Re: [PATCH v2] leds: qcom-lpg: Fix PWM period limits

From: Johan Hovold <johan@kernel.org>
To: Bjorn Andersson <quic_bjorande@quicinc.com>,
	Pavel Machek <pavel@ucw.cz>, Lee Jones <lee@kernel.org>
Cc: "Anjelique Melendez" <quic_amelende@quicinc.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	linux-leds@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-msm@vger.kernel.org,
	"Caleb Connolly" <caleb.connolly@linaro.org>,
	"Steev Klimaszewski" <steev@kali.org>
Subject: Re: [PATCH v2] leds: qcom-lpg: Fix PWM period limits
Date: Wed, 24 May 2023 16:32:36 +0200	[thread overview]
Message-ID: <ZG4gBE1rqkMIllV3@hovoldconsulting.com> (raw)
In-Reply-To: <20230515162604.649203-1-quic_bjorande@quicinc.com>

On Mon, May 15, 2023 at 09:26:04AM -0700, Bjorn Andersson wrote:
> The introduction of high resolution PWM support changed the order of the
> operations in the calculation of min and max period. The result in both
> divisions is in most cases a truncation to 0, which limits the period to
> the range of [0, 0].
> 
> 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")
> Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
> Tested-by: Steev Klimaszewski <steev@kali.org>
> Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>

Tested-by: Johan Hovold <johan+linaro@kernel.org>

Pavel or Lee, could you pick this one up for 6.4 as it fixes a
regression (e.g. broken backlight on a number of laptops like the X13s)?

> ---
> 
> Changes since v1:
> - Reworded first sentence to express that it's the order and not the
>   previously non-existent parenthesis that changed...
> - Picked up review tags.
> 
>  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;

Johan