From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lee Jones Subject: Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero Date: Fri, 17 Jun 2016 15:17:19 +0100 Message-ID: <20160617141719.GH21702@dell> References: <1465294429-8570-1-git-send-email-LW@KARO-electronics.de> <20160609135125.GA2385@dell> <20160610072346.51b0a5d5@ipc1.ka-ro> <20160610074449.GB1537@dell> <20160610123453.3a8ee14e@ipc1.ka-ro> <20160610145449.GA7351@dell> <20160611090803.42aeaf5f@ipc1.ka-ro> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <20160611090803.42aeaf5f@ipc1.ka-ro> Sender: linux-pwm-owner@vger.kernel.org To: Lothar =?iso-8859-1?Q?Wa=DFmann?= Cc: Jean-Christophe Plagniol-Villard , Jingoo Han , Thierry Reding , Tomi Valkeinen , linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org, Marcel Ziswiler , Ian Campbell , Kumar Gala , Mark Rutland , Pawel Moll , Rob Herring , devicetree@vger.kernel.org List-Id: devicetree@vger.kernel.org On Sat, 11 Jun 2016, Lothar Wa=C3=9Fmann wrote: > On Fri, 10 Jun 2016 15:54:49 +0100 Lee Jones wrote: > > On Fri, 10 Jun 2016, Lothar Wa=C3=9Fmann wrote: > > > On Fri, 10 Jun 2016 08:44:49 +0100 Lee Jones wrote: > > > > On Fri, 10 Jun 2016, Lothar Wa=C3=9Fmann wrote: > > > >=20 > > > > > Hi, > > > > >=20 > > > > > On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote: > > > > > > On Tue, 07 Jun 2016, Lothar Wa=C3=9Fmann wrote: > > > > > >=20 > > > > > > > 'brightness' is usually an index into a table of duty_cyc= le values, > > > > > > > where the value at index 0 may well be non-zero > > > > > > > (tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts = are real-life > > > > > > > examples). > > > > > > > Thus brightness =3D=3D 0 does not necessarily mean that t= he PWM output > > > > > > > will be inactive. > > > > > > > Check for 'duty_cycle =3D=3D 0' rather than 'brightness =3D= =3D 0' to decide > > > > > > > whether to disable the PWM. > > > > > > >=20 > > > > > > > Signed-off-by: Lothar Wa=C3=9Fmann > > > > > > > --- > > > > > > > Changes wrt. v1: > > > > > > > - update binding docs to reflect the change > > > > > > >=20 > > > > > > > .../devicetree/bindings/leds/backlight/pwm-backlight.txt= | 9 ++++++--- > > > > > > > drivers/video/backlight/pwm_bl.c = | 4 ++-- > > > > > > > 2 files changed, 8 insertions(+), 5 deletions(-) > > > > > > >=20 > > > > > > > diff --git a/Documentation/devicetree/bindings/leds/backl= ight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlig= ht/pwm-backlight.txt > > > > > > > index 764db86..95fa8a9 100644 > > > > > > > --- a/Documentation/devicetree/bindings/leds/backlight/pw= m-backlight.txt > > > > > > > +++ b/Documentation/devicetree/bindings/leds/backlight/pw= m-backlight.txt > > > > > > > @@ -4,10 +4,13 @@ Required properties: > > > > > > > - compatible: "pwm-backlight" > > > > > > > - pwms: OF device-tree PWM specification (see PWM bind= ing[0]) > > > > > > > - brightness-levels: Array of distinct brightness leve= ls. Typically these > > > > > > > - are in the range from 0 to 255, but any range star= ting at 0 will do. > > > > > > > + are in the range from 0 to 255, but any range will= do. > > > > > > > The actual brightness level (PWM duty cycle) will = be interpolated > > > > > > > - from these values. 0 means a 0% duty cycle (darkes= t/off), while the > > > > > > > - last value in the array represents a 100% duty cyc= le (brightest). > > > > > > > + from these values. 0 means a 0% duty cycle, while = the highest value in > > > > > > > + the array represents a 100% duty cycle. > > > > > > > + The range may be in reverse order (starting with t= he maximum duty cycle > > > > > > > + value) to create a PWM signal with the 100% duty c= ycle representing > > > > > > > + minimum and 0% duty cycle maximum brigthness. > > > > > > > - default-brightness-level: the default brightness lev= el (index into the > > > > > > > array defined by the "brightness-levels" property) > > > > > > > - power-supply: regulator for supply voltage > > > > > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/v= ideo/backlight/pwm_bl.c > > > > > > > index b2b366b..80b2b52 100644 > > > > > > > --- a/drivers/video/backlight/pwm_bl.c > > > > > > > +++ b/drivers/video/backlight/pwm_bl.c > > > > > > > @@ -103,8 +103,8 @@ static int pwm_backlight_update_statu= s(struct backlight_device *bl) > > > > > > > if (pb->notify) > > > > > > > brightness =3D pb->notify(pb->dev, brightness); > > > > > > > =20 > > > > > > > - if (brightness > 0) { > > > > > > > - duty_cycle =3D compute_duty_cycle(pb, brightness); > > > > > > > + duty_cycle =3D compute_duty_cycle(pb, brightness); > > > > > > > + if (duty_cycle > 0) { > > > > > >=20 > > > > > > How does this work in the aforementioned: > > > > > >=20 > > > > > > "The range may be in reverse order" > > > > > >=20 > > > > > > ... case? Surely when duty_cycle is when the screen should= be at it's > > > > > > brightest? Wouldn't it confuse the user if they turn their= brightness > > > > > > *up* and the screen goes *off*? > > > > > >=20 > > > > > Assuming that the PWM output is inactive (LOW) when the duty_= cycle is > > > > > set to zero, there will be no difference between operating th= e PWM at > > > > > duty_cycle 0 or disabling it. > > > > >=20 > > > > > Currently, the screen will go bright when it should be off in= this > > > > > case. > > > >=20 > > > > It sounds like we need something that lets the framework know i= f > > > > duty_cycle =3D MAX is the brightest or if duty_cycle =3D 0 is. = Either way > > > > someone is going to get screwed by this logic. > > > >=20 > > > The backlight framework does not (and does not need to) know anyt= hing > > > about PWM duty cycles. Its 'brightness' values are consistently 0= =3D=3D > > > dark, max =3D=3D brightest in either case. > >=20 > > What I'm getting at is; by the look of the documentation, the > > brightest setting can either be a duty cycle of 0 or 255. So what > > happens with your new semantics when the duty cycle of 0 represents > > the brightest setting and you reach 0? Didn't you just turn the > > backlight off? > >=20 > As mentioned earlier, disabling the PWM has generally the same result= as > setting the duty cycle to 0. The current behaviour is broken in this > case, since setting brightness to 0 with a non-zero duty_cycle as the > first element of brightness-levels, the PWM will be disabled rather t= han > switched to the given duty cycle. > Disabling the PWM should have the same effect as setting the duty cyc= le > to 0, so it is safe to check for duty_cycle =3D=3D 0 to decide whethe= r to > disable the PWM. I agree with this. BUT, that's not what you're doing is it? Look at the code you're trying to write: duty_cycle =3D compute_duty_cycle(pb, brightness); if (duty_cycle > 0) { pwm_config(pb->pwm, duty_cycle, pb->period); pwm_backlight_power_on(pb, brightness); } else pwm_backlight_power_off(pb); Let's say duty_cycle =3D=3D 0. In some cases this can mean "turn brightness up to the *maximum*", but with your new logic you just turned the backlight *off*. Conversely, let's say duty_cycle =3D=3D 255. In some cases this can me= an "turn the brightness to the *lowest* setting" i.e. *off*. Well your logic just turned the backlight *on*. If there is something I'm missing, you're going to have to find a better way to explain it to me. --=20 Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org =E2=94=82 Open source software for ARM SoCs =46ollow Linaro: Facebook | Twitter | Blog