Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic
 configuration
To:     Sean Young <sean@mess.org>
Cc:     =?UTF-8?Q?Uwe_Kleine-K=c3=b6nig?= <u.kleine-koenig@pengutronix.de>,
        thierry.reding@gmail.com, lee.jones@linaro.org,
        nsaenzjulienne@suse.de, f.fainelli@gmail.com, rjui@broadcom.com,
        sbranden@broadcom.com, bcm-kernel-feedback-list@broadcom.com,
        linux-pwm@vger.kernel.org, linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <202011281128.54eLfMWr-lkp@intel.com>
 <1606564926-19555-1-git-send-email-LinoSanfilippo@gmx.de>
 <20201129181050.p6rkif5vjoumvafm@pengutronix.de>
 <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de>
 <20201204084417.GA2154@gofer.mess.org>
From:   Lino Sanfilippo <LinoSanfilippo@gmx.de>
Message-ID: <e4a124a8-0905-4504-62e0-a809163775fe@gmx.de>
Date:   Sat, 5 Dec 2020 00:16:01 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <20201204084417.GA2154@gofer.mess.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Hi Sean,

On 04.12.20 at 09:44, Sean Young wrote:

>> What about an extra check then to make sure that the period has not bee=
n truncated,
>> e.g:
>>
>> 	value =3D DIV_ROUND_CLOSEST_ULL(state->period, scaler);
>>
>> 	/* dont accept a period that is too small or has been truncated */
>> 	if ((value < PERIOD_MIN) ||
>> 	    (value !=3D DIV_ROUND_CLOSEST_ULL(state->period, scaler)))
>> 		return -EINVAL;
>
> Rather than doing another 64 bit division which is expensive (esp on 32 =
bit
> kernels), you could assign to u64 and check:
>
> 	if (value < PERIOD || value > U32_MAX)
> 		return -EINVAL;
>

Sound reasonable, I will adjust this.

>
> There was a problem where the carrier is incorrect for some IR hardware
> which uses a carrier of 455kHz. With periods that small, rounding errors
> do really matter and rounding down might cause problems.
>
> A policy of rounding down the carrier is not the right thing to do
> for pwm-ir-tx, and such a change will probably break pwm-ir-tx in some
> edge cases.
>


Thanks for this background information.

Regards,
Lino