1# SPDX-License-Identifier: GPL-2.0+ 2 3Blob Lists - bloblist 4===================== 5 6Introduction 7------------ 8 9A bloblist provides a way to store collections of binary information (blobs) in 10a central structure. Each record of information is assigned a tag so that its 11owner can find it and update it. Each record is generally described by a C 12structure defined by the code that owns it. 13 14 15Passing state through the boot process 16-------------------------------------- 17 18The bloblist is created when the first U-Boot component runs (often SPL, 19sometimes TPL). It is passed through to each successive part of the boot and 20can be accessed as needed. This provides a way to transfer state from one part 21to the next. For example, TPL may determine that a watchdog reset occurred by 22reading an SoC register. Reading the register may reset the value, so that it 23cannot be read a second time. So TPL can store that in a bloblist record which 24can be passed through to SPL and U-Boot proper, which can print a message 25indicating that something went wrong and the watchdog fired. 26 27 28Blobs 29----- 30 31While each blob in the bloblist can be of any length, bloblists are designed to 32hold small amounts of data, typically a few KB at most. It is not possible to 33change the length of a blob once it has been written. Each blob is normally 34created from a C structure which can beused to access its fields. 35 36 37Blob tags 38--------- 39 40Each blob has a tag which is a 32-bit number. This uniquely identifies the 41owner of the blob. Blob tags are listed in enum blob_tag_t and are named 42with a BLOBT_ prefix. 43 44 45Single structure 46---------------- 47 48There is normally only one bloblist in U-Boot. Since a bloblist can store 49multiple blobs it does not seem useful to allow multiple bloblists. Of course 50there could be reasons for this, such as needing to spread the blobs around in 51different memory areas due to fragmented memory, but it is simpler to just have 52a single bloblist. 53 54 55API 56--- 57 58Bloblist provides a fairly simple API which allows blobs to be created and 59found. All access is via the blob's tag. 60 61 62Finishing the bloblist 63---------------------- 64 65When a part of U-Boot is about to jump to the next part, it can 'finish' the 66bloblist in preparation for the next stage. This involves adding a checksum so 67that the next stage can make sure that the data arrived safely. While the 68bloblist is in use, changes can be made which will affect the checksum, so it 69is easier to calculate the checksum at the end after all changes are made. 70 71 72Future work 73----------- 74 75Bootstage has a mechanism to 'stash' its records for passing to the next part. 76This should move to using bloblist, to avoid having its own mechanism for 77passing information between U-Boot parts. 78 79 80Simon Glass 81sjg@chromium.org 8212-Aug-2018 83