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[217.229.28.220]) by smtp.gmail.com with ESMTPSA id r3sm28241814wrm.35.2020.03.31.13.49.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2020 13:49:30 -0700 (PDT) Date: Tue, 31 Mar 2020 22:49:29 +0200 From: Thierry Reding To: Guru Das Srinagesh Cc: Arnd Bergmann , Linux PWM List , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= , Subbaraman Narayanamurthy , "linux-kernel@vger.kernel.org" , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , NXP Linux Team Subject: Re: [PATCH v11 06/12] pwm: imx27: Use 64-bit division macro and function Message-ID: <20200331204929.GC2954599@ulmo> References: <5aae102e21c0e63ad2588ae1e174b48b06d25e96.1584667964.git.gurus@codeaurora.org> <20200330204359.GB5107@codeaurora.org> <20200331202058.GB25781@codeaurora.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="PuGuTyElPB9bOcsM" Content-Disposition: inline In-Reply-To: <20200331202058.GB25781@codeaurora.org> User-Agent: Mutt/1.13.1 (2019-12-14) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --PuGuTyElPB9bOcsM Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Mar 31, 2020 at 01:20:58PM -0700, Guru Das Srinagesh wrote: > On Tue, Mar 31, 2020 at 05:24:52PM +0200, Arnd Bergmann wrote: > > On Mon, Mar 30, 2020 at 10:44 PM Guru Das Srinagesh > > wrote: > > > > > > On Fri, Mar 20, 2020 at 06:09:39PM +0100, Arnd Bergmann wrote: > > > > On Fri, Mar 20, 2020 at 2:42 AM Guru Das Srinagesh wrote: > > > > > > > > > @@ -240,8 +240,7 @@ static int pwm_imx27_apply(struct pwm_chip *c= hip, struct pwm_device *pwm, > > > > > > > > > > period_cycles /=3D prescale; > > > > > c =3D (unsigned long long)period_cycles * state->duty_cyc= le; > > > > > - do_div(c, state->period); > > > > > - duty_cycles =3D c; > > > > > + duty_cycles =3D div64_u64(c, state->period); > > > > > > > > > > > > > This change looks fine, but I wonder if the code directly above it > > > > > > > > c =3D clk_get_rate(imx->clk_per); > > > > c *=3D state->period; > > > > do_div(c, 1000000000); > > > > period_cycles =3D c; > > > > > > > > might run into an overflow when both the clock rate and the period > > > > are large numbers. > > > > > > Hmm. Seems to me like addressing this would be outside the scope of t= his > > > patch series. > >=20 > > I think it should be part of the same series, addressing bugs that > > were introduced > > by the change to 64-bit period. If it's not getting fixed along with > > the other regressions, > > I fear nobody is going to go back and fix it later. >=20 > Makes sense, I agree. Would this be an acceptable fix? >=20 > Instead of multiplying c and state->period first and then dividing by > 10^9, first divide state->period by 10^9 and then multiply the quotient > of that division with c and assign it to period_cycles. Like so: >=20 > c =3D clk_get_rate(imx->clk_per); > c *=3D div_u64(state->period, 1000000000); > period_cycles =3D c; >=20 > This should take care of overflow not happening because state->period is > converted from nanoseconds to seconds early on and so becomes a small > number. Doesn't that mean that anything below a 1 second period will be clamped to just 0? 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