From patchwork Mon Mar 27 08:32:01 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Hans Verkuil X-Patchwork-Id: 13188808 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A66A0C76195 for ; Mon, 27 Mar 2023 08:38:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233188AbjC0Iib (ORCPT ); Mon, 27 Mar 2023 04:38:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56966 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233196AbjC0Ihq (ORCPT ); Mon, 27 Mar 2023 04:37:46 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8D42C76AF for ; Mon, 27 Mar 2023 01:32:21 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 4BD59B80EBC for ; Mon, 27 Mar 2023 08:32:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB940C4339C for ; Mon, 27 Mar 2023 08:32:02 +0000 (UTC) Message-ID: <4a188eb2-19c4-d9b3-634e-9dd96cc5cac9@xs4all.nl> Date: Mon, 27 Mar 2023 10:32:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Content-Language: en-US To: Linux Media Mailing List From: Hans Verkuil Subject: [PATCH] Documentation: admin-guide: cec.rst: document NTP issue Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org The CEC pin framework is affected by NTP daemons speeding up or slowing down the system clock. Document this and explain how to fix this for chronyd. Signed-off-by: Hans Verkuil diff --git a/Documentation/admin-guide/media/cec.rst b/Documentation/admin-guide/media/cec.rst index 656dec7e0182..6b30e355cf23 100644 --- a/Documentation/admin-guide/media/cec.rst +++ b/Documentation/admin-guide/media/cec.rst @@ -55,6 +55,15 @@ Miscellaneous: you can control the CEC line through this driver. This supports error injection as well. +- cec-gpio and Allwinner A10 (or any other driver that uses the CEC pin + framework to drive the CEC pin directly): the CEC pin framework uses + high-resolution timers. These timers are affected by NTP daemons that + speed up or slow down the clock to sync with the official time. The + chronyd server will by default increase or decrease the clock by + 1/12th. This will cause the CEC timings to go out of spec. To fix this, + add a 'maxslewrate 40000' line to chronyd.conf. This limits the clock + frequency change to 1/25th, which keeps the CEC timings within spec. + Utilities =========