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To:     Miquel Raynal <miquel.raynal@bootlin.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Rob Herring <robh+dt@kernel.org>,
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Cc:     Mikhail Zhilkin <csharper2005@gmail.com>,
        Christian Marangi <ansuelsmth@gmail.com>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Jens Axboe <axboe@kernel.dk>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mtd@lists.infradead.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        =?UTF-8?q?Rafa=C5=82=20Mi=C5=82ecki?= <rafal@milecki.pl>
Subject: [PATCH 1/2] dt-bindings: mtd: partitions: support marking root device partition
Date:   Wed, 19 Oct 2022 13:48:54 +0200
Message-Id: <20221019114855.31639-1-zajec5@gmail.com>
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From: Rafał Miłecki <rafal@milecki.pl>

Linux needs to know what to use as root device. On embedded devices with
flash the only common way to specify that is cmdline & root= parameter.

That solution works with U-Boot which is Linux & cmdline aware but isn't
available with all market bootloaders. Also that method is fragile:
1. Requires specific probing order on multi-flash devices
2. Uses hardcoded partitions indexes

A lot of devices use different partitioning methods. It may be
"fixed-partitions" or some dynamic partitioning (e.g. based on parts
table). For such cases allow "linux,root-device" property to mark
correct flash partition.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
---
 .../devicetree/bindings/mtd/partitions/fixed-partitions.yaml  | 1 +
 .../devicetree/bindings/mtd/partitions/partition.yaml         | 4 ++++
 2 files changed, 5 insertions(+)

diff --git a/Documentation/devicetree/bindings/mtd/partitions/fixed-partitions.yaml b/Documentation/devicetree/bindings/mtd/partitions/fixed-partitions.yaml
index ad3ccd250802..e627085df4fc 100644
--- a/Documentation/devicetree/bindings/mtd/partitions/fixed-partitions.yaml
+++ b/Documentation/devicetree/bindings/mtd/partitions/fixed-partitions.yaml
@@ -84,6 +84,7 @@ examples:
         partition@0 {
             label = "filesystem";
             reg = <0x00000000 0x1 0x00000000>;
+            linux,root-device;
         };
     };
 
diff --git a/Documentation/devicetree/bindings/mtd/partitions/partition.yaml b/Documentation/devicetree/bindings/mtd/partitions/partition.yaml
index f1a02d840b12..671906acf1bd 100644
--- a/Documentation/devicetree/bindings/mtd/partitions/partition.yaml
+++ b/Documentation/devicetree/bindings/mtd/partitions/partition.yaml
@@ -52,6 +52,10 @@ properties:
       immune to paired-pages corruptions
     type: boolean
 
+  linux,root-device:
+    description: Marks partition that is a root device with filesystem to mount
+      and boot user space from
+
 if:
   not:
     required: [ reg ]
-- 
2.34.1