From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from galahad.ideasonboard.com ([185.26.127.97]:37326 "EHLO galahad.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753119AbcKUJ7d (ORCPT ); Mon, 21 Nov 2016 04:59:33 -0500 From: Laurent Pinchart To: Geert Uytterhoeven Cc: Laurent Pinchart , DRI Development , Linux-Renesas , Tomi Valkeinen Subject: Re: [PATCH v2 09/13] arm64: dts: r8a7795: salvator-x: Add panel backlight support Date: Mon, 21 Nov 2016 11:59:47 +0200 Message-ID: <2227128.532ZZ8dc1T@avalon> In-Reply-To: References: <1479526093-7014-1-git-send-email-laurent.pinchart+renesas@ideasonboard.com> <1686355.2zT21Pz9pV@avalon> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-renesas-soc-owner@vger.kernel.org List-ID: Hi Geert, On Monday 21 Nov 2016 10:23:46 Geert Uytterhoeven wrote: > On Mon, Nov 21, 2016 at 10:19 AM, Laurent Pinchart wrote: > > On Monday 21 Nov 2016 09:36:22 Geert Uytterhoeven wrote: > >> On Sat, Nov 19, 2016 at 4:28 AM, Laurent Pinchart wrote: > >>> The panel backlight is controlled through a GPIO and a PWM channel. > >>> > >>> --- a/arch/arm64/boot/dts/renesas/r8a7795-salvator-x.dts > >>> +++ b/arch/arm64/boot/dts/renesas/r8a7795-salvator-x.dts > >>> @@ -178,6 +178,16 @@ > >>> }; > >>> }; > >>> }; > >>> + > >>> + backlight: backlight { > >>> + compatible = "pwm-backlight"; > >>> + pwms = <&pwm1 0 50000>; > >>> + > >>> + brightness-levels = <256 128 64 16 8 4 0>; > >> > >> Would it make sense to define more and/or linear levels? > > > > Possibly, this is pretty arbitrary. Linear levels might not be the best > > option given that the human eye doesn't have a linear response to light > > power, but we > > It not only depends on the human eye, but also on the backlight hardware > (is the conversion from voltage (L_VBRT) to light linear?). So we need to specify transfer functions in DT ;-) > > could certainly have more levels. In that case I'd prefer modifying the > > pwm- backlight DT bindings though, and specifying the PWM resolution > > instead of discrete levels. > > > > Note that the LVDS panel backlight PWM control signal is multiplexed with > > the external memory A21 signal on the Salvator-X board, with SW5 > > selecting which how to route the signal. When using backlight control we > > can't access the whole NOR flash anymore, so I'm not sure this patch > > should be merged. > > That NOR flash is also optional, right? > My Ex Memory Connector is not populated. That's correct. The Salvator-X DT file in mainline is just an example anyway, and we should pick the most useful peripherals for that purpose. -- Regards, Laurent Pinchart