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 C3689C433EF for ; Fri, 4 Mar 2022 15:49:38 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 27D0010E3F1; Fri, 4 Mar 2022 15:49:38 +0000 (UTC) Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) by gabe.freedesktop.org (Postfix) with ESMTPS id 510F910EDA7 for ; Fri, 4 Mar 2022 15:49:37 +0000 (UTC) Received: from pendragon.ideasonboard.com (62-78-145-57.bb.dnainternet.fi [62.78.145.57]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id ADAAC51C; Fri, 4 Mar 2022 16:49:35 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1646408975; bh=kLQmwl/+vPNRO8xx7xBk2B3LJG9VaxVZC9LOvPp9fQA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=EdNwZ6D9NPGyzWVZHNT3+8PqzH1cG8XCGrsEwPF9zKez8+LD3WlVUUqrrtrtOx54A dCXPwihgf1ooNsSdBbRsuFG+qXs9t/rmFSMKknrozGGzFEK4mx0e/Qy0CFJeiaWIE1 Rdv0QW6BuC+NyBx7Xb2cXo38j1LlR8HHtHk3NmNA= Date: Fri, 4 Mar 2022 17:49:23 +0200 From: Laurent Pinchart To: Doug Anderson Subject: Re: [RFC PATCH 10/11] drm/bridge: ti-sn65dsi86: Support DisplayPort (non-eDP) mode Message-ID: References: <20210322030128.2283-1-laurent.pinchart+renesas@ideasonboard.com> <20210322030128.2283-11-laurent.pinchart+renesas@ideasonboard.com> <164563949999.4066078.2399611738908533224@Monstersaurus> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: 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: Jernej Skrabec , Neil Armstrong , Jonas Karlman , Kieran Bingham , dri-devel , Stephen Boyd , Linux-Renesas , Andrzej Hajda Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, Feb 23, 2022 at 10:20:18AM -0800, Doug Anderson wrote: > On Wed, Feb 23, 2022 at 10:05 AM Kieran Bingham wrote: > > > > > > > + /* For DisplayPort, disable scrambling mode. */ > > > > > + if (pdata->bridge.type == DRM_MODE_CONNECTOR_DisplayPort) > > > > > + regmap_update_bits(pdata->regmap, SN_TRAINING_SETTING_REG, > > > > > + SCRAMBLE_DISABLE, SCRAMBLE_DISABLE); > > > > > > > > I'm assuming that this is the important part of your patch? Would be > > > > sorta nice to include the "why" in your comment. Why do you want to > > > > disable scrambling mode for DP but not for eDP? Maybe you care about > > > > compatibility but not EMI if you're hooking up to random DP things? > > > > > > I'll investigate and include proper documentation in v2 (or drop the > > > change altogether if it's not required). > > > > And indeed, this part is important. If I drop this hunk - then I get no > > display output. > > > > I'm afraid I don't (yet) know the reasons 'why' to extend the comment, > > beyond "Scrambling is not supported for DP". > > > > If anyone already does, please feel free to provide the text, and I'll > > include it in the next revision, or I'll try to do some more digging > > into this part. > > I don't know _tons_ about it, but I later learned that the "alternate" > scrambler is used for eDP and the normal scrambler is used for DP. I > don't have any background about why they are different other than what > looks to be intentionally making the two things incompatible. I think it was done for DRM purpose, to prevent signals meant for a panel to be connected to a device that could capture video from a DP source. > ...so I guess that would make it pretty clear why you can't use the > alternate scrambler for DP. I haven't personally done the research to > know if you can be officially DP compliant with the scrambler > disabled. I also don't know why the ti-sn65dsi86 makes it so difficult > to switch to the standard scrambler or if it works at all... ;-) -- Regards, Laurent Pinchart