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From: Guru Das Srinagesh <gurus@codeaurora.org>
To: David Laight <David.Laight@ACULAB.COM>
Cc: "linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <uwe@kleine-koenig.org>,
	"Subbaraman Narayanamurthy" <subbaram@codeaurora.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Michael Turquette" <mturquette@baylibre.com>,
	"Stephen Boyd" <sboyd@kernel.org>,
	"linux-clk@vger.kernel.org" <linux-clk@vger.kernel.org>
Subject: Re: [PATCH v8 01/12] clk: pwm: Use 64-bit division function
Date: Thu, 19 Mar 2020 13:53:07 -0700	[thread overview]
Message-ID: <20200319205307.GB17255@codeaurora.org> (raw)
In-Reply-To: <20200312190859.GA19605@codeaurora.org>

On Thu, Mar 12, 2020 at 12:09:12PM -0700, Guru Das Srinagesh wrote:
> On Thu, Mar 12, 2020 at 09:14:09AM +0000, David Laight wrote:
> > From: Guru Das Srinagesh
> > > Sent: 12 March 2020 02:10
> > > On Wed, Mar 11, 2020 at 04:58:24PM +0000, David Laight wrote:
> > > > From: Guru Das Srinagesh
> > > > > Sent: 11 March 2020 01:41
> > > > >
> > > > > Since the PWM framework is switching struct pwm_args.period's datatype
> > > > > to u64, prepare for this transition by using div64_u64 to handle a
> > > > > 64-bit divisor.
> > > > >
> > ...
> > > > > --- a/drivers/clk/clk-pwm.c
> > > > > +++ b/drivers/clk/clk-pwm.c
> > > > > @@ -89,7 +89,7 @@ static int clk_pwm_probe(struct platform_device *pdev)
> > > > >  	}
> > > > >
> > > > >  	if (of_property_read_u32(node, "clock-frequency", &clk_pwm->fixed_rate))
> > > > > -		clk_pwm->fixed_rate = NSEC_PER_SEC / pargs.period;
> > > > > +		clk_pwm->fixed_rate = div64_u64(NSEC_PER_SEC, pargs.period);
> > > >
> > > > That cannot be needed, a 32 bit division is fine.
> > > 
> > > Could you please explain why? I think the use of this function is
> > > warranted in order to handle the division properly with a 64-bit
> > > divisor.
> > ...
> > > > I'd assign pargs.period to an 'unsigned int' variable
> > > > prior to the division (I hate casts - been bitten by them in the past.).
> > > 
> > > Wouldn't this truncate the 64-bit value? The intention behind this patch
> > > is to allow the processing of 64-bit values in full.
> > 
> > You are dividing a 32bit constant by a value.
> > If pargs.period is greater than 2^32 the result is zero.
> 
> Thanks for the explanation. 
> 
> > I think you divide by 'fixed_rate' a bit later on - better not be zero.
> 
> Good point, but this issue exists with or without this patch, and fixing
> it is beyond this patch's scope.
> 
> Just to check if this patch can be dropped, I tested out compilation
> with this patch reverted and there were no errors, so I'm leaning
> towards dropping this patch unless you have any further comments on how
> to proceed.

Turns out I couldn't drop this patch after all - kbuild test robot
complained [1]. Accordingly, I've brought this patch back in my v10
patchset with the modifications you suggested. Could you kindly review it?

[1] https://www.spinics.net/lists/linux-pwm/msg11906.html

Thank you.

Guru Das.

  reply	other threads:[~2020-03-19 20:53 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <cover.1583889178.git.gurus@codeaurora.org>
2020-03-11  1:41 ` Guru Das Srinagesh
2020-03-11 16:58   ` David Laight
2020-03-12  2:09     ` Guru Das Srinagesh
2020-03-12  9:14       ` David Laight
2020-03-12 19:09         ` Guru Das Srinagesh
2020-03-19 20:53           ` Guru Das Srinagesh [this message]
2020-04-09  2:40         ` Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 02/12] drm/i915: Use 64-bit division macro Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 03/12] hwmon: pwm-fan: " Guru Das Srinagesh
2020-03-11 15:08   ` Guenter Roeck
2020-03-11  1:41 ` [PATCH v8 04/12] ir-rx51: " Guru Das Srinagesh
2020-03-11  2:11   ` Sean Young
2020-03-11  1:41 ` [PATCH v8 05/12] pwm: clps711x: " Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 06/12] pwm: pwm-imx-tpm: " Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 07/12] pwm: imx27: Use 64-bit division macro and function Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 08/12] pwm: sifive: Use 64-bit division macro Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 09/12] pwm: stm32-lp: Use %llu format specifier for period Guru Das Srinagesh
2020-03-11  1:41 ` [PATCH v8 10/12] pwm: sun4i: Use 64-bit division function Guru Das Srinagesh
2020-03-11  3:35   ` Chen-Yu Tsai
2020-03-11  1:41 ` [PATCH v8 11/12] backlight: pwm_bl: " Guru Das Srinagesh
2020-03-11 10:05   ` Daniel Thompson
2020-03-11  1:41 ` [PATCH v8 12/12] pwm: core: Convert period and duty cycle to u64 Guru Das Srinagesh

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