[PATCHv5 2/2] leds: tlc591xx: Driver for the TI 8/16 Channel i2c LED driver

[Peter Ujfalusi]

On 01/27/2015 01:11 PM, Tomi Valkeinen wrote:
> Hi,
> 
> On 26/01/15 19:10, Andrew Lunn wrote:
>>> So... To me it's still slightly unclear when should one write a PWM
>>> driver and when a LED driver. But I would say that as the TLC591xx
>>> outputs a PWM signal, it should be a PWM driver. Then the different
>>> users of this PWM signal could be made on top of that (LED, backlight, GPO).
>>>
>>> What would be the technical drawbacks with having the TLC591xx driver as
>>> a PWM, instead of LED?
>>  
>> Hi Tomi
>>
>> We have been through this once, but the big technical drawback is that
>> this hardware cannot do what the Linux Kernel defines as PWM.
>>
>> It cannot correctly implement the PMW call:
>>
>> int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
>>
>> This hardware has a fixed period, since it is clocked at 97-kHz.  So
>> you cannot set the period. The duty is also somewhat restrictive, in
>> that it only allows 1/256 increments of the 97Khz.

The duty_ns and period_ns will give you the on and off time your user is
asking for. If you can not change frequency, then you don't (97KHz that is).
1/256 is the resolution you can configure the on/off times, nothing
exceptional about it.
The twl4030/5030's LED drivers have 128 clock cycle to play with while twl6030
has 256 clock cycles. These PMICs also have PWM drivers which has 128 steps.
drivers/pwm/pwm-twl.c and pwm-twl-led.c

and these just work fine when used via pwm-led or the pwm backlight or
whatever driver.

There's no issue to have PWM driver for tlc591xx IMHO.

> Surely other PWM devices also have restrictions in the period or duty cycle?
> 
>> This hardware does however perfectly fit the LED API:
>>
>> enum led_brightness {
>>         LED_OFF         = 0,
>>         LED_HALF        = 127,
>>         LED_FULL        = 255,
>> };
>>
>>         void            (*brightness_set)(struct led_classdev *led_cdev,
>>                                           enum led_brightness brightness);
>>
>> So we can model it perfectly as an LED driver, or badly as a PWM
>> driver.
> 
> Maybe so, but what does it mean in practice? If it's implemented as a
> PWM driver, and the existing leds-pwm driver is used for the led
> functionality, shall we miss some brightness values? Is the
> configuration more difficult? Or what?
> 
> So my point here is that it outputs a PWM signal, so it makes sense to
> have it as a PWM driver. It has restricted configurability compared to
> more versatile PWM hardware, but I so far don't see why that would be an
> issue.
> 
> If it is a PWM driver, we get backlight support for free via the
> existing pwm_bl driver, and LED support via leds-pwm. And there has been
> a clear acceptance on GPO functionality with PWM outputs (in the Peter's
> mail thread), whereas I would bet that a LED based GPO functionality
> would encounter resistance, because that doesn't quite make sense.
> 
>  Tomi
> 
> 


-- 
Péter