From patchwork Thu Nov 30 20:07:40 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Andr=C3=A9_Almeida?= X-Patchwork-Id: 13474986 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 B9601C4167B for ; Thu, 30 Nov 2023 20:08:05 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 168DC10E038; Thu, 30 Nov 2023 20:08:05 +0000 (UTC) Received: from fanzine2.igalia.com (fanzine.igalia.com [178.60.130.6]) by gabe.freedesktop.org (Postfix) with ESMTPS id 5341710E03C for ; Thu, 30 Nov 2023 20:08:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Content-Transfer-Encoding:Content-Type:MIME-Version:Message-ID: Date:Subject:Cc:To:From:Sender:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: In-Reply-To:References:List-Id:List-Help:List-Unsubscribe:List-Subscribe: List-Post:List-Owner:List-Archive; bh=pyLKFFJL0+QHAkvFUr9epX+zDms0kHGRY+0nNCweE+4=; b=VRCEDbxyM4FF5Dt9ADAKKchvzy K5FxoPzCUxIch4XBRnoCKBVF4kaTMkl00Pi7kuUWTciw8vZwbQr7NOF5IHA9qYSbrkCIANQ/OrjyK pfzIhvTkDb8r72PrdWrTG20n7y9aLnVpk+Zcjf9NT2joLBNPRsg6RrlDomc4WbtGy8Ou1nfIi3LB2 h7rIziMyU2ojrcQ5SCnLKZx9dCLZZDTviZK0GIb9h66ltoLbuhBZdDGzsa6h9Qs4B0bf8PX2lFbRj wDoAQgQaBlB4SsYGXAb25JNXN1c8HjPRH6MKkx78LDf6qTbSvx725N6gmUGtzFUext3qMALJTcxsI LFt/SvWg==; Received: from [191.193.131.178] (helo=steammachine.lan) by fanzine2.igalia.com with esmtpsa (Cipher TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim) id 1r8nKC-008wQ8-W1; Thu, 30 Nov 2023 21:07:53 +0100 From: =?utf-8?q?Andr=C3=A9_Almeida?= To: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: [PATCH] drm/doc: Define KMS atomic state set Date: Thu, 30 Nov 2023 17:07:40 -0300 Message-ID: <20231130200740.53454-1-andrealmeid@igalia.com> X-Mailer: git-send-email 2.43.0 MIME-Version: 1.0 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: Maxime Ripard , Jonathan Corbet , Pekka Paalanen , =?utf-8?b?J01hcmVrIE9sxaE=?= =?utf-8?b?w6FrJw==?= , =?utf-8?q?Michel_D=C3=A4nzer?= , Randy Dunlap , linux-doc@vger.kernel.org, Pekka Paalanen , Thomas Zimmermann , kernel-dev@igalia.com, alexander.deucher@amd.com, =?utf-8?q?Andr=C3=A9_Almeida?= , christian.koenig@amd.com Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" From: Pekka Paalanen Specify how the atomic state is maintained between userspace and kernel, plus the special case for async flips. Signed-off-by: Pekka Paalanen Signed-off-by: André Almeida Reviewed-by: Bagas Sanjaya --- This is a standalone patch from the following serie, the other patches are already merged: https://lore.kernel.org/lkml/20231122161941.320564-1-andrealmeid@igalia.com/ Documentation/gpu/drm-uapi.rst | 47 ++++++++++++++++++++++++++++++++++ 1 file changed, 47 insertions(+) diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst index 370d820be248..d0693f902a5c 100644 --- a/Documentation/gpu/drm-uapi.rst +++ b/Documentation/gpu/drm-uapi.rst @@ -570,3 +570,50 @@ dma-buf interoperability Please see Documentation/userspace-api/dma-buf-alloc-exchange.rst for information on how dma-buf is integrated and exposed within DRM. + +KMS atomic state +================ + +An atomic commit can change multiple KMS properties in an atomic fashion, +without ever applying intermediate or partial state changes. Either the whole +commit succeeds or fails, and it will never be applied partially. This is the +fundamental improvement of the atomic API over the older non-atomic API which is +referred to as the "legacy API". Applying intermediate state could unexpectedly +fail, cause visible glitches, or delay reaching the final state. + +An atomic commit can be flagged with DRM_MODE_ATOMIC_TEST_ONLY, which means the +complete state change is validated but not applied. Userspace should use this +flag to validate any state change before asking to apply it. If validation fails +for any reason, userspace should attempt to fall back to another, perhaps +simpler, final state. This allows userspace to probe for various configurations +without causing visible glitches on screen and without the need to undo a +probing change. + +The changes recorded in an atomic commit apply on top the current KMS state in +the kernel. Hence, the complete new KMS state is the complete old KMS state with +the committed property settings done on top. The kernel will try to avoid +no-operation changes, so it is safe for userspace to send redundant property +settings. However, not every situation allows for no-op changes, due to the +need to acquire locks for some attributes. Userspace needs to be aware that some +redundant information might result in oversynchronization issues. No-operation +changes do not count towards actually needed changes, e.g. setting MODE_ID to a +different blob with identical contents as the current KMS state shall not be a +modeset on its own. As a special exception for VRR needs, explicitly setting +FB_ID to its current value is not a no-op. + +A "modeset" is a change in KMS state that might enable, disable, or temporarily +disrupt the emitted video signal, possibly causing visible glitches on screen. A +modeset may also take considerably more time to complete than other kinds of +changes, and the video sink might also need time to adapt to the new signal +properties. Therefore a modeset must be explicitly allowed with the flag +DRM_MODE_ATOMIC_ALLOW_MODESET. This in combination with +DRM_MODE_ATOMIC_TEST_ONLY allows userspace to determine if a state change is +likely to cause visible disruption on screen and avoid such changes when end +users do not expect them. + +An atomic commit with the flag DRM_MODE_PAGE_FLIP_ASYNC is allowed to +effectively change only the FB_ID property on any planes. No-operation changes +are ignored as always. Changing any other property will cause the commit to be +rejected. Each driver may relax this restriction if they have guarantees that +such property change doesn't cause modesets. Userspace can use TEST_ONLY commits +to query the driver about this.