| Message ID | 20220125123429.3490883-3-max.kellermann@gmail.com |
|---|---|
| State | Accepted |
| Commit | 8246b478a23aafdef0e1deb2ac2f21eeee97e877 |
| Headers | show |
| Series | [1/3] pwm-sun4i: convert "next_period" to local variable | expand |
On Tue, Jan 25, 2022 at 01:34:29PM +0100, Max Kellermann wrote: > This fixes a problem that was supposed to be addressed by commit > 6eefb79d6f5bc ("pwm: sun4i: Remove erroneous else branch") - backlight > could not be switched off on some Allwinner A20. The commit was > correct, but was not a reliable fix for the problem, which was timing > related. > > The real problem for the backlight switching problem was that sleeping > for a full period did not work, because delay_us is always zero. > > It is zero because the period (plus 1 microsecond) is rounded down to > the next "jiffies", but the period is less than one jiffy. > > On my Cubieboard 2, the period is 5ms, and 1 jiffy (at the default > HZ=100) is 10ms, so nsecs_to_jiffies(10ms+1us)=0. > > The roundtrip from nanoseconds to jiffies and back to microseconds is > an unnecessary loss of precision; always rounding down (via > nsecs_to_jiffies()) then causes the breakage. > > This patch eliminates this roundtrip, and directly converts from > nanoseconds to microseconds (for usleep_range()), using > DIV_ROUND_UP_ULL() to force rounding up. This way, the sleep time is > never zero, and after the sleep, we are guaranteed to be in a > different period, and the device is ready for another control command > for sure. > > Cc: stable@vger.kernel.org > Signed-off-by: Max Kellermann <max.kellermann@gmail.com> Sounds reasonable Acked-by; Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Best regards Uwe
diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c index b44deececb8b..8527a655034c 100644 --- a/drivers/pwm/pwm-sun4i.c +++ b/drivers/pwm/pwm-sun4i.c @@ -234,7 +234,6 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, struct pwm_state cstate; u32 ctrl, duty = 0, period = 0, val; int ret; - unsigned int delay_jiffies; unsigned int delay_us, prescaler = 0; bool bypass; @@ -302,8 +301,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, return 0; /* We need a full period to elapse before disabling the channel. */ - delay_jiffies = nsecs_to_jiffies(cstate.period + 1000); - delay_us = jiffies_to_usecs(delay_jiffies); + delay_us = DIV_ROUND_UP_ULL(cstate.period, NSEC_PER_USEC); if ((delay_us / 500) > MAX_UDELAY_MS) msleep(delay_us / 1000 + 1); else
This fixes a problem that was supposed to be addressed by commit 6eefb79d6f5bc ("pwm: sun4i: Remove erroneous else branch") - backlight could not be switched off on some Allwinner A20. The commit was correct, but was not a reliable fix for the problem, which was timing related. The real problem for the backlight switching problem was that sleeping for a full period did not work, because delay_us is always zero. It is zero because the period (plus 1 microsecond) is rounded down to the next "jiffies", but the period is less than one jiffy. On my Cubieboard 2, the period is 5ms, and 1 jiffy (at the default HZ=100) is 10ms, so nsecs_to_jiffies(10ms+1us)=0. The roundtrip from nanoseconds to jiffies and back to microseconds is an unnecessary loss of precision; always rounding down (via nsecs_to_jiffies()) then causes the breakage. This patch eliminates this roundtrip, and directly converts from nanoseconds to microseconds (for usleep_range()), using DIV_ROUND_UP_ULL() to force rounding up. This way, the sleep time is never zero, and after the sleep, we are guaranteed to be in a different period, and the device is ready for another control command for sure. Cc: stable@vger.kernel.org Signed-off-by: Max Kellermann <max.kellermann@gmail.com> --- drivers/pwm/pwm-sun4i.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)