From patchwork Thu May 19 23:51:05 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Doug Anderson X-Patchwork-Id: 575060 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 3DA69C433F5 for ; Thu, 19 May 2022 23:51:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343776AbiESXvc (ORCPT ); Thu, 19 May 2022 19:51:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58332 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242895AbiESXva (ORCPT ); Thu, 19 May 2022 19:51:30 -0400 Received: from mail-pg1-x52e.google.com (mail-pg1-x52e.google.com [IPv6:2607:f8b0:4864:20::52e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D1B9631353 for ; Thu, 19 May 2022 16:51:27 -0700 (PDT) Received: by mail-pg1-x52e.google.com with SMTP id v10so6334168pgl.11 for ; Thu, 19 May 2022 16:51:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=O68zOwYSR6b5bIhMSQfgIGQAc/cdqbvjZPUOTjfh09U=; b=gfq/frvCvea+Jl6JVCs1ZnXx5YZ0mp2A2abNlzF2s1fF+s7AzqcGazJmmNCb0KNec1 SEy2fOrPkGUDjf4hzrsvvImF+GP4koXcKJDVLDEZW3cy9mDDq8vBNdh3K8GxLU+l48SL uY3ACHgkzO5q9VYZ5ncO9sHAMFGRBHXLhaB60= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=O68zOwYSR6b5bIhMSQfgIGQAc/cdqbvjZPUOTjfh09U=; b=wMOyc0McvjeDZk18HIynMN3iqwwE1QVhq2mCTzy7GXQfRaB9VRGkbCtPkEWrfaNYhv 493HdDHXcEznk59Ru925ZwKneGknvFLY9S4S/1CCkGrR5hCBANKM9e1Ulf0S7d1TObg2 zV9chhaB+FcZN6RSIJ2kF5axsOYj3C+X5j3dLeUhYfTsOVKvqz1Uab41dRg3XfwXCten KfU8E8YO1VVGFSc2MldzgwRMzfiBzlR87DGm68VOMfBVWpYoKnlW45iedY9UfjNKDLnN B1f9N7Zeq4dxr1w95j2f4s2nTxK2T2wzqLOar/H8Ny4Xbtj1239ly5LJICSlJHXY61ra Qz4w== X-Gm-Message-State: AOAM532hHW1nEJGTB9M7IdGntKMC+/NwwK11dE98gsKHcWDH/dUaIYqz KhpSoh67xepkGthcbcxtdWePdA== X-Google-Smtp-Source: ABdhPJyamfEkeHtpOdtcg6ZxrEdQGZCexKa9myBcRzDRrg9mGVcijU6CctbM2iy+F++LJtTegbPK0A== X-Received: by 2002:aa7:83d0:0:b0:50c:eb2b:8e8a with SMTP id j16-20020aa783d0000000b0050ceb2b8e8amr6950116pfn.31.1653004287251; Thu, 19 May 2022 16:51:27 -0700 (PDT) Received: from tictac2.mtv.corp.google.com ([2620:15c:202:201:e45f:1f3c:299b:4d86]) by smtp.gmail.com with ESMTPSA id m2-20020a6545c2000000b003c18ab7389asm4128992pgr.36.2022.05.19.16.51.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 May 2022 16:51:26 -0700 (PDT) From: Douglas Anderson To: Rob Herring , Bjorn Andersson Cc: Stephen Boyd , linux-arm-msm@vger.kernel.org, Matthias Kaehlcke , Andy Gross , Alexandru M Stan , "Joseph S . Barrera III" , patches@lists.linux.dev, devicetree@vger.kernel.org, Rajendra Nayak , Krzysztof Kozlowski , Julius Werner , Douglas Anderson , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v3 1/5] dt-bindings: Document how Chromebooks with depthcharge boot Date: Thu, 19 May 2022 16:51:05 -0700 Message-Id: <20220519164914.v3.1.I71e42c6174f1cec17da3024c9f73ba373263b9b6@changeid> X-Mailer: git-send-email 2.36.1.124.g0e6072fb45-goog MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org This documents how many Chromebooks pick the device tree that will be passed to the OS and can help understand the revisions / skus listed as the top-level "compatible" in many Chromebooks. Signed-off-by: Douglas Anderson --- In my opinion this could land through the Qualcomm dts64 tree, mostly because I want to land bindings patches in that tree that refer to it. Since it's a new file it seems like there ought to be few objections? Changes in v3: - Fix up typos as per Matthias. - Move under Documentation/arm/google/ as per Krzysztof. - Add missing newline at end of file. Changes in v2: - ("Document how Chromebooks with depthcharge boot") new for v2. .../arm/google/chromebook-boot-flow.rst | 63 +++++++++++++++++++ 1 file changed, 63 insertions(+) create mode 100644 Documentation/arm/google/chromebook-boot-flow.rst diff --git a/Documentation/arm/google/chromebook-boot-flow.rst b/Documentation/arm/google/chromebook-boot-flow.rst new file mode 100644 index 000000000000..92d8a658ceaa --- /dev/null +++ b/Documentation/arm/google/chromebook-boot-flow.rst @@ -0,0 +1,63 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================================== +Chromebook Boot Flow +====================================== + +Most recent Chromebooks that use device tree are using the opensource +depthcharge bootloader. Depthcharge expects the OS to be packaged as a "FIT +Image" which contains an OS image as well as a collection of device trees. It +is up to depthcharge to pick the right device tree from the FIT Image and +provide it to the OS. + +The scheme that depthcharge uses to pick the device tree takes into account +three variables: +- Board name, specified at compile time. +- Board revision number, read from GPIO strappings at boot time. +- SKU number, read from GPIO strappings at boot time. + +For recent Chromebooks, depthcharge creates a match list that looks like this: +- google,$(BOARD)-rev$(REV)-sku$(SKU) +- google,$(BOARD)-rev$(REV) +- google,$(BOARD)-sku$(SKU) +- google,$(BOARD) + +Note that some older Chromebooks use a slightly different list that may +not include sku matching or may prioritize sku/rev differently. + +Note that for some boards there may be extra board-specific logic to inject +extra compatibles into the list, but this is uncommon. + +Depthcharge will look through all device trees in the FIT image trying to +find one that matches the most specific compatible. It will then look +through all device trees in the FIT image trying to find the one that +matches the _second most_ specific compatible, etc. + +When searching for a device tree, depthcharge doesn't care where the +compatible falls within a given device tree. As an example, if we're on +board "lazor", rev 4, sku 0 and we have two device trees: +- "google,lazor-rev5-sku0", "google,lazor-rev4-sku0", "qcom,sc7180" +- "google,lazor", "qcom,sc7180" + +Then depthcharge will pick the first device tree even though +"google,lazor-rev4-sku0" was the second compatible listed in that device tree. +This is because it is a more specific compatible than "google,lazor". + +It should be noted that depthcharge does not have any smarts to try to +match board or SKU revisions that are "close by". That is to say that +if depthcharge knows it's on "rev4" of a board but there is no "rev4" +device tree then depthcharge _won't_ look for a "rev3" device tree. + +In general when any significant changes are made to a board the board +revision number is increased even if none of those changes need to +be reflected in the device tree. Thus it's fairly common to see device +trees with multiple revisions. + +It should be noted that, taking into account the above system that +depthcharge has, the most flexibility is achieved if the device tree +supporting the newest revision(s) of a board omits the "-rev{REV}" +compatible strings. When this is done then if you get a new board +revision and try to run old software on it then we'll at pick the most +reasonable device tree. If it turns out that the new revision actually +has no device-tree visible changes then we'll not only pick the most +reasonable device tree, we'll pick the exact right one.