On Thu, Feb 24, 2022 at 05:51:35PM +0100, Uwe Kleine-König wrote:
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
/me had problems with is keyboard, the Shift-key didn't work and so I typed a ; instead a :. To make patchwork pick up my tag, I'll repeat it here:
Acked-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de
Thanks Uwe