st7735r: activate PWM backlight

[pdp7]

I am using Adafruit 1.8" TFT LCD with a BeagleBone Black. I created a device tree overlay, BB-LCD-ADAFRUIT-18-SPI1-00A0.dts, which will load the tinydrm st7735r driver by David Lechner (@dlech).

I ran into a problem when trying to enable PWM backlight control. I was able to turn on the backlight on by setting bl_power to 0 (FB_BLANK_UNBLACK) per comment by @notro:

echo 0 > /sys/devices/platform/backlight_pwm/backlight/backlight_pwm/bl_power

NOTE: FB_BLANK_UNBLACK is specified in sysfs-class-backlight

@notro also commented that:

the tinydrm driver should make sure the backlight turns on when the display pipeline is enabled.

I am creating this issue to discussion a patch to st7735r that would set bl_power correctly for PWM backlight.

[pdp7]

I added mipi->backlight->props.power = FB_BLANK_UNBLANK to st7735r_probe() and it appears to solve the issue.

Is this is a good approach?

diff --git a/drivers/gpu/drm/tinydrm/st7735r.c b/drivers/gpu/drm/tinydrm/st7735r.c
index 98ff447f40b4..65e6cd4e801d 100644
--- a/drivers/gpu/drm/tinydrm/st7735r.c
+++ b/drivers/gpu/drm/tinydrm/st7735r.c
@@ -171,6 +171,7 @@ static int st7735r_probe(struct spi_device *spi)
        mipi->backlight = tinydrm_of_find_backlight(dev);
        if (IS_ERR(mipi->backlight))
                return PTR_ERR(mipi->backlight);
+       mipi->backlight->props.power = FB_BLANK_UNBLANK;
 
        device_property_read_u32(dev, "rotation", &rotation);
 

Related dmesg output with printk()'s for debugging:

[   25.908483] tinydrm/st7735r: st7735r_probe(): mipi->backlight->props=574c5eb5
[   25.915658] tinydrm/st7735r: st7735r_probe(): mipi->backlight->props.power=0x4
[   25.922919] tinydrm/st7735r: st7735r_probe(): mipi->backlight->props.power = FB_BLANK_UNBLANK
[   25.931492] tinydrm/st7735r: st7735r_probe(): mipi->backlight->props.power=0x0
[   25.938754] tinydrm/st7735r: st7735r_probe(): device_property_read_u32(dev, "rotation", &rotation)
[   25.947765] tinydrm/st7735r: st7735r_probe(): call mipi_dbi_spi_init()
[   25.954338] tinydrm/st7735r: st7735r_probe(): mipi_dbi_spi_init(): ret=0
[   25.961077] tinydrm/st7735r: st7735r_probe(): call mipi_dbi_init()

[notro]

Did you try the backlight patchset I mentioned in the other thread?
It looks like it should solve your problem. This is how backlight is enabled now (in linux-next):

static inline int backlight_enable(struct backlight_device *bd)
{
	if (!bd)
		return 0;

	bd->props.power = FB_BLANK_UNBLANK;
	bd->props.fb_blank = FB_BLANK_UNBLANK;
	bd->props.state &= ~BL_CORE_FBBLANK;

	return backlight_update_status(bd);
}

One problem with the backlight subsystem is that is has 3 properties to enable/disable backlight...