On Wed, Jun 14, 2017 at 7:47 AM, Adam Ford <aford173@xxxxxxxxx> wrote: > On Wed, Jun 14, 2017 at 6:16 AM, Tony Lindgren <tony@xxxxxxxxxxx> wrote: >> * Adam Ford <aford173@xxxxxxxxx> [170614 03:53]: >>> I have a DM3730 with the dmtimer (timer10) controlling the pwm >>> backlight for my LCD. >>> >>> pwms = <&pwm10 0 5000000 0>; >>> brightness-levels = <0 10 20 30 40 50 60 70 80 90 100>; >>> default-brightness-level = <7>; >>> >>> If I cat /dev/urandom > /dev/fb0 to setup a test pattern then start to >>> change the brightness in user-space, there is a momentary flicker on >>> the backlight when the backlight changes intensity. Screens that are >>> predominantly black are not as noticeable. >>> >>> We also have a Win CE implementation on the same board which does not >>> do this, so I believe it to be software related and not hardware. Has >>> anyone else noticed this? If so, might you have any suggestions to >>> reducing this? I checked 4.9.y and 4.11.y and both seem to exhibit >>> the same behavior. I don't have other pwm options, so I cannot >>> determine if this is a problem with dmtimer or the pwm-bl driver. >> >> Maybe some issue changing the timer rate? Could it be that we're >> missing stopping of the dmtimer in the pwm driver during the rate >> change? >> > > I think we do stop the timer during the chage. > > Looking at the code in pwm_omap_dmtimer_config: > > /* > * We MUST stop the associated dual-mode timer before attempting to > * write its registers, but calls to omap_dm_timer_start/stop must > * be balanced so check if timer is active before calling timer_stop. > */ > timer_active = pm_runtime_active(&omap->dm_timer_pdev->dev); > > if (timer_active) > omap->pdata->stop(omap->dm_timer); > > [ Change a bunch of stuff] > > /* If config was called while timer was running it must be reenabled. */ > if (timer_active) > pwm_omap_dmtimer_start(omap); > > I am wondering if we need to somehow sync the stop with the pwm when > the duty cycle is in the off position so it doesn't stay on while we > change the registers. I am not really sure how to go about that. Some > LED's may be active high while others are active low. > Reading the DM3730 Technical Reference manual, it reads: The timer counter register (GPTi.TCRR) can be loaded when stopped or on-the-fly (while counting). I removed the protection code (what shown above) around the changing of the registers, and the flicker went away. I am hoping Neil might have some comment on this. If removing this is deemed acceptable, I can push a patch, but I don't want to arbitrarily assume it would work. > adam > >> Regards, >> >> Tony -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
