From: Nicolas Ferre <nicolas.ferre@microchip.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>,
"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: <cgel.zte@gmail.com>, <linux-pwm@vger.kernel.org>,
<alexandre.belloni@bootlin.com>, Zeal Robot <zealci@zte.com.cn>,
<linux-kernel@vger.kernel.org>, <ludovic.desroches@microchip.com>,
<thierry.reding@gmail.com>,
Changcheng Deng <deng.changcheng@zte.com.cn>,
<lee.jones@linaro.org>, <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] pwm: Use div64_ul instead of do_div
Date: Thu, 18 Nov 2021 14:19:37 +0100 [thread overview]
Message-ID: <a9a62f9d-7227-0421-e36f-44222b79bbe7@microchip.com> (raw)
In-Reply-To: <YZYmZecp8WPkFY2F@shell.armlinux.org.uk>
On 18/11/2021 at 11:09, Russell King (Oracle) wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>
> On Wed, Nov 17, 2021 at 12:24:00PM +0100, Uwe Kleine-König wrote:
>> Hello,
>>
>> On Wed, Nov 17, 2021 at 02:04:26AM +0000, cgel.zte@gmail.com wrote:
>>> From: Changcheng Deng <deng.changcheng@zte.com.cn>
>>>
>>> do_div() does a 64-by-32 division. If the divisor is unsigned long, using
>>> div64_ul can avoid truncation to 32-bit.
>>
>> After some research I understood your commit log. I'd write:
>>
>> do_div() does a 64-by-32 division. Here the divsor is an
>> unsigned long which on some platforms is 64 bit wide. So use
>> div64_ul instead of do_div to avoid a possible truncation.
>>
>> The priority of this patch seems to be low, as the device seems to exist
>> only on (32bit) arm.
>
> ... where unsigned long is 32-bit.
>
> In any case, for this to overflow, we would need to have a clock in
> excess of 2^32-1 Hz, or around 4GHz - and if we had such a situation
> on 32-bit devices, we need to change the type for holding the frequency
> in the clk API, and probably a lot of code in the CCF as well.
>
> Unless there is a real reason for this change, I would suggest leaving
> the code as is - there is absolutely no point in making these divisions
> more expensive unless there is a real reason.
Thanks for the technical demonstration Russell. With this in mind:
NACK to the patch, sorry Changcheng Deng.
Best regards,
Nicolas
--
Nicolas Ferre
prev parent reply other threads:[~2021-11-18 13:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-17 2:04 [PATCH] pwm: Use div64_ul instead of do_div cgel.zte
2021-11-17 11:24 ` Uwe Kleine-König
2021-11-17 12:46 ` [PATCH V2] " cgel.zte
2021-11-17 14:54 ` Uwe Kleine-König
2021-11-18 2:52 ` [PATCH V3] " cgel.zte
2021-11-18 9:54 ` Uwe Kleine-König
2021-11-18 10:09 ` [PATCH] " Russell King (Oracle)
2021-11-18 13:19 ` Nicolas Ferre [this message]
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