[PATCH RESEND] pwm: mxs: fix period divider computation

[PATCH RESEND] pwm: mxs: fix period divider computation

[Gaetan Hug]

The driver computes which clock divider it sould be using from the
requested period. This computation assumes that the link between the
register value and the actual divider value is raising 2 to the power of
the registry value.

    div = 1 << regvalue

This is true only for the first 5 values out of 8. Next values are 64,
256 and, 1024 - instead of 32, 64, 128.
This affects only the users requesting a period > 0.04369s.

Replace the computation with a look-up table.

Signed-off-by: Gaetan Hug <ghug@induct.be>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
---
 drivers/pwm/pwm-mxs.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-mxs.c b/drivers/pwm/pwm-mxs.c
index f75ecb0..c65e183 100644
--- a/drivers/pwm/pwm-mxs.c
+++ b/drivers/pwm/pwm-mxs.c
@@ -35,6 +35,8 @@
 #define  PERIOD_CDIV(div)	(((div) & 0x7) << 20)
 #define  PERIOD_CDIV_MAX	8
 
+static unsigned const int cdiv[PERIOD_CDIV_MAX] = {1, 2, 4, 8, 16, 64, 256, 1024};
+
 struct mxs_pwm_chip {
 	struct pwm_chip chip;
 	struct clk *clk;
@@ -54,13 +56,13 @@ static int mxs_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	rate = clk_get_rate(mxs->clk);
 	while (1) {
-		c = rate / (1 << div);
+		c = rate / cdiv[div];
 		c = c * period_ns;
 		do_div(c, 1000000000);
 		if (c < PERIOD_PERIOD_MAX)
 			break;
 		div++;
-		if (div > PERIOD_CDIV_MAX)
+		if (div >= PERIOD_CDIV_MAX)
 			return -EINVAL;
 	}
 
-- 
1.7.10.4

Re: [PATCH RESEND] pwm: mxs: fix period divider computation

[Thierry Reding]

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On Wed, Mar 11, 2015 at 01:08:12PM +0100, Gaetan Hug wrote:
> The driver computes which clock divider it sould be using from the
> requested period. This computation assumes that the link between the
> register value and the actual divider value is raising 2 to the power of
> the registry value.
> 
>     div = 1 << regvalue
> 
> This is true only for the first 5 values out of 8. Next values are 64,
> 256 and, 1024 - instead of 32, 64, 128.
> This affects only the users requesting a period > 0.04369s.
> 
> Replace the computation with a look-up table.
> 
> Signed-off-by: Gaetan Hug <ghug@induct.be>
> Acked-by: Shawn Guo <shawn.guo@linaro.org>
> ---
>  drivers/pwm/pwm-mxs.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)

Applied, though I made some last-minute changes, see below.

> diff --git a/drivers/pwm/pwm-mxs.c b/drivers/pwm/pwm-mxs.c
> index f75ecb0..c65e183 100644
> --- a/drivers/pwm/pwm-mxs.c
> +++ b/drivers/pwm/pwm-mxs.c
> @@ -35,6 +35,8 @@
>  #define  PERIOD_CDIV(div)	(((div) & 0x7) << 20)
>  #define  PERIOD_CDIV_MAX	8
>  
> +static unsigned const int cdiv[PERIOD_CDIV_MAX] = {1, 2, 4, 8, 16, 64, 256, 1024};

I turned this into static const unsigned int, which is the more
canonical form and wrapped this over multiple lines so as not to exceed
the 80-character limit.

Thanks,
Thierry

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