68ff6d3655
Add a comment explaining the design goals of bloblist, to make it easier for people to understand and comment on the structure. Signed-off-by: Simon Glass <sjg@chromium.org>
107 lines
3.6 KiB
ReStructuredText
107 lines
3.6 KiB
ReStructuredText
.. 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.
|
|
|
|
For the design goals of bloblist, please see the comments at the top of the
|
|
`bloblist.h` header file.
|
|
|
|
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 be used 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.
|
|
|
|
|
|
Placing the bloblist
|
|
--------------------
|
|
|
|
The bloblist is typically positioned at a fixed address by TPL, or SPL. This
|
|
is controlled by `CONFIG_BLOBLIST_ADDR`. But in some cases it is preferable to
|
|
allocate the bloblist in the malloc() space. Use the `CONFIG_BLOBLIST_ALLOC`
|
|
option to enable this.
|
|
|
|
The bloblist is automatically relocated as part of U-Boot relocation. Sometimes
|
|
it is useful to expand the bloblist in U-Boot proper, since it may want to add
|
|
information for use by Linux. Note that this does not mean that Linux needs to
|
|
know anything about the bloblist format, just that it is convenient to use
|
|
bloblist to place things contiguously in memory. Set
|
|
`CONFIG_BLOBLIST_SIZE_RELOC` to define the expanded size, if needed.
|
|
|
|
|
|
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.
|
|
|
|
|
|
API documentation
|
|
-----------------
|
|
|
|
.. kernel-doc:: include/bloblist.h
|
|
|
|
|
|
Simon Glass
|
|
sjg@chromium.org
|
|
12-Aug-2018
|