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/drivers/pwm/pwm-iqs620a.c b/drivers/pwm/pwm-iqs620a.c > index 5ede8255926e..eb03f60c5db8 100644 > --- a/drivers/pwm/pwm-iqs620a.c > +++ b/drivers/pwm/pwm-iqs620a.c > @@ -46,7 +46,9 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, > { > struct iqs620_pwm_private *iqs620_pwm; > struct iqs62x_core *iqs62x; > - u64 duty_scale; > + unsigned duty_cycle; > + unsigned duty_scale; > + > int ret; > Nit: there is a rogue newline here. I'm also not such a fan of mixing shorthand 'unsigned' with existing 'unsigned int' (as in probe). > if (state->polarity != PWM_POLARITY_NORMAL) > @@ -70,7 +72,8 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, > * For lower duty cycles (e.g. 0), the PWM output is simply disabled to > * allow an external pull-down resistor to hold the GPIO3/LTX pin low. > */ > - duty_scale = div_u64(state->duty_cycle * 256, IQS620_PWM_PERIOD_NS); > + duty_cycle = min_t(u64, state->duty_cycle, IQS620_PWM_PERIOD_NS); > + duty_scale = duty_cycle * 256 / IQS620_PWM_PERIOD_NS; > > mutex_lock(&iqs620_pwm->lock); > > @@ -82,7 +85,7 @@ static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, > } > > if (duty_scale) { > - u8 duty_val = min_t(u64, duty_scale - 1, 0xff); > + u8 duty_val = duty_scale - 1; > > ret = regmap_write(iqs62x->regmap, IQS620_PWM_DUTY_CYCLE, > duty_val); > -- > 2.29.2 > That being said, the patch is functionally correct and I validated all corner cases with actual hardware. Feel free to add to future cosmetic revisions: Tested-by: Jeff LaBundy Kind regards, Jeff LaBundy