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 Sounds reasonable Acked-by; Uwe Kleine-König Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ |