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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 0F6C5F31E45 for ; Thu, 9 Apr 2026 15:49:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Cc:To:In-Reply-To:References :Message-Id:Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date: From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Vagc0tmyjg2FL6gLoVKwlajXp8Z6jZqxj7RPoyoM6Ak=; b=KFARaBVDPqBC1LpLtYIhkTWrLQ Ief81Y5CphQgOSDkvYp9nI6koL3Hzr7i03wu112C2SKZB0O3tmHWs79mhgLUUXFevcIZEbkkDQOp1 oln9lMcsbdvP+FNkQJfWUrpPZV1L3jzV8Vo0ms21mnOlCceTpuiJs1mpO+cQLXy0sQo+mj6O9iarh TmerEaSi86B7+xHHs8EeGNTosKZHVRpYkj6XU3FyQiluBf3XPfbmSk6n3YLBS4D/b3pAiOfuj4lfr rgH14l9QCnKSSARY+Z2lm09Hnw1LV68PwCVlbCq4htPmcwpBU1TyvkPnmOoMsSTmBExp5F25OezJY fa9BCt5Q==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1wArco-0000000ArOD-3mFu; Thu, 09 Apr 2026 15:48:59 +0000 Received: from sender4-pp-f112.zoho.com ([136.143.188.112]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1wArck-0000000ArI3-2NaM; Thu, 09 Apr 2026 15:48:56 +0000 ARC-Seal: i=1; a=rsa-sha256; t=1775749702; cv=none; d=zohomail.com; s=zohoarc; b=ZiEgVLRNQcWII/Do+KJyyb0aSH+5I+KH+dIqB9Ss8rbBaNwbuc3Cbm6/x/Xpn0smWnzQICGKxG7fD8L4nLrqH3ghSC7iGXYZQmEbfKJbzWK8PoUA41mea9+8IsyE9Yhj3fDoj26V2Cd2xJcwlwrsnuHWOslDwvz0VQ7PSAnk5po= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1775749702; h=Content-Type:Content-Transfer-Encoding:Cc:Cc:Date:Date:From:From:In-Reply-To:MIME-Version:Message-ID:References:Subject:Subject:To:To:Message-Id:Reply-To; bh=Vagc0tmyjg2FL6gLoVKwlajXp8Z6jZqxj7RPoyoM6Ak=; b=Y/sILt67IpWJPYhY+KiVRhlInN3t+JUPDV1ITq6uLAWyQ1iEHSuY+m1Hx+36uw8DeqBattdloXKlJiAuguZrOsSBOsPr5VwmtA3gcon6ZbubioADLCF4dQs3+HQmnvmPkSldS37bIM2NVPFboHAfx9SMlheN/vQDEd6mg343C94= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass header.i=collabora.com; spf=pass smtp.mailfrom=nicolas.frattaroli@collabora.com; dmarc=pass header.from= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; t=1775749702; s=zohomail; d=collabora.com; i=nicolas.frattaroli@collabora.com; h=From:From:Date:Date:Subject:Subject:MIME-Version:Content-Type:Content-Transfer-Encoding:Message-Id:Message-Id:References:In-Reply-To:To:To:Cc:Cc:Reply-To; bh=Vagc0tmyjg2FL6gLoVKwlajXp8Z6jZqxj7RPoyoM6Ak=; b=IpmxzxoyOo1AHCcs21nFIgYR58IMscNbfjCDHGlqsxc1wtWn3txUImdBW5ztJ0tN Xch5cKFsLms813nz/C/mjw5b58Mcq7VjtEmsbnXcMEl+xn4ZcsTB31dDbAVytFZ+Nbf h+7/hwpHn9ufRRFM/GcRacKSDihu9wSeUObJUF+A= Received: by mx.zohomail.com with SMTPS id 1775749699769229.6567478819677; Thu, 9 Apr 2026 08:48:19 -0700 (PDT) From: Nicolas Frattaroli Date: Thu, 09 Apr 2026 17:45:14 +0200 Subject: [PATCH v12 24/25] drm/bridge: Document bridge chain format selection MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260409-color-format-v12-24-ce84e1817a27@collabora.com> References: <20260409-color-format-v12-0-ce84e1817a27@collabora.com> In-Reply-To: <20260409-color-format-v12-0-ce84e1817a27@collabora.com> To: Harry Wentland , Leo Li , Rodrigo Siqueira , Alex Deucher , =?utf-8?q?Christian_K=C3=B6nig?= , David Airlie , Simona Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Andrzej Hajda , Neil Armstrong , Robert Foss , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , Sandy Huang , =?utf-8?q?Heiko_St=C3=BCbner?= , Andy Yan , Jani Nikula , Rodrigo Vivi , Joonas Lahtinen , Tvrtko Ursulin , Dmitry Baryshkov , Sascha Hauer , Rob Herring , Jonathan Corbet , Shuah Khan Cc: kernel@collabora.com, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org, intel-gfx@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-doc@vger.kernel.org, Nicolas Frattaroli X-Mailer: b4 0.15.1 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260409_084854_735679_8C92984C X-CRM114-Status: GOOD ( 18.07 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org The bridge chain format selection behaviour was, until now, undocumented. With the addition of the "color format" DRM property, it's not sufficiently complex enough that documentation is warranted, especially for driver authors trying to do the right thing. Add a high-level overview of how the process is supposed to work, and mention what the display driver is supposed to do if it wants to make use of this functionality. Reviewed-by: Maxime Ripard Signed-off-by: Nicolas Frattaroli --- Documentation/gpu/drm-kms-helpers.rst | 6 ++++++ drivers/gpu/drm/drm_bridge.c | 40 +++++++++++++++++++++++++++++++++++ 2 files changed, 46 insertions(+) diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst index b4a9e5ae81f6..bf5a9d909cf3 100644 --- a/Documentation/gpu/drm-kms-helpers.rst +++ b/Documentation/gpu/drm-kms-helpers.rst @@ -169,6 +169,12 @@ Bridge Operations .. kernel-doc:: drivers/gpu/drm/drm_bridge.c :doc: bridge operations +Bridge Chain Format Selection +----------------------------- + +.. kernel-doc:: drivers/gpu/drm/drm_bridge.c + :doc: bridge chain format selection + Bridge Connector Helper ----------------------- diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c index 7c1516864d96..5cc7d281ef7f 100644 --- a/drivers/gpu/drm/drm_bridge.c +++ b/drivers/gpu/drm/drm_bridge.c @@ -198,6 +198,46 @@ * driver. */ +/** + * DOC: bridge chain format selection + * + * A bridge chain, from display output processor to connector, may contain + * bridges capable of converting between bus formats on their inputs, and + * output formats on their outputs. For example, a bridge may be able to convert + * from RGB to YCbCr 4:4:4, and pass through YCbCr 4:2:0 as-is, but not convert + * from RGB to YCbCr 4:2:0. This means not all input formats map to all output + * formats. + * + * Further adding to this, a desired output color format, as specified with the + * "color format" DRM property, might not correspond 1:1 to what the display + * driver should set at its output. The bridge chain it feeds into may only be + * able to reach the desired output format, if a conversion from a different + * starting format is performed. + * + * To deal with this complexity, the recursive bridge chain bus format selection + * logic starts with the last bridge in the chain, usually the connector, and + * then recursively walks the chain of bridges backwards to the first bridge, + * trying to find a path. + * + * For a display driver to work in such a scenario, it should read the first + * bridge's bridge state to figure out which bus format the chain resolved to. + * If the first bridge's input format resolved to %MEDIA_BUS_FMT_FIXED, then its + * output format should be used. + * + * Special handling is done for HDMI as it relates to format selection. Instead + * of directly using the "color format" DRM property for bridge chains that end + * in HDMI bridges, the bridge chain format selection logic will trust the logic + * that set the HDMI output format. For the common HDMI state helper + * functionality, this means that %DRM_CONNECTOR_COLOR_FORMAT_AUTO will allow + * fallbacks to YCBCr 4:2:0 if the bandwidth requirements would otherwise be too + * high but the mode and connector allow it. + * + * For bridge chains that do not end in an HDMI bridge, + * %DRM_CONNECTOR_COLOR_FORMAT_AUTO will be satisfied with the first output + * format on the last bridge for which it can find a path back to the first + * bridge. + */ + /* Protect bridge_list and bridge_lingering_list */ static DEFINE_MUTEX(bridge_lock); static LIST_HEAD(bridge_list); -- 2.53.0