From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 3247DC04A68 for ; Wed, 27 Jul 2022 06:25:25 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 78C8DC1981; Wed, 27 Jul 2022 06:25:24 +0000 (UTC) Received: from mail-qt1-x833.google.com (mail-qt1-x833.google.com [IPv6:2607:f8b0:4864:20::833]) by gabe.freedesktop.org (Postfix) with ESMTPS id EC136C1981 for ; Wed, 27 Jul 2022 06:25:23 +0000 (UTC) Received: by mail-qt1-x833.google.com with SMTP id a9so12011345qtw.10 for ; Tue, 26 Jul 2022 23:25:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=3TPo7Ien9QeOugxuEK5vZ4jEbRvQ6L+0JHSHIFL79V8=; b=Ju/fvdD+2tscdEHB4jMKbtF6QmWjmZhx4K1G8NkrcLADIpn1JSvESTmxlpt3R4JNiz UREKmBz7PU2Y4uvwc4LZREeVj+DUEo2k1jrd7hNHdeyJaPFWuyOYHw16/YljzGwcQaoU IH9nnG8x+s+fp3W9UU/Nffn3iiGxPKEQbqR3X0123vmqEq4yNkiC0A2VH4Sesf85arfJ RzfM19DJEhBx9KsylbfN02wrMkNXOO4yndJf2oYfq2BmSNFuvo9Rdv3pkq/RJsxGlZB+ ENmHhdFgodOhI3rw8EXt41BmEN6clXlYa2qisFRSVLQ9+VWdxO1/quC1BwvjDvoQk+pS Zcuw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=3TPo7Ien9QeOugxuEK5vZ4jEbRvQ6L+0JHSHIFL79V8=; b=VBxd+nNMnvJkO8+3jpnuqrLok3SM+Y+JxvJab3FmMU7gHOlUkJpctZ3q/JdpPSs0Rh VHNYjhuEep65FcedDdTeNYBtd9p/9+gMcqaQuGz9AQUERQCRIssIXjq/y3mNs4BDbNdM geuu/qAhMAkOFc3QBajuulrkElrDoBrMjtleLR4mD5+zabSNNLH9p1i27tJan/QdPlgV kky3iojbADTSyvGGgaoEeEPPdMHBZBCHiUxFOvCwp3s1ASFB3pTxAMGzhObTRYb4km2K bkzlzlRgqucltyEvXGWQSPGvl7JlXFZjgM/X/Bwp/hWCFYGdj1CZ4llZnzwhjNVhNMbT Yqew== X-Gm-Message-State: AJIora+u3QLgK6SLTsDOsNT4qB2R7zybZVbNsFZg0dc1dzoiUadXYr9h YgmgBS/0z9r2dgFq2eTtQjoiV97MCNz+wzuXH1E= X-Google-Smtp-Source: AGRyM1tfME3ecCaCTfMzeCQrYhvxtlKmxqJRxH1lMRN/F0J8TcAZeGQUzQp4E/o76yd4w8Asm4EShy68HPneD7bl0pE= X-Received: by 2002:a05:622a:1742:b0:31f:ef:f6eb with SMTP id l2-20020a05622a174200b0031f00eff6ebmr17635908qtk.548.1658903123036; Tue, 26 Jul 2022 23:25:23 -0700 (PDT) MIME-Version: 1.0 References: <20220722102407.2205-1-peterwu.pub@gmail.com> <20220722102407.2205-14-peterwu.pub@gmail.com> <20220725103128.xtaw2c4y5fobowg7@maple.lan> <20220726093058.2fz2p2vg7xpfsnfe@maple.lan> <20220726115954.kpkmidrk3zo3dpbq@maple.lan> In-Reply-To: <20220726115954.kpkmidrk3zo3dpbq@maple.lan> From: ChiaEn Wu Date: Wed, 27 Jul 2022 14:24:46 +0800 Message-ID: Subject: Re: [PATCH v6 13/13] video: backlight: mt6370: Add MediaTek MT6370 support To: Daniel Thompson Content-Type: text/plain; charset="UTF-8" X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "open list:FRAMEBUFFER LAYER" , "Krogerus, Heikki" , Krzysztof Kozlowski , Alice Chen , linux-iio , dri-devel , Liam Girdwood , ChiYuan Huang , Pavel Machek , Lee Jones , Linux LED Subsystem , Helge Deller , Rob Herring , Andy Shevchenko , Chunfeng Yun , Guenter Roeck , devicetree , Linux PM , szuni chen , Mark Brown , "moderated list:ARM/Mediatek SoC support" , Matthias Brugger , linux-arm Mailing List , Jingoo Han , USB , Sebastian Reichel , Linux Kernel Mailing List , ChiaEn Wu , Greg Kroah-Hartman , Jonathan Cameron Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Tue, Jul 26, 2022 at 7:59 PM Daniel Thompson wrote: ... > > > > > Does the MT6372 support more steps than this? In other words does it use > > > > > a fourteen bit scale or does it use an 11-bit scale at a different > > > > > register location? > > > > > > > > Hi Daniel, > > > > > > > > Thanks for your reply. > > > > Yes, MT6372 can support 16384 steps and uses a 14-bit scale register > > > > location. But the maximum current of each > > > > channel of MT6372 is the same as MT6370 and MT6371, both 30mA. > > > > The main reason why MT6372 is designed this way is that one of the > > > > customers asked for a more delicate > > > > adjustment of the backlight brightness. But other customers actually > > > > do not have such requirements. > > > > Therefore, we designed it this way for maximum compatibility in software. > > > > Sorry for I used of the wrong word, I mean is 'driver', not > > higher-level software > > > > > > > > I don't think that is an acceptable approach for the upstream kernel. > > > > > > To be "compatible" with (broken) software this driver ends up reducing > > > the capability of the upstream kernel to the point it becomes unable to > > > meet requirements for delicate adjustment (requirements that were > > > sufficiently important to change the hardware design so you could meet > > > them). > > > > Originally, we just wanted to use one version of the driver to cover > > all the SubPMIC of the 6370 series(6370~6372). > > And, the users who use this series SubPMIC can directly apply this > > driver to drive the backlight device without knowing the underlying > > hardware. > > To achieve this goal, we have designed it to look like this. > > You don't need a second driver to support two different values for > max-brightness. The same driver can support both ranges with nothing but > a small tweak during the driver probe function. > > > > ... > > > > > > + > > > > > > + if (brightness) { > > > > > > + brightness_val[0] = (brightness - 1) & MT6370_BL_DIM2_MASK; > > > > > > + brightness_val[1] = (brightness - 1) >> fls(MT6370_BL_DIM2_MASK); > > > > > > + > > > > > > + /* > > > > > > + * To make MT6372 using 14 bits to control the brightness > > > > > > + * backward compatible with 11 bits brightness control > > > > > > + * (like MT6370 and MT6371 do), we left shift the value > > > > > > + * and pad with 1 to remaining bits. Hence, the MT6372's > > > > > > + * backlight brightness will be almost the same as MT6370's > > > > > > + * and MT6371's. > > > > > > + */ > > > > > > + if (priv->vid_type == MT6370_VID_6372) { > > > > > > + brightness_val[0] <<= MT6370_BL_DIM2_6372_SHIFT; > > > > > > + brightness_val[0] |= MT6370_BL_DUMMY_6372_MASK; > > > > > > + } > > > > > > > > > > This somewhat depends on the answer to the first question above, but > > > > > what is the point of this shifting? If the range is 14-bit then the > > > > > driver should set max_brightness to 16384 and present the full range of > > > > > the MT6372 to the user. > > > > > > > > So should we make all 16384 steps of MT6372 available to users? > > > > > > Yes. > > > > > > > > > > Does that mean the DTS needs to be modified as well? > > > > > > Yes... the property to set initial brightness needs a 14-bit range. > > > > > > It would also be a good idea to discuss with the DT maintainers whether > > > you should introduce a second compatible string (ending 6372) in order > > > to allow the DT validation checks to detect accidental use of MT6372 > > > ranges on MT6370 hardware. > > > > hmmm... I have just thought about it, > > maybe I can just modify the maximum value of default-brightness and > > max-brightness in DT to 16384, > > modify the description and add some comments. > > What for? > > All the other backlight drivers (there are >130 of them) expose the hardware > range[1]. Most userspaces will already know how to handle that (by reading > the max_brightness and, if it is recent enough, also the scale > properties in sysfs). > > I'm still don't understand why one should fix a bug in the userspace by > implementing a hack in the driver. > > > [1] Well almost. The PWM backlight driver does contain support for > dead-spot avoidance and to allow the adoption of exponential scale. > However this purpose of these is based on how PWM backlights work > > > And then on the driver side, > > we can use mt6370_check_vendor_info() to determine whether it is MT6372. > > If no, then in mt6370_bl_update_status(), first brightness_val / 8 and then set. > > In mt6370_bl_get_brightness(), first brightness_val * 8 and then return; > > > > If I do this change, does this meet your requirements? > > Not really. > > It's still misleading a sophisticated userspace, which may make bad > rounding decisions for backlight animation, in order to support a broken > one. > > > > > > Or, for the reasons, I have just explained (just one customer has this > > > > requirement), then we do not make any changes for compatibility > > > > reasons? > > > > > > I'd be curious what the compatiblity reasons are. In other words what > > > software breaks? > > > > The reason is as above. We just hope the users who use this series SubPMIC can > > directly apply this driver to drive the backlight device without > > knowing the underlying hardware. > > Not software breaks. > > As above, ignoring the max_brightness property is a bug in the > userspace. I'm still unclear why sending faked ranges to userspace > it a better solution than fixing the userspace. Ok, I got it! If I add a second compatible string (like 'mediatek,mt6372-backlight') in the DT section, and append 'if-then-else' to determine the correct maximum value of 'default-brightness' and 'max-brightness', Also, I will append 'bled exponential mode' to make user control using linear or exponential mode. These changes I will explain to DT's maintainer again. Back to the driver section, do I still need to use the register to confirm again whether this SubPMIC used now is MT6372 and record this information? (using 'mt6370_check_vendor_info()') I am afraid that the user who uses the MT6370 hardware, but the DT compatible string is set to 'mediatek,mt6372-backlight'. This may cause errors when update/get brightness values. So I hope the driver here can check again to make sure the 'default-brightness', 'max-brightness', can be updated/got correctly. I don't know if this will make you feel redundant if I do this?? Thank you so much! -- Best Regards, ChiaEn Wu