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SubjectRe: [PATCH v2 1/1] pwm: mediatek: add longer period support
On Tue, Mar 03, 2020 at 06:19:15PM +0800, Sam Shih wrote:
> The pwm clock source could be divided by 1625 with PWM_CON
> BIT(3) setting in mediatek hardware.
>
> This patch add support for longer pwm period configuration,
> which allowing blinking LEDs via pwm interface.
>
> Signed-off-by: Sam Shih <sam.shih@mediatek.com>
> ---
> drivers/pwm/pwm-mediatek.c | 34 ++++++++++++++++++++++++++++++----
> 1 file changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
> index b94e0d09c300..c64ecff6c550 100644
> --- a/drivers/pwm/pwm-mediatek.c
> +++ b/drivers/pwm/pwm-mediatek.c
> @@ -121,8 +121,11 @@ static int pwm_mediatek_config(struct pwm_chip *chip, struct pwm_device *pwm,
> int duty_ns, int period_ns)
> {
> struct pwm_mediatek_chip *pc = to_pwm_mediatek_chip(chip);
> - u32 clkdiv = 0, cnt_period, cnt_duty, reg_width = PWMDWIDTH,
> - reg_thres = PWMTHRES;
> + /* The source clock is divided by 2^clkdiv or iff the clksel bit
> + * is set by (2^clkdiv*1625)
> + */

Please put the /* on it's own line. See
https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting

> + u32 clkdiv = 0, clksel = 0, cnt_period, cnt_duty,
> + reg_width = PWMDWIDTH, reg_thres = PWMTHRES;
> u64 resolution;
> int ret;
>
> @@ -133,12 +136,30 @@ static int pwm_mediatek_config(struct pwm_chip *chip, struct pwm_device *pwm,
>
> /* Using resolution in picosecond gets accuracy higher */
> resolution = (u64)NSEC_PER_SEC * 1000;
> + /* Calculate resolution based on current clock frequency */
> do_div(resolution, clk_get_rate(pc->clk_pwms[pwm->hwpwm]));
> -
> + /* Using resolution to calculate cnt_period which represents
> + * the effective range of the PWM period counter
> + */
> cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * 1000, resolution);

The existing code is rather bad. A better approach to calulate
cnt_period (with a single division and higher accuracy):

cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * clk_get_rate(..), NSEC_PER_SEC);


> while (cnt_period > 8191) {
> + /* Using clkdiv to reduce clock frequency and calculate
> + * new resolution based on new clock speed
> + */
> resolution *= 2;
> clkdiv++;
> + if (clkdiv > PWM_CLK_DIV_MAX && !clksel) {
> + /* Using clksel to divide the pwm source clock by
> + * an additional 1625, and recalculate new clkdiv
> + * and resolution
> + */
> + clksel = 1;
> + clkdiv = 0;
> + resolution = (u64)NSEC_PER_SEC * 1000 * 1625;
> + do_div(resolution,
> + clk_get_rate(pc->clk_pwms[pwm->hwpwm]));
> + }
> + /* Calculate cnt_period based on resolution */
> cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * 1000,
> resolution);
> }
> @@ -158,8 +179,13 @@ static int pwm_mediatek_config(struct pwm_chip *chip, struct pwm_device *pwm,
> reg_thres = PWM45THRES_FIXUP;
> }
>
> + /* Calculate cnt_duty based on resolution */
> cnt_duty = DIV_ROUND_CLOSEST_ULL((u64)duty_ns * 1000, resolution);

Using resolution as divisor is bad here, too, so the way to calculate
cnt_duty should be changed accordingly.

I think if the driver is simplified first, maybe even getting rid of the
while loop, your change will get considerably easier, too.

> - pwm_mediatek_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | clkdiv);
> + if (clksel)
> + pwm_mediatek_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | BIT(3) |
> + clkdiv);
> + else
> + pwm_mediatek_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | clkdiv);
> pwm_mediatek_writel(pc, pwm->hwpwm, reg_width, cnt_period);
> pwm_mediatek_writel(pc, pwm->hwpwm, reg_thres, cnt_duty);

Best regards
Uwe

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

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