From patchwork Fri May 20 21:38:41 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Doug Anderson X-Patchwork-Id: 12857493 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 B31C0C433FE for ; Fri, 20 May 2022 21:38:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232937AbiETVi5 (ORCPT ); Fri, 20 May 2022 17:38:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54544 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1353698AbiETVi4 (ORCPT ); Fri, 20 May 2022 17:38:56 -0400 Received: from mail-pg1-x532.google.com (mail-pg1-x532.google.com [IPv6:2607:f8b0:4864:20::532]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 29EBA17B851 for ; Fri, 20 May 2022 14:38:54 -0700 (PDT) Received: by mail-pg1-x532.google.com with SMTP id a38so5898496pgl.9 for ; Fri, 20 May 2022 14:38:54 -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=JY2yFL0uwa0h1jwIIpYY9zlYj6tua2HV5MA+NWkzrlM=; b=VKf459qgtfIN2nwURKwdzW9/x2MsK2XhWFkIQpwhoxQ8B5sQdZZgm7CeI51WyA1W8i bjiZRrIlXQ35qjT+7Fa/pZWRw9ZTLZ6PtWfv9obMY1QnNlnruEZ141wZD3m34+6puxNG uvZJ/F6VbPYrW+4mXStLdi94M3px/tar/ZhFE= 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=JY2yFL0uwa0h1jwIIpYY9zlYj6tua2HV5MA+NWkzrlM=; b=zVm+iQz7SWvdGyQoTkbKBOGRQ+UOL4bFVVMPfTyTrTRKS9eNgXkiSO3/kuAO3Ybtxb 2+PHvgnH/OoPo1I/Efeb6yHHcQedOgRCuolOdjnsnLRyXH2ddUhAXHKOXMsDqL8afiPF yuEW/v2S3BglhiDl9w/XJGihmLrQJItyeapCbCyLIV+YJg1amz/FJfwt4P+JLPECbRwZ dDinXRQOGGjgaXFNnuQF/uPqJSgkEEATGQec/dRVu5oeD8jpOucjWbDree1Ico3/Omic 6961OlttARYOC4tEinS3EKCllEB3t0iz8kx0xYssI+zeGc4l0t+8j28r2hO7WVvZZArk qyMQ== X-Gm-Message-State: AOAM531MbT1CZVbWEZtttwg8QXkOeRFonBOI0/vaEXJMAXHlmRoXlS7U 7qiZyoRQaH+BILXhkJo0H+ATRQ== X-Google-Smtp-Source: ABdhPJyQ7sTZ4EOkBnkyPfFAtpjkbiEhqkikYoQc6iCiuVDywMgjCXw9CxDj9gsimAlD7Cxsu5AJnA== X-Received: by 2002:a63:2b11:0:b0:3f6:5f7f:9e67 with SMTP id r17-20020a632b11000000b003f65f7f9e67mr6384417pgr.492.1653082733609; Fri, 20 May 2022 14:38:53 -0700 (PDT) Received: from tictac2.mtv.corp.google.com ([2620:15c:202:201:b090:3896:c312:c4df]) by smtp.gmail.com with ESMTPSA id u13-20020a17090a450d00b001df955c28f6sm2070864pjg.37.2022.05.20.14.38.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 May 2022 14:38:52 -0700 (PDT) From: Douglas Anderson To: Rob Herring , Bjorn Andersson Cc: Matthias Kaehlcke , Alexandru M Stan , patches@lists.linux.dev, linux-arm-msm@vger.kernel.org, Julius Werner , Andy Gross , Stephen Boyd , Krzysztof Kozlowski , Rajendra Nayak , "Joseph S . Barrera III" , devicetree@vger.kernel.org, Douglas Anderson , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 1/5] dt-bindings: Document how Chromebooks with depthcharge boot Date: Fri, 20 May 2022 14:38:41 -0700 Message-Id: <20220520143502.v4.1.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 v4: - Add reference to depthcharge and FIT Image. - A few rst syntax fixups found by using a different rst preview. - Updated wording as per Stephen. 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 | 69 +++++++++++++++++++ 1 file changed, 69 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..36da77684bba --- /dev/null +++ b/Documentation/arm/google/chromebook-boot-flow.rst @@ -0,0 +1,69 @@ +.. 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 depthcharge_ compile time. This is $(BOARD) below. +- Board revision number, determined at runtime (perhaps by reading GPIO + strappings, perhaps via some other method). This is $(REV) below. +- SKU number, read from GPIO strappings at boot time. This is $(SKU) below. + +For recent Chromebooks, depthcharge_ creates a match list that looks like this: + +- google,$(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 string falls within a device tree's root compatible string array. +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 +newest device tree we know about. + +.. _depthcharge: https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/depthcharge/ +.. _`FIT Image`: https://doc.coreboot.org/lib/payloads/fit.html