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: Mon, 20 Jun 2016 09:18:30 +0100 Message-ID: <20160620081830.GK21702@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> <20160617141719.GH21702@dell> <20160620082118.10a639d5@ipc1.ka-ro> <9d79cc78-ef51-9461-c6dc-8a28945440ee@electromag.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <9d79cc78-ef51-9461-c6dc-8a28945440ee@electromag.com.au> Sender: linux-pwm-owner@vger.kernel.org To: Phil Reid Cc: Lothar =?iso-8859-1?Q?Wa=DFmann?= , 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 Mon, 20 Jun 2016, Phil Reid wrote: > On 20/06/2016 14:21, Lothar Wa=C3=9Fmann wrote: > >Hi, > > > >On Fri, 17 Jun 2016 15:17:19 +0100 Lee Jones wrote: > >>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: > >>>>>> > >>>>>>>Hi, > >>>>>>> > >>>>>>>On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote: > >>>>>>>>On Tue, 07 Jun 2016, Lothar Wa=C3=9Fmann wrote: > >>>>>>>> > >>>>>>>>>'brightness' is usually an index into a table of duty_cycle = 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 the = 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. > >>>>>>>>> > >>>>>>>>>Signed-off-by: Lothar Wa=C3=9Fmann > >>>>>>>>>--- > >>>>>>>>>Changes wrt. v1: > >>>>>>>>> - update binding docs to reflect the change > >>>>>>>>> > >>>>>>>>> .../devicetree/bindings/leds/backlight/pwm-backlight.txt = | 9 ++++++--- > >>>>>>>>> drivers/video/backlight/pwm_bl.c = | 4 ++-- > >>>>>>>>> 2 files changed, 8 insertions(+), 5 deletions(-) > >>>>>>>>> > >>>>>>>>>diff --git a/Documentation/devicetree/bindings/leds/backligh= t/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/= pwm-backlight.txt > >>>>>>>>>index 764db86..95fa8a9 100644 > >>>>>>>>>--- a/Documentation/devicetree/bindings/leds/backlight/pwm-b= acklight.txt > >>>>>>>>>+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-b= acklight.txt > >>>>>>>>>@@ -4,10 +4,13 @@ Required properties: > >>>>>>>>> - compatible: "pwm-backlight" > >>>>>>>>> - pwms: OF device-tree PWM specification (see PWM binding= [0]) > >>>>>>>>> - brightness-levels: Array of distinct brightness levels.= Typically these > >>>>>>>>>- are in the range from 0 to 255, but any range startin= g at 0 will do. > >>>>>>>>>+ are in the range from 0 to 255, but any range will do= =2E > >>>>>>>>> The actual brightness level (PWM duty cycle) will be = interpolated > >>>>>>>>>- from these values. 0 means a 0% duty cycle (darkest/o= ff), while the > >>>>>>>>>- last value in the array represents a 100% duty cycle = (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 the = maximum duty cycle > >>>>>>>>>+ value) to create a PWM signal with the 100% duty cycl= e representing > >>>>>>>>>+ minimum and 0% duty cycle maximum brigthness. > >>>>>>>>> - default-brightness-level: the default brightness level = (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/vide= o/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_status(s= truct backlight_device *bl) > >>>>>>>>> if (pb->notify) > >>>>>>>>> brightness =3D pb->notify(pb->dev, brightness); > >>>>>>>>> > >>>>>>>>>- if (brightness > 0) { > >>>>>>>>>- duty_cycle =3D compute_duty_cycle(pb, brightness); > >>>>>>>>>+ duty_cycle =3D compute_duty_cycle(pb, brightness); > >>>>>>>>>+ if (duty_cycle > 0) { > >>>>>>>> > >>>>>>>>How does this work in the aforementioned: > >>>>>>>> > >>>>>>>> "The range may be in reverse order" > >>>>>>>> > >>>>>>>>... case? Surely when duty_cycle is when the screen should b= e at it's > >>>>>>>>brightest? Wouldn't it confuse the user if they turn their b= rightness > >>>>>>>>*up* and the screen goes *off*? > >>>>>>>> > >>>>>>>Assuming that the PWM output is inactive (LOW) when the duty_c= ycle is > >>>>>>>set to zero, there will be no difference between operating the= PWM at > >>>>>>>duty_cycle 0 or disabling it. > >>>>>>> > >>>>>>>Currently, the screen will go bright when it should be off in = this > >>>>>>>case. > >>>>>> > >>>>>>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. > >>>>>> > >>>>>The backlight framework does not (and does not need to) know any= thing > >>>>>about PWM duty cycles. Its 'brightness' values are consistently = 0 =3D=3D > >>>>>dark, max =3D=3D brightest in either case. > >>>> > >>>>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 wha= t > >>>>happens with your new semantics when the duty cycle of 0 represen= ts > >>>>the brightest setting and you reach 0? Didn't you just turn the > >>>>backlight off? > >>>> > >>>As mentioned earlier, disabling the PWM has generally the same res= ult as > >>>setting the duty cycle to 0. The current behaviour is broken in th= is > >>>case, since setting brightness to 0 with a non-zero duty_cycle as = the > >>>first element of brightness-levels, the PWM will be disabled rathe= r than > >>>switched to the given duty cycle. > >>>Disabling the PWM should have the same effect as setting the duty = cycle > >>>to 0, so it is safe to check for duty_cycle =3D=3D 0 to decide whe= ther 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*. > >> > >Huh? Please think again! > > - duty_cycle =3D=3D 0 means a CONSTANT LOW level on the PWM output.= Agreed? > > - Disabling the PWM usually achieves a CONSTANT LOW level on the PW= M > > output. Agreed? > >So duty_cycle =3D=3D 0 <=3D> disable the PWM no matter whether the = backlight > >is darkest or brightest at this duty cycle setting! > > > >The backlight controller does not know anything about the value of t= he > >'brightness' variable in the code but only sees the 'duty_cycle' val= ue. > >When brightness =3D=3D 0 translates into max. duty cycle, the origin= al code > >will switch the PWM OFF (which is equivalent to a ZERO duty cycle), = when > >it rather should operate at the max. duty cycle. > >When duty_cycle is '0', this is equivalent to the PWM output being a= t > >constant LOW level which is the same as being switched OFF in the us= ual > >cases. > > > >When the brightness is maximum at duty_cycle =3D=3D 0, that means, t= hat the > >backlight is brightest when the control pin is constantly LOW, which > >is usually the case when the PWM is disabled. This is exactly what t= he > >patch does achieve! > >With the current code a backlight that is brightest at a constant '0= ' > >level will turn to max. brightness rather than off when selecting > >brightness level 0 (max. PWM duty cycle). > > > >>Conversely, let's say duty_cycle =3D=3D 255. In some cases this ca= n mean > >>"turn the brightness to the *lowest* setting" i.e. *off*. Well your > >>logic just turned the backlight *on*. > >> > >OK. Let's try a sequence of brightness levels and duty cycles: > >For simplicity assume a range of brightness levels from 0..100, so > >that the 'brightness' value directly represents the duty cycle of th= e > >PWM. So either: brightness =3D=3D 0 =3D> duty cycle =3D=3D 0% =3D> c= onstant LOW > >Or: brightnes =3D=3D 0 =3D> duty cycle =3D=3D 100% =3D> constant HIG= H. > > > >Normal range with current and patched code: > > brightness duty_cycle > > 0 0 PWM disabled =3D> constant LOW > > 1 1 PWM active > > ... > > 100 100 PWM active =3D> constant HIGH > > > >Inverted range (backlight brightest at duty cycle 0) > >Current code: > > brightness duty_cycle > > 0 100 PWM disabled (OUTPUT CONSTANT LOW!) > > 1 99 PWM active with near full duty cycle > > ... > > 99 1 PWM active with near ZERO duty cycle > > 100 0 PWM active with 0% duty cycle =3D> cons= tant LOW > > > >With my patch: > > brightness duty_cycle > > 0 100 PWM active with 100% duty cycle (consta= nt HIGH) > > 1 99 PWM active with near full duty cycle > > ... > > 99 1 PWM active with near ZERO duty cycle > > 100 0 PWM disabled =3D> constant LOW You're repleting yourself. I already told you I agreed with this. I think you need to re-read my previous reply. Your code is buggy. > pwm_backlight_power_off() disables the regulator. > So the supply to Backlight disappears, regardless of constant low... Yes exactly. Look at this again: > 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*. When I say that you turned the backlight *off*. I didn't mean you turned it right down using the duty_cycle mechanism, I mean you ripped the socket from the wall. The IP is trying to turn the backlight to the highest setting, but it no longer has power. It's *off*, *off*! --=20 Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org =E2=94=82 Open source software for ARM SoCs =46ollow Linaro: Facebook | Twitter | Blog