state: Update documentation

- explain what buckets are
- rework text about storage backends
- explain redundancy concept

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 26 insertions, 7 deletions

diff --git a/Documentation/user/state.rst b/Documentation/user/state.rst
index 5dd5c486e2..73c4be8159 100644
--- a/Documentation/user/state.rst
+++ b/Documentation/user/state.rst
@@ -23,16 +23,35 @@ available, ``raw`` and ``dtb``. Both format the state data differently.
 Basically these are serializers. The raw serializer additionally supports a
 HMAC algorithm to detect manipulations.
 
+The data is always stored in a logical unit called ``bucket``. A ``bucket`` has
+its own size depending on some external contraints. These contraints are listed
+in more detail below depending on the used memory type and storage backend. A
+``bucket`` stores exactly one state. A default number of three buckets is used
+to store data redundantely.
+
+Redundancy
+----------
+
+The state framework is safe against powerfailures during write operations. To
+archieve that multiple buckets are stored to disk. When writing all buckets are
+written in order. When reading, the buckets are read in order and the first
+one found that passes CRC tests is used. When all data is read the buckets
+containing invalid or outdated data are written with the data just read. Also
+NAND blocks need cleanup due to excessive bitflips are rewritten in this step.
+With this it is made sure that after successful initialization of a state the
+data on the storage device is consistent and redundant.
+
 Storage Backends
 ----------------
 
-The serialized data can be stored to different backends which are automatically
-selected depending on the defined backend in the devicetree. Currently two
-implementations exist, ``circular`` and ``direct``. ``circular`` writes the
-data sequentially on the backend storage device. Each save is appended until
-the storage area is full. It then erases the block and starts from offset 0.
-``circular`` is used for MTD devices with erase functionality. ``direct``
-writes the data directly to the file without erasing.
+The serialized data can be stored to different backends. Currently two
+implementations exist, ``circular`` and ``direct``. The state framework automatically
+selects the correct backend depending on the storage medium. Media requiring
+erase operations (NAND, NOR flash) use the ``circular`` backend, others use the ``direct``
+backend. The purpose of the ``circular`` backend is to save erase cycles which may
+wear out the flash blocks. It continuously fills eraseblocks with updated data
+and only when an eraseblock if fully written erases it and starts over writing
+new data to the same eraseblock again.
 
 For all backends multiple copies are written to handle read errors.