linux-kernel - Re: [PATCH v2 5/6] pwm: mxs: avoid a division in mxs_pwm

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Message-ID: <20191004140834.4idddyohf52ha4ao@pengutronix.de>
Date:   Fri, 4 Oct 2019 16:08:34 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] pwm: mxs: avoid a division in mxs_pwm_apply()

On Fri, Oct 04, 2019 at 03:32:06PM +0200, Rasmus Villemoes wrote:
> Since the divisor is not a compile-time constant (unless gcc somehow
> decided to unroll the loop PERIOD_CDIV_MAX times), this does a
> somewhat expensive 32/32 division. Replace that with a right shift.
> 
> We still have a 64/32 division just below, but at least in that
> case the divisor is compile-time constant.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

> ---
>  drivers/pwm/pwm-mxs.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-mxs.c b/drivers/pwm/pwm-mxs.c
> index 57562221c439..f2e57fcf8f8b 100644
> --- a/drivers/pwm/pwm-mxs.c
> +++ b/drivers/pwm/pwm-mxs.c
> @@ -33,8 +33,8 @@
>  #define  PERIOD_CDIV(div)	(((div) & 0x7) << 20)
>  #define  PERIOD_CDIV_MAX	8
>  
> -static const unsigned int cdiv[PERIOD_CDIV_MAX] = {
> -	1, 2, 4, 8, 16, 64, 256, 1024
> +static const u8 cdiv_shift[PERIOD_CDIV_MAX] = {
> +	0, 1, 2, 3, 4, 6, 8, 10

One small nitpick: I would like to see this name have a mxs_pwm_ prefix.
But even without this change:

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |