[v2,5/6] pwm: mxs: avoid a division in mxs_pwm_apply()
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Message ID 20191004133207.6663-6-linux@rasmusvillemoes.dk
State Accepted
Headers show
Series
  • pwm: mxs: add support for setting polarity via DT
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Commit Message

Rasmus Villemoes Oct. 4, 2019, 1:32 p.m. UTC
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@rasmusvillemoes.dk>
---
 drivers/pwm/pwm-mxs.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Comments

Uwe Kleine-König Oct. 4, 2019, 2:08 p.m. UTC | #1
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@rasmusvillemoes.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@pengutronix.de>

Thanks
Uwe

Patch
diff mbox series

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
 };
 
 struct mxs_pwm_chip {
@@ -71,7 +71,7 @@  static int mxs_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	rate = clk_get_rate(mxs->clk);
 	while (1) {
-		c = rate / cdiv[div];
+		c = rate >> cdiv_shift[div];
 		c = c * state->period;
 		do_div(c, 1000000000);
 		if (c < PERIOD_PERIOD_MAX)