Re: [PATCH 15/16] backlight: remove pandora_bl

[H. Nikolaus Schaller]

> Am 10.08.2026 um 15:23 schrieb H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>:
> 
> 
> 
>> Am 10.08.2026 um 14:39 schrieb Andreas Kemnade <andreas@xxxxxxxxxxxx>:
>> 
>> On Mon, 10 Aug 2026 12:45:16 +0200
>> "H. Nikolaus Schaller" <hns@xxxxxxxxxxxxx> wrote:
>> 
>>> Hi Daniel,
>>> 
>>>> Am 10.08.2026 um 12:35 schrieb Daniel Thompson <danielt@xxxxxxxxxx>:
>>>> 
>>>> On Sat, Jul 11, 2026 at 08:02:02AM +0200, H. Nikolaus Schaller wrote:  
>>>>> Retire the platform specific pandora backlight driver since
>>>>> it can now be replaced by twl_pm and device tree.
>>>>> 
>>>>> Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>  
>>>> 
>>>> Yay!
>>>> 
>>>> Reviewed-by: Daniel Thompson (RISCstar) <danielt@xxxxxxxxxx>  
>>> 
>>> Well, there were some good comments by Sahiko to the prerequisites of this patch,
>>> because it has one speciality the PWM BL driver does not handle (avoid
>>> interpretation of PWM signals as W1 protocol).

Here is the report: https://sashiko.dev/#/patchset/cover.1783749722.git.hns@xxxxxxxxxxxxx?part=15

>>> 
>>> I haven't found time to work on this, so we can't remove this yet.
>>> 
>>> In the end it may either remain a pandora_bl driver (converted to DT)
>>> or become a patch for something else.
>>> 
>> But then it is a TPS61161 on top of PWM, not a pandora driver.
>> And then there might be just a quirk for that chip in the pwm_bl using
>> triggered by a compatible. I guess the quirk is this:
>> 
>> 
>> if (priv->old_state == PANDORABL_WAS_OFF) {
>> - /*
>> - * set PWM duty cycle to max. TPS61161 seems to use this
>> - * to calibrate it's PWM sensitivity when it starts.
>> - */
>> - twl_i2c_write_u8(TWL_MODULE_PWM, MAX_VALUE, TWL_PWM0_OFF);
>> -
>> - /* first enable clock, then PWM0 out */
>> - twl_i2c_read_u8(TWL4030_MODULE_INTBR, &r, TWL_INTBR_GPBR1);
>> - r &= ~PWM0_ENABLE;
>> - r |= PWM0_CLK_ENABLE;
>> - twl_i2c_write_u8(TWL4030_MODULE_INTBR, r, TWL_INTBR_GPBR1);
>> - r |= PWM0_ENABLE;
>> - twl_i2c_write_u8(TWL4030_MODULE_INTBR, r, TWL_INTBR_GPBR1);
>> -
>> - /*
>> - * TI made it very easy to enable digital control, so easy that
>> - * it often triggers unintentionally and disabes PWM control,
>> - * so wait until 1 wire mode detection window ends.
>> - */
>> - usleep_range(2000, 10000);
>> - }
>> 

Exactly. The Sashiko comment to this section was:

When switching to the generic pwm-backlight driver, how is the TPS61161
initialized into PWM mode? Because the TPS61161 shares its EN pin for both 1-wire
EasyScale digital control and PWM control, the removed pandora_bl driver included this
hardware workaround to force a 100% PWM duty cycle and sleep for 2-10ms at
startup. Without this initialization pulse, could the TPS61161 misinterpret the initial
PWM signal as a 1-wire EasyScale command and disable PWM control, preventing
the backlight from turning on properly?

Well, I didn't see this but it would be on the safe side to have it.

> 
> Yes that is what I mean. It needs to be ported somewhere. If it can be ported to
> some compatible = "pwm_bl_for_tps61161" or what fits best it will be fine. Or it
> could be in some compatible = "pandora_bl" driver.
> 
> Has to be worked out when the development machines are not blocked by 3 or 4 other
> projects already running in parallel :)

Nevertheless I did a quick look into the pandora_bl driver structure.

Here are my findings:
- the code fragment is part of pandora_backlight_update_status()
- priv->old_state remembers if brighness was set to 0 or not
- so this code triggers on any change from bright = 0 to bright > 0
- the pandora_bl driver is the only one to use TWL_MODULE_PWM besides twl-pwm

So the most natural way of transplanting this appears to me:
- add this logic to the twl-pwm driver
- add a propery to escribe this special need (e.g. "disable-easyscale")
- use this to triger the extra code in case this flag is set
- and if the pwm duty cycle changes from permanently off to real PWM
- this should obviously go into twl_pwm_config() to detect through duty_ns
- alternativey it might be possible to handle everything in twl4030_pwm_enable

Suggestions welcome.

BR,
Nikolaus