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[82.132.213.94]) by smtp.gmail.com with ESMTPSA id y10sm5524508wmj.2.2019.07.05.03.30.05 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Fri, 05 Jul 2019 03:30:07 -0700 (PDT) Date: Fri, 5 Jul 2019 11:29:59 +0100 From: Daniel Thompson To: Pavel Machek Cc: Jean-Jacques Hiblot , jacek.anaszewski@gmail.com, robh+dt@kernel.org, mark.rutland@arm.com, lee.jones@linaro.org, jingoohan1@gmail.com, dmurphy@ti.com, linux-leds@vger.kernel.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, tomi.valkeinen@ti.com Subject: Re: [PATCH 0/4] Add a generic driver for LED-based backlight Message-ID: <20190705102959.cmqhzpzikqjsydlx@holly.lan> References: <20190701151423.30768-1-jjhiblot@ti.com> <20190705101434.fw5rpctnqt6dwg6e@devuan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190705101434.fw5rpctnqt6dwg6e@devuan> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 05, 2019 at 12:14:34PM +0200, Pavel Machek wrote: > On Mon 2019-07-01 17:14:19, Jean-Jacques Hiblot wrote: > > This series aims to add a led-backlight driver, similar to pwm-backlight, > > but using a LED class device underneath. > > > > A few years ago (2015), Tomi Valkeinen posted a series implementing a > > backlight driver on top of a LED device: > > https://patchwork.kernel.org/patch/7293991/ > > https://patchwork.kernel.org/patch/7294001/ > > https://patchwork.kernel.org/patch/7293981/ > > > > The discussion stopped because Tomi lacked the time to work on it. > > > > This series takes it from there and implements the binding that was > > discussed in https://patchwork.kernel.org/patch/7293991/. In this new > > binding the backlight device is a child of the LED controller instead of > > being another platform device that uses a phandle to reference a LED. > > Other option would be to have backlight trigger. What are > advantages/disadvantages relative to that? I spent a little time thinking about that during the recent round of reviews. My rough thoughts were that LED triggers would be a good way to handle it in the kernel code and would trivially solve a backlight composed of multiple LED devices (since all could attach to the same trigger). However I think it would be difficult to use the existing DT bindings for triggers to express things like brightness curves and to handle systems with multiple heads. I'm not *too* worried about conflict with the existing backlight trigger (which isn't actually a backlight... just a hook into the framebuffer code to operate a binary LED). People like Daniel Vetter are labouring diligently to get rid of the notifier it depends on so turning it into a proper backlight device would probably help with that effort. Anyhow... having thought the above I then shook myself a bit and figured it was more important to focus on sane bindings that on what the kernel does under the covers to realize them ;-) and decided to keep quiet until the next set of bindings is proposed. However... since you asked... Daniel.