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 X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 346E7ECDFD0 for ; Fri, 14 Sep 2018 09:55:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CFB6821477 for ; Fri, 14 Sep 2018 09:55:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="j2rds43h" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CFB6821477 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ideasonboard.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728093AbeINPIz (ORCPT ); Fri, 14 Sep 2018 11:08:55 -0400 Received: from perceval.ideasonboard.com ([213.167.242.64]:33026 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726883AbeINPIz (ORCPT ); Fri, 14 Sep 2018 11:08:55 -0400 Received: from avalon.localnet (unknown [IPv6:2a02:a03f:44f6:3500:d929:375b:d608:66c7]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id C7A5DCE; Fri, 14 Sep 2018 11:55:07 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1536918908; bh=pR47P1U1WNheQZjXhZNb1InKDSlpVV8sU8B6WT66JLE=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=j2rds43hjYy2MtC+xsoi5C2oqANF6ucHb5Lib/wHF8l1bS48hOprO8MiWtH8c9kfW d7eK29G2TGTe1kH6QfRdfFrjr8fCYOYhd1pjIYQmeJtJ3rynic+K9PcaAPvLB8cDTv mfdLnyYxbeYu6V3sO59ojCpOeAT7aQjIXQFwc3Fk= From: Laurent Pinchart To: Stefan Agner Cc: Linus Walleij , Dave Airlie , Rob Herring , Mark Rutland , Shawn Guo , Sascha Hauer , Philipp Zabel , Sascha Hauer , Fabio Estevam , NXP Linux Team , Archit Taneja , Andrzej Hajda , Gustavo Padovan , Maarten Lankhorst , sean@poorly.run, Marcel Ziswiler , max.krummenacher@toradex.com, "open list:DRM PANEL DRIVERS" , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , Linux ARM , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/6] drm/bridge: use bus flags in bridge timings Date: Fri, 14 Sep 2018 12:55:21 +0300 Message-ID: <2058139.n7KhRLb94F@avalon> Organization: Ideas on Board Oy In-Reply-To: <3c048938d12a070c03f0cbdbc89c86e8@agner.ch> References: <20180905052113.21262-1-stefan@agner.ch> <3c048938d12a070c03f0cbdbc89c86e8@agner.ch> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Stefan, On Friday, 7 September 2018 21:25:40 EEST Stefan Agner wrote: > On 07.09.2018 00:10, Linus Walleij wrote: > > On Thu, Sep 6, 2018 at 10:25 PM Stefan Agner wrote: > >> Ok, I read a bit up on the history of bridge timing, especially: > >> https://www.spinics.net/lists/dri-devel/msg155618.html > >> > >> IMHO, this got overengineered. For displays we do not need all that > >> setup/sample delay timing information, and much longer cables are in > >> use. So why is all that needed for bridges? > > > > I also avoided the overhead of creating this abstraction initially. > > > > But after doing it I have this Stockholm syndrome that I start > > liking what Laurent told me to do. > > > >> For Linus case, the THS8134(A/B) data sheet I found (revised March 2010) > >> clearly states: > >> Clock input. A rising edge on CLK latches RPr0-7, GY0-7, BPb0-7. > >> > >> So we need to drive on negative edge, hence DRM_BUS_FLAG_PIXDATA_NEGEDGE > >> should be used, which makes the pl111 driver setting TIM2_IPC: > >> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0121d/inde > >> x.html > > > > That is easy to say, but if I just set that up in code, even with a good > > comment it is hard for the next reader to understand what is going > > on. The central question will be, why does PL111 need to do this > > but not R-Car though they are using the same bridge? > > That is something I have experienced before. Depending on drive > strength, and I guess sampling delay etc, it might sample just late > enough that signals are stable and no issues are really visible on the > screen. In those cases different cabling, changes in drive strength, or > even production variations might suddenly show artefacts though... My > point being, just because it supposedly works, does not mean that it is > right. > > I did introduce the bus flags a while ago: > https://patchwork.kernel.org/patch/8254801/ > https://patchwork.kernel.org/patch/8254821/ > > And then had to debug quite some controller and display issues around > pixel clock polarity: > https://patchwork.kernel.org/patch/8117181/ > https://patchwork.kernel.org/patch/9465485/ > > E.g. in that last case, the pixel clock polarity flags were interpreted > wrong on both controller and display side. Hence this worked fine for > the case already upstream, but it did not work in my case... > > In the end, best thing is to look on the bus. Parallel display signals > are still rather slow, and usually rather easy to scope. > > PL111 needs to do this because its standard setting is driving on rising > edge. However, most displays as well as that bridge sample data on > rising edge. > > I guess R-Car DU drives on negative edge by default, but can't say for > sure without hardware/data sheet. No, it drives on the rising edge by default. > As far as I can tell the R-Car DU driver also does not use the > DRM_BUS_FLAG_PIXDATA_*EDGE flags currently. That's because the hardware default happens to work. As soon as I'll need to support a system that requires the other edge, I'll have to add the corresponding logic to the driver. So far it hasn't been needed, so I haven't bothered. > > So this elaborate model gives a better transfer of abstract concepts > > to whoever needs to touch that code next. The code is not just > > logic, but also our map of the world and the documentation of our > > problem space. > > > > Donald Knuth has this idea about literate programming which even > > turns the documentation/implementation process around. We are > > not there, not even remotely, but IMO the more complex the problem. > > the more we need to convey our thinking, not just our solution. > > Fully agree. > > My argument here is, that the map of the world (code) and the problem > are not aligned. -- Regards, Laurent Pinchart