From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759598AbcILOmE (ORCPT ); Mon, 12 Sep 2016 10:42:04 -0400 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:54839 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759140AbcILOl6 (ORCPT ); Mon, 12 Sep 2016 10:41:58 -0400 X-Sasl-enc: EtezrMyyzN0+IvbarB0ryg1CKG7rmrj73IR7FZFtKiLx 1473691316 Date: Mon, 12 Sep 2016 10:41:55 -0400 From: David Rivshin To: Neil Armstrong Cc: Matthijs van Duin , "H. Nikolaus Schaller" , =?UTF-8?Q?Beno=C3=AEt?= Cousson , Tony Lindgren , Rob Herring , Mark Rutland , Russell King , marek@goldelico.com, linux-omap@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, letux-kernel@openphoenux.org Subject: Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz Message-ID: <20160912104155.267876a2.drivshin@awxrd.com> In-Reply-To: <57D40F2C.7000805@baylibre.com> References: <954d43b570092fe92b8b49cf8f475c027d3d8342.1473066997.git.hns@goldelico.com> <20160910031736.xwnepaoifpedqfbr@squirrel.local> <57D40F2C.7000805@baylibre.com> Organization: Allworx X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 10 Sep 2016 15:48:28 +0200 Neil Armstrong wrote: > Le 10/09/2016 05:17, Matthijs van Duin a écrit : > > On Mon, Sep 05, 2016 at 11:16:38AM +0200, H. Nikolaus Schaller wrote: > >> This helps to get 100% intensity closer to "always on". > >> > >> It compensates for an effect of dmtimer which at 100% still emits short > >> "off" impulses and the startup-time of the DC/DC converter makes > >> backlight intensity not reach full scale. The lower the PWM frequency > >> is, the smaller is this effect. > > > > Sounds to me like you're working around something that should be fixed > > in the pwm-omap-dmtimer driver instead? > > > > Looking at the (baremetal) dmtimer pwm code I wrote ages ago, which > > supports fully off to fully on, I do seem to be handling both endpoints > > in a special way. A rough conversion of my code into C: > > > > // period in timer cycles > > void pwm_init( volatile struct dmtimer *timer, u32 period, bool invert ) > > { > > assert( period >= 2 ); > > timer->if_ctrl = 2; // reset timer, configure as non-posted > > timer->reload = -period; > > timer->trigger = 0; > > timer->config = 0x1043 | invert << 7; // pwm initially disabled > > } > > > > // value in timer cycles, 0 <= value <= period > > void pwm_set( volatile struct dmtimer *timer, u32 value ) > > { > > if( value == 0 ) { > > timer->config &= ~0x800; // disable pwm > > return; > > } > > u32 period = -timer->reload; > > if( value >= period ) > > timer->match = 0; > > else > > timer->match = value - period - 1; > > timer->config |= 0x800; // enable pwm > > } > > > > At the time I used a scope to check the exact behaviour of dmtimer pwm > > on a dm814x. My notes mention (when pwm enabled): > > match < reload output on continuous > > match == reload output on 1 cycle, off period-1 cycles > > match == -2 output on period-1 cycles, off 1 cycle > > match == -1 output freezes > > > > Hope this helps > > Hi, > > I think these corner cases should definitely be handled in the dmtimer driver. Do you mean to modify the dmtimer driver itself, or the pwm-omap-dmtimer driver? IIRC from the last time I was in the pwm-omap-dmtimer driver, it seemed to me that the 0% and 100% cases could/should be handled as simple special cases there. I think the dmtimer driver itself has the necessary API to the HW, but I'd need to re-familiarize myself with it to remember the details of what I was thinking. Actually, I did mention some thoughts on this a previous thread where Adam Ford was using pwm-omap-dmtimer for a backlight: http://www.spinics.net/lists/linux-omap/msg126006.html So it may be as simple as using PWM_OMAP_DMTIMER_TRIGGER_NONE and passing def_on according to whether 0 or 100% duty were requested (and polarity). > > I'll try to post a fix to handle these, thanks for the original code dump. > > > > > Matthijs > > > > Neil > -- > To unsubscribe from this list: send the line "unsubscribe linux-omap" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html