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[82.37.195.13]) by smtp.gmail.com with ESMTPSA id j6-20020a05600c488600b004080f0376a0sm9320997wmp.42.2023.10.23.05.49.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Oct 2023 05:49:11 -0700 (PDT) Date: Mon, 23 Oct 2023 13:49:09 +0100 From: Daniel Thompson To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: Philipp Zabel , Thierry Reding , Lee Jones , Jingoo Han , Helge Deller , linux-pwm@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel@pengutronix.de Subject: Re: [PATCH] backlight: pwm_bl: Avoid backlight flicker applying initial PWM state Message-ID: <20231023124909.GC49511@aspen.lan> References: <20230608-backlight-pwm-avoid-flicker-v1-1-afd380d50174@pengutronix.de> <20231018210741.6t3yfj6qgmpwhhlo@pengutronix.de> <20231020112727.GF23755@aspen.lan> <20231020121148.3g6t3v5uuyubifpb@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20231020121148.3g6t3v5uuyubifpb@pengutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org On Fri, Oct 20, 2023 at 02:11:48PM +0200, Uwe Kleine-König wrote: > Hello Daniel, > > On Fri, Oct 20, 2023 at 12:27:27PM +0100, Daniel Thompson wrote: > > On Wed, Oct 18, 2023 at 11:07:41PM +0200, Uwe Kleine-König wrote: > > > Hello Philipp, > > > > > > On Thu, Jun 08, 2023 at 04:11:14PM +0200, Philipp Zabel wrote: > > > > The initial PWM state returned by pwm_init_state() has a duty cycle > > > > of 0 ns. > > > > > > This is only true for drivers without a .get_state() callback, isn't it? > > > > pwm_init_state() explicitly zeros the duty-cycle in order to avoid > > problems when the default args have a different period to the currently > > applied config: > > https://elixir.bootlin.com/linux/latest/source/include/linux/pwm.h#L174 > > Ah right, pwm_init_state() is strange in a different way than I > remembered :-) pwm_get_state() is only called to get .enabled set > appropriately. > > Looking at the callers: > > > - drivers/video/backlight/lm3630a_bl.c > explictily sets .enabled before calling pwm_apply_state() > > - drivers/video/backlight/lp855x_bl.c > explictily sets .enabled before calling pwm_apply_state() > > - drivers/video/backlight/pwm_bl.c > This is the one we currently discuss. I think even with the patch > applied it uses the .enabled value returned by pwm_init_state() but > it shouldn't. Agreed. > So all consumers using pwm_init_state() either don't use the .enabled > value returned by pwm_init_state() or at least shouldn't do that. Looking a little deeper in the PWM code, it looks to me like pwm_bl.c could just use pwm_adjust_config() during probe to transition between bootloader settings and kernel settings! Daniel.