doc/develop/bloblist.rst - platform/external/u-boot - Gitiles

 .. 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. Blob records are zeroed when added.


 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
	.. 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. Blob records are zeroed when added.


	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