[PATCH v3 08/15] pwm: crc: Fix period changes not having any effect

Hans de Goede
2020-06-20T12:19:10Z

The pwm-crc code is using 2 different enable bits:
1. bit 7 of the PWM0_CLK_DIV (PWM_OUTPUT_ENABLE)
2. bit 0 of the BACKLIGHT_EN register

I strongly suspect that the BACKLIGHT_EN register at address 0x51 really
controls a separate output-only GPIO which is connected to the LCD panels
backlight-enable input. Like how the PANEL_EN register at address 0x52
controls an output-only GPIO which is earmarked for the LCD panel's
enable pin. If this is correct then this GPIO should really be added to
the gpio-crystalcove.c driver and the PWM driver should stop poking it.
But I've been unable to come up with a definitive answer here, so I'm
keeping this as is for now.

As the comment in the old code already indicates we must disable the PWM
before we can change the clock divider. But the crc_pwm_disable() and
crc_pwm_enable() calls the old code make for this only change the
BACKLIGHT_EN register; and the value of that register does not matter for
changing the period / the divider. What does matter is that the
PWM_OUTPUT_ENABLE bit must be cleared before a new value can be written.

This commit modifies crc_pwm_config() to clear PWM_OUTPUT_ENABLE instead
when changing the period, so that period changes actually work.

Note this fix will cause a significant behavior change on some devices
using the CRC PWM output to drive their backlight. Before the PWM would
always run with the output frequency configured by the BIOS at boot, now
the period time specified by the i915 driver will actually be honored.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
drivers/pwm/pwm-crc.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/pwm/pwm-crc.c b/drivers/pwm/pwm-crc.c
index 44ec7d5b63e1..81232da0c767 100644
--- a/drivers/pwm/pwm-crc.c
+++ b/drivers/pwm/pwm-crc.c
@@ -82,14 +82,11 @@ static int crc_pwm_config(struct pwm_chip *c, struct pwm_device *pwm,
if (pwm_get_period(pwm) != period_ns) {
int clk_div = crc_pwm_calc_clk_div(period_ns);

- /* changing the clk divisor, need to disable fisrt */
- crc_pwm_disable(c, pwm);
+ /* changing the clk divisor, clear PWM_OUTPUT_ENABLE first */
+ regmap_write(crc_pwm->regmap, PWM0_CLK_DIV, 0);

regmap_write(crc_pwm->regmap, PWM0_CLK_DIV,
clk_div | PWM_OUTPUT_ENABLE);
-
- /* enable back */
- crc_pwm_enable(c, pwm);
}

/* change the pwm duty cycle */
--
2.26.2