Re: pwm: Introduce the pwm_args concept regression

["Mika Penttilä" <mika.penttila@nextfour.com>]

On 17.05.2016 15:05, Thierry Reding wrote:
> On Tue, May 17, 2016 at 12:16:32PM +0200, Boris Brezillon wrote:
>> On Tue, 17 May 2016 12:48:19 +0300
>> Mika Penttilä <mika.penttila@nextfour.com> wrote:
>>
>>> On 05/17/2016 11:31 AM, Boris Brezillon wrote:
>>>> +Thierry
>>>>
>>>> Hi Mika,
>>>>
>>>> On Tue, 17 May 2016 08:39:59 +0300
>>>> Mika Penttilä <mika.penttila@nextfour.com> wrote:
>>>>   
>>>>> Hi,
>>>>>
>>>>> commit e39c0df1be5a97e0910b09af1530bdf3de057a06, pwm: Introduce the pwm_args concept
>>>>> causes pwm backlight to fail on our imx6 based board. I suspect it affects all pwm backlight setups.
>>>>> With this reverted, everything works as normal.  
>>>> Hm, just tried using a PWM backlight on an atmel board (at91sama5d4ek),
>>>> and after applying the following patch, it worked correctly (that's actually
>>>> a bug in the PWM driver: the HLCDC PWM device is configured with inversed
>>>> polarity by default, and the driver was not taking that into account).
>>>> I doubt you have the same problem here since your PWM device is not
>>>> supporting polarity configuration.
>>>>
>>>> Could you tell me more about this bug? What are the symptoms? Which
>>>> board are you testing on (if it's not mainlined, can you share your
>>>> DT)?
>>>>
>>>> Thanks,
>>>>
>>>> Boris
>>>>
>>>> ---
>>>> diff --git a/drivers/pwm/pwm-atmel-hlcdc.c b/drivers/pwm/pwm-atmel-hlcdc.c
>>>> index f994c7e..14fc011 100644
>>>> --- a/drivers/pwm/pwm-atmel-hlcdc.c
>>>> +++ b/drivers/pwm/pwm-atmel-hlcdc.c
>>>> @@ -272,7 +272,7 @@ static int atmel_hlcdc_pwm_probe(struct platform_device *pdev)
>>>>         chip->chip.of_pwm_n_cells = 3;
>>>>         chip->chip.can_sleep = 1;
>>>>  
>>>> -       ret = pwmchip_add(&chip->chip);
>>>> +       ret = pwmchip_add_with_polarity(&chip->chip, PWM_POLARITY_INVERSED);
>>>>         if (ret) {
>>>>                 clk_disable_unprepare(hlcdc->periph_clk);
>>>>                 return ret;
>>>>   
>>> Symptom is entirely dark screen.
>>> The board is not mainlined but the backlight part is defined like : 
>>>
>>>   backlight: backlight {
>>>                 compatible = "pwm-backlight";
>>>                 pwms = <&pwm2 0 1000000 PWM_POLARITY_INVERTED>;
>>>                 /*
>>>                  * a poor man's way to create a 1:1 relationship between
>>>                  * the PWM value and the actual duty cycle
>>>                  */
>>>                 brightness-levels = < 0  1  2  3  4  5  6  7  8  9
>>>                                      10 11 12 13 14 15 16 17 18 19
>>>                                      20 21 22 23 24 25 26 27 28 29
>>>                                      30 31 32 33 34 35 36 37 38 39
>>>                                      40 41 42 43 44 45 46 47 48 49
>>>                                      50 51 52 53 54 55 56 57 58 59
>>>                                      60 61 62 63 64 65 66 67 68 69
>>>                                      70 71 72 73 74 75 76 77 78 79
>>>                                      80 81 82 83 84 85 86 87 88 89
>>>                                      90 91 92 93 94 95 96 97 98 99
>>>                                     100>;  
>>>                 default-brightness-level = <50>;
>>>         };
>>>
>>>
>>> so the polarity is in there defined.
>>> The call call graphs is like : devm_pwm_get() -> pwm_get() -> of_pwm_get() 
>>> -> of_pwm_xlate_with_flags which used to call pwm_set_period and pwm_set_polarity but now  
>>> just update the struct values.
> Adding the linux-pwm mailing list on Cc for archiving.
>
> Do you have any local modifications? of_pwm_xlate_with_flags() is only
> used if explicitly set up by the PWM driver and those that explicitly do
> so are:
>
> 	$ git grep -n '=.*of_pwm_xlate_with_flags' -- drivers/pwm/
> 	drivers/pwm/pwm-atmel-hlcdc.c:271:      chip->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-atmel-tcb.c:397:        tcbpwm->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-atmel.c:378:            atmel_pwm->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-bcm-kona.c:277: kp->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-berlin.c:181:   pwm->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-fsl-ftm.c:445:  fpc->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-lpc18xx-sct.c:383:      lpc18xx_pwm->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-omap-dmtimer.c:318:     omap->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-renesas-tpu.c:421:      tpu->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-rockchip.c:269:         pc->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-samsung.c:517:          chip->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-sun4i.c:335:    pwm->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-tiecap.c:229:   pc->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-tiehrpwm.c:460: pc->chip.of_xlate = of_pwm_xlate_with_flags;
> 	drivers/pwm/pwm-vt8500.c:219:   chip->chip.of_xlate = of_pwm_xlate_with_flags;
>
> pwm-imx is not among them and since it doesn't set anything else it
> should be using of_pwm_simple_xlate(). Also, the pwm-backlight driver
> now calls pwm_apply_args() so it should still end up calling
> pwm_set_polarity().
>
> Can you paste the DT node that has the pwm2 label? I suspect it's the
> one in arch/arm/boot/dts/imx6qdl.dtsi (or something similar). That node
> has #pwm-cells = <2>, so providing the polarity in a third cell is not
> correct. The PWM core /should/ be able to deal with it correctly, but I
> think it might be worth correcting the DTS. There are a bunch of other
> DTS files that contain the same mistake.
>
> If that doesn't help, perhaps you can attach dmesg output to help debug
> this further? The PWM core is fairly silent, though, so you might need
> to add some debugging messages in order to be able to see anything. I'm
> thinking a first step would be to make sure all of the driver callbacks
> get called in the sequence that they should (and with the expected
> parameters).
>
> Let me know if you need help with that.
>
> Thierry

Yes you're right the pwm-cells is <2> and polarity in dts has no effect, and yes it
uses pwm_simple_xlate of course. The polarity is ok, but something broke with this patch,
looks like the pwm hw state is stuck to zero even when modified thru /sys/class/backlight/backlight/brightness
(the values in sysfs do change). I can debug this further tomorrow.

--Mika

it's polarity issue, we have it right in hw and it all worked before this patch.