From patchwork Mon Dec 21 15:25:29 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sakari Ailus X-Patchwork-Id: 346436 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=-16.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable 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 7D2CFC433E9 for ; Mon, 21 Dec 2020 15:33:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5556522CAF for ; Mon, 21 Dec 2020 15:33:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725782AbgLUPcs (ORCPT ); Mon, 21 Dec 2020 10:32:48 -0500 Received: from retiisi.eu ([95.216.213.190]:52220 "EHLO hillosipuli.retiisi.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725497AbgLUPcs (ORCPT ); Mon, 21 Dec 2020 10:32:48 -0500 Received: from lanttu.localdomain (lanttu-e.localdomain [192.168.1.64]) by hillosipuli.retiisi.eu (Postfix) with ESMTP id E1FCB634C87; Mon, 21 Dec 2020 17:29:57 +0200 (EET) From: Sakari Ailus To: linux-media@vger.kernel.org Cc: devicetree@vger.kernel.org, linux-acpi@vger.kernel.org, linux-clk@vger.kernel.org Subject: [PATCH 1/1] Documentation: media: Document clock handling in camera sensor drivers Date: Mon, 21 Dec 2020 17:25:29 +0200 Message-Id: <20201221152529.25417-1-sakari.ailus@linux.intel.com> X-Mailer: git-send-email 2.29.2 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org Document pratices of handling clocks in camera sensor drivers on both DT and ACPI. Signed-off-by: Sakari Ailus --- .../driver-api/media/camera-sensor.rst | 20 ++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst index ffb0cad8137a..4e8e92fdd8b4 100644 --- a/Documentation/driver-api/media/camera-sensor.rst +++ b/Documentation/driver-api/media/camera-sensor.rst @@ -15,7 +15,7 @@ Camera sensors have an internal clock tree including a PLL and a number of divisors. The clock tree is generally configured by the driver based on a few input parameters that are specific to the hardware:: the external clock frequency and the link frequency. The two parameters generally are obtained from system -firmware. No other frequencies should be used in any circumstances. +firmware. **No other frequencies should be used in any circumstances.** The reason why the clock frequencies are so important is that the clock signals come out of the SoC, and in many cases a specific frequency is designed to be @@ -23,6 +23,24 @@ used in the system. Using another frequency may cause harmful effects elsewhere. Therefore only the pre-determined frequencies are configurable by the user. +ACPI +~~~~ + +Read the "clock-frequency" _DSD property to denote the frequency. The driver can +rely this frequency being used. + +Devicetree +~~~~~~~~~~ + +The currently preferred way to achieve this is using "assigned-clock-rates" +property. See Documentation/devicetree/bindings/clock/clock-bindings.txt for +more information. + +This approach has the drawback that there's no guarantee that the frequency +hasn't been modified directly or indirectly by another driver, or supported by +the board's clock tree to begin with. Changes to the Common Clock Framework API +are required to ensure reliability. + Frame size ----------