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

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Date:   Sat, 13 May 2023 10:09:49 +0000
From:   Caleb Connolly <caleb.connolly@...aro.org>
To:     Bjorn Andersson <quic_bjorande@...cinc.com>,
        Pavel Machek <pavel@....cz>, Lee Jones <lee@...nel.org>,
        Anjelique Melendez <quic_amelende@...cinc.com>
Cc:     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 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.
> 
> 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)