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authorBoris Brezillon <boris.brezillon@free-electrons.com>2016-06-14 11:13:11 +0200
committerThierry Reding <thierry.reding@gmail.com>2016-07-11 12:49:26 +0200
commit12f9ce4a519845070d338253ab9528b5d7e2df34 (patch)
treeadcbeed54bd74eb7480f44f9bdfa491c28618f4a /drivers/pwm/pwm-rockchip.c
parentdaa5abc41c80e32ebaf069bd482b7561e0ada71d (diff)
pwm: rockchip: Fix period and duty cycle approximation
The current implementation always round down the duty and period values, while it would be better to round them to the closest integer. These changes are needed in preparation of atomic update support to prevent a period/duty cycle drift when executing several times the 'pwm_get_state() / modify / pwm_apply_state()' sequence. Say you have an expected period of 3.333 us and a clk rate of 112.666667 MHz -- the clock frequency doesn't divide evenly, so the period (stashed in nanoseconds) shrinks when we convert to the register value and back, as follows: pwm_apply_state(): register = period * 112666667 / 1000000000; pwm_get_state(): period = register * 1000000000 / 112666667; or in other words: period = period * 112666667 / 1000000000 * 1000000000 / 112666667; which yields a sequence like: 3333 -> 3328 3328 -> 3319 3319 -> 3310 3310 -> 3301 3301 -> 3292 3292 -> ... (etc) ... With this patch, we'd see instead: period = div_round_closest(period * 112666667, 1000000000) * 1000000000 / 112666667; which yields a stable sequence: 3333 -> 3337 3337 -> 3337 3337 -> ... (etc) ... Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com> Reviewed-by: Brian Norris <briannorris@chromium.org> Tested-by: Brian Norris <briannorris@chromium.org> Tested-by: Heiko Stuebner <heiko@sntech.de> Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Diffstat (limited to 'drivers/pwm/pwm-rockchip.c')
-rw-r--r--drivers/pwm/pwm-rockchip.c7
1 files changed, 3 insertions, 4 deletions
diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c
index 7d9cc9049522..68d72ce8fba5 100644
--- a/drivers/pwm/pwm-rockchip.c
+++ b/drivers/pwm/pwm-rockchip.c
@@ -114,12 +114,11 @@ static int rockchip_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
* default prescaler value for all practical clock rate values.
*/
div = clk_rate * period_ns;
- do_div(div, pc->data->prescaler * NSEC_PER_SEC);
- period = div;
+ period = DIV_ROUND_CLOSEST_ULL(div,
+ pc->data->prescaler * NSEC_PER_SEC);
div = clk_rate * duty_ns;
- do_div(div, pc->data->prescaler * NSEC_PER_SEC);
- duty = div;
+ duty = DIV_ROUND_CLOSEST_ULL(div, pc->data->prescaler * NSEC_PER_SEC);
ret = clk_enable(pc->clk);
if (ret)

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