The previous behavior was a little unexpected, its properties/problems:
1. It was designed to generate strictly increasing values (no repeats)
2. It had quantization errors when calculating step size. Resulting in
unexpected jumps near the end of some segments.

Example settings:
	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
	num-interpolated-steps = <16>;

Whenever num-interpolated-steps was larger than the distance
between 2 consecutive brightness levels the table would get really
discontinuous. The slope of the interpolation would stick with
integers only and if it was 0 the whole line segment would get skipped.

The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
and only starting with 16 it would start to interpolate properly.

Property #1 is not enough. The goal here is more than just monotonically
increasing. We should still care about the shape of the curve. Repeated
points might be desired if we're in the part of the curve where we want
to go slow (aka slope near 0).

Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
the calculated slope on the 32-63 segment will be almost half as it
should be.

The most expected and simplest algorithm for interpolation is linear
interpolation, which would handle both problems.
Let's just implement that!

Take pairs of points from the brightness-levels array and linearly
interpolate between them. On the X axis (what userspace sees) we'll
now have equally sized intervals (num-interpolated-steps sized,
as opposed to before where we were at the mercy of quantization).

Signed-off-by: Alexandru Stan <amstan@chromium.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Pull backlight updates from Lee Jones:
 "New Drivers:
   - Add support for KTD253

  Fix-ups:
   - Add Device Tree documentation; common, kinetic,ktd253
   - Use correct header(s); tosa_lcd, tosa_bl

  Bug Fixes:
   - Fix refcount imbalance; sky81452-backlight"

* tag 'backlight-next-5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight:
  backlight: tosa_bl: Include the right header
  backlight: tosa_lcd: Include the right header
  backlight: Add Kinetic KTD253 backlight driver
  dt-bindings: backlight: Add Kinetic KTD253 bindings
  dt-bindings: backlight: Add some common backlight properties
  backlight: sky81452-backlight: Fix refcount imbalance on error

The Tosa backlight driver was converted to use GPIO descriptors
in 0b0cb52bd80eda76c4b9921f5cf9c1b709d44e83
("video: backlight: tosa: Use GPIO lookup table") but
still includes <linux/gpio.h> rather than <linux/gpio/consumer.h>.
Fix it.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

The Tosa backlight LCDE driver was converted to use GPIO descriptors
in 0b0cb52bd80eda76c4b9921f5cf9c1b709d44e83
("video: backlight: tosa: Use GPIO lookup table") but
still includes <linux/gpio.h> rather than <linux/gpio/consumer.h>.
Fix it.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

The Kinetic KTD253 backlight driver is controlled with a
single GPIO line, but still supports a range of brightness
settings by sending fast pulses on the line.

This is based off the source code release for the Samsung
GT-S7710 mobile phone.

Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

When of_property_read_u32_array() returns an error code, a
pairing refcount decrement is needed to keep np's refcount
balanced.

Fixes: f705806c9f355 ("backlight: Add support Skyworks SKY81452 backlight driver")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Pull pwm updates from Thierry Reding:
 "The majority of this batch is conversion of the PWM period and duty
  cycle to 64-bit unsigned integers, which is required so that some
  types of hardware can generate the full range of signals that they're
  capable of.

  The remainder is mostly minor fixes and cleanups"

* tag 'pwm/for-5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/thierry.reding/linux-pwm:
  pwm: bcm-iproc: handle clk_get_rate() return
  pwm: Replace HTTP links with HTTPS ones
  pwm: omap-dmtimer: Repair pwm_omap_dmtimer_chip's broken kerneldoc header
  pwm: mediatek: Provide missing kerneldoc description for 'soc' arg
  pwm: bcm-kona: Remove impossible comparison when validating duty cycle
  pwm: bcm-iproc: Remove impossible comparison when validating duty cycle
  pwm: iqs620a: Use lowercase hexadecimal literals for consistency
  pwm: Convert period and duty cycle to u64
  clk: pwm: Use 64-bit division function
  backlight: pwm_bl: Use 64-bit division function
  pwm: sun4i: Use nsecs_to_jiffies to avoid a division
  pwm: sifive: Use 64-bit division macro
  pwm: iqs620a: Use 64-bit division
  pwm: imx27: Use 64-bit division macro
  pwm: imx-tpm: Use 64-bit division macro
  pwm: clps711x: Use 64-bit division macro
  hwmon: pwm-fan: Use 64-bit division macro
  drm/i915: Use 64-bit division macro

Pull backlight updates from Lee Jones:
 "Core Framework:
   - Trivial: Code refactoring
   - New API backlight_is_blank()
   - New API backlight_get_brightness()
   - Additional/reworked documentation
   - Remove 'extern' labels from prototypes
   - Drop backlight_put()
   - Staticify of_find_backlight()

  Driver Removal:
   - Removal of unused OT200 driver
   - Removal of unused Generic Backlight driver

  Fix-ups
   - Bunch of W=1 warning fixes
   - Convert to GPIO descriptors; sky81452
   - Move platform data handling into driver; sky81452
   - Remove superfluous code; lms501kf03
   - Many instances of using new APIs"

* tag 'backlight-next-5.9' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight: (34 commits)
  video: backlight: cr_bllcd: Remove unused variable 'intensity'
  backlight: backlight: Make of_find_backlight static
  backlight: backlight: Drop backlight_put()
  backlight: Use backlight_get_brightness() throughout
  backlight: jornada720_bl: Introduce backlight_is_blank()
  backlight: gpio_backlight: Simplify update_status()
  backlight: cr_bllcd: Introduce gpio-backlight semantics
  backlight: as3711_bl: Simplify update_status
  backlight: backlight: Introduce backlight_get_brightness()
  doc-rst: Wire-up Backlight kernel-doc documentation
  backlight: backlight: Add overview and update existing doc
  backlight: backlight: Drop extern from prototypes
  backlight: generic_bl: Remove this driver as it is unused
  backlight: backlight: Document enums in backlight.h
  backlight: backlight: Document inline functions in backlight.h
  backlight: backlight: Improve backlight_device documentation
  backlight: backlight: Improve backlight_properties documentation
  backlight: backlight: Improve backlight_ops documentation
  backlight: backlight: Add backlight_is_blank()
  backlight: backlight: Refactor fb_notifier_callback()
  ...

Fixes the following kernel build warning:

 drivers/video/backlight/cr_bllcd.c: In function ‘cr_backlight_set_intensity’:
 drivers/video/backlight/cr_bllcd.c:62:6: warning: unused variable ‘intensity’ [-Wunused-variable]
 62 | int intensity = bd->props.brightness;
 | ^~~~~~~~~

Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Fixes: 24d34617c24f ("backlight: cr_bllcd: Introduce gpio-backlight semantics")
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>