linux-kernel - Re: [PATCH] leds: qcom-lpg: Fix PWM period limits

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Message-ID: <20230515022041.4i2w36vubzggpdt6@ripper>
Date:   Sun, 14 May 2023 19:20:41 -0700
From:   Bjorn Andersson <andersson@...nel.org>
To:     Caleb Connolly <caleb.connolly@...aro.org>
Cc:     Bjorn Andersson <quic_bjorande@...cinc.com>,
        Pavel Machek <pavel@....cz>, Lee Jones <lee@...nel.org>,
        Anjelique Melendez <quic_amelende@...cinc.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Luca Weiss <luca@...tu.xyz>,
        linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] leds: qcom-lpg: Fix PWM period limits

On Sat, May 13, 2023 at 10:09:49AM +0000, Caleb Connolly wrote:
> 
> 
> On 12/05/2023 16:55, Bjorn Andersson wrote:
> > The introduction of high resolution PWM support moved the parenthesis of
> > the divisions in the calculation of min and max period. The result in
> > both divisions is in most cases truncation to 0, which limits the period
> > to the range of [0, 0].
> 
> Huh, TIL C gives multiplication and division the same precedence when
> deciding order of operations.

There where no explicit parenthesis in the original implementation. So
I guess it would be more correct to state that parenthesis was
introduced around part of the expression?

Let me know if you think the wording matters and you would prefer me to
rewrite it.

Regards,
Bjorn

> > 
> > Both numerators (and denominators) are within 64 bits, so the whole
> > expression can be put directly into the div64_u64, instead of doing it
> > partially.
> > 
> > Fixes: b00d2ed37617 ("leds: rgb: leds-qcom-lpg: Add support for high resolution PWM")
> > Signed-off-by: Bjorn Andersson <quic_bjorande@...cinc.com>
> 
> Reviewed-by: Caleb Connolly <caleb.connolly@...aro.org>
> > ---
> > 
> > This fixes the regression in v6.4-rc1.
> > 
> >  drivers/leds/rgb/leds-qcom-lpg.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/leds/rgb/leds-qcom-lpg.c b/drivers/leds/rgb/leds-qcom-lpg.c
> > index c9cea797a697..7287fadc00df 100644
> > --- a/drivers/leds/rgb/leds-qcom-lpg.c
> > +++ b/drivers/leds/rgb/leds-qcom-lpg.c
> > @@ -312,14 +312,14 @@ static int lpg_calc_freq(struct lpg_channel *chan, uint64_t period)
> >  		max_res = LPG_RESOLUTION_9BIT;
> >  	}
> >  
> > -	min_period = (u64)NSEC_PER_SEC *
> > -			div64_u64((1 << pwm_resolution_arr[0]), clk_rate_arr[clk_len - 1]);
> > +	min_period = div64_u64((u64)NSEC_PER_SEC * (1 << pwm_resolution_arr[0]),
> > +			       clk_rate_arr[clk_len - 1]);
> >  	if (period <= min_period)
> >  		return -EINVAL;
> >  
> >  	/* Limit period to largest possible value, to avoid overflows */
> > -	max_period = (u64)NSEC_PER_SEC * max_res * LPG_MAX_PREDIV *
> > -			div64_u64((1 << LPG_MAX_M), 1024);
> > +	max_period = div64_u64((u64)NSEC_PER_SEC * max_res * LPG_MAX_PREDIV * (1 << LPG_MAX_M),
> > +			       1024);
> >  	if (period > max_period)
> >  		period = max_period;
> >  
> 
> -- 
> Kind Regards,
> Caleb (they/them)