# SPDX-License-Identifier: GPL-2.0+
  
  Blob Lists - bloblist
  =====================
  
  Introduction
  ------------
  
  A bloblist provides a way to store collections of binary information (blobs) in
  a central structure. Each record of information is assigned a tag so that its
  owner can find it and update it. Each record is generally described by a C
  structure defined by the code that owns it.
  
  
  Passing state through the boot process
  --------------------------------------
  
  The bloblist is created when the first U-Boot component runs (often SPL,
  sometimes TPL). It is passed through to each successive part of the boot and
  can be accessed as needed. This provides a way to transfer state from one part
  to the next. For example, TPL may determine that a watchdog reset occurred by
  reading an SoC register. Reading the register may reset the value, so that it
  cannot be read a second time. So TPL can store that in a bloblist record which
  can be passed through to SPL and U-Boot proper, which can print a message
  indicating that something went wrong and the watchdog fired.
  
  
  Blobs
  -----
  
  While each blob in the bloblist can be of any length, bloblists are designed to
  hold small amounts of data, typically a few KB at most. It is not possible to
  change the length of a blob once it has been written. Each blob is normally
  created from a C structure which can beused to access its fields.
  
  
  Blob tags
  ---------
  
  Each blob has a tag which is a 32-bit number. This uniquely identifies the
  owner of the blob. Blob tags are listed in enum blob_tag_t and are named
  with a BLOBT_ prefix.
  
  
  Single structure
  ----------------
  
  There is normally only one bloblist in U-Boot. Since a bloblist can store
  multiple blobs it does not seem useful to allow multiple bloblists. Of course
  there could be reasons for this, such as needing to spread the blobs around in
  different memory areas due to fragmented memory, but it is simpler to just have
  a single bloblist.
  
  
  API
  ---
  
  Bloblist provides a fairly simple API which allows blobs to be created  and
  found. All access is via the blob's tag.
  
  
  Finishing the bloblist
  ----------------------
  
  When a part of U-Boot is about to jump to the next part, it can 'finish' the
  bloblist in preparation for the next stage. This involves adding a checksum so
  that the next stage can make sure that the data arrived safely. While the
  bloblist is in use, changes can be made which will affect the checksum, so it
  is easier to calculate the checksum at the end after all changes are made.
  
  
  Future work
  -----------
  
  Bootstage has a mechanism to 'stash' its records for passing to the next part.
  This should move to using bloblist, to avoid having its own mechanism for
  passing information between U-Boot parts.
  
  
  Simon Glass
  sjg@chromium.org
  12-Aug-2018