1 # SPDX-License-Identifier: GPL-2.0+ 2 3 Blob Lists - bloblist 4 ===================== 5 6 Introduction 7 ------------ 8 9 A bloblist provides a way to store collections of binary information (blobs) in 10 a central structure. Each record of information is assigned a tag so that its 11 owner can find it and update it. Each record is generally described by a C 12 structure defined by the code that owns it. 13 14 15 Passing state through the boot process 16 -------------------------------------- 17 18 The bloblist is created when the first U-Boot component runs (often SPL, 19 sometimes TPL). It is passed through to each successive part of the boot and 20 can be accessed as needed. This provides a way to transfer state from one part 21 to the next. For example, TPL may determine that a watchdog reset occurred by 22 reading an SoC register. Reading the register may reset the value, so that it 23 cannot be read a second time. So TPL can store that in a bloblist record which 24 can be passed through to SPL and U-Boot proper, which can print a message 25 indicating that something went wrong and the watchdog fired. 26 27 28 Blobs 29 ----- 30 31 While each blob in the bloblist can be of any length, bloblists are designed to 32 hold small amounts of data, typically a few KB at most. It is not possible to 33 change the length of a blob once it has been written. Each blob is normally 34 created from a C structure which can beused to access its fields. 35 36 37 Blob tags 38 --------- 39 40 Each blob has a tag which is a 32-bit number. This uniquely identifies the 41 owner of the blob. Blob tags are listed in enum blob_tag_t and are named 42 with a BLOBT_ prefix. 43 44 45 Single structure 46 ---------------- 47 48 There is normally only one bloblist in U-Boot. Since a bloblist can store 49 multiple blobs it does not seem useful to allow multiple bloblists. Of course 50 there could be reasons for this, such as needing to spread the blobs around in 51 different memory areas due to fragmented memory, but it is simpler to just have 52 a single bloblist. 53 54 55 API 56 --- 57 58 Bloblist provides a fairly simple API which allows blobs to be created and 59 found. All access is via the blob's tag. 60 61 62 Finishing the bloblist 63 ---------------------- 64 65 When a part of U-Boot is about to jump to the next part, it can 'finish' the 66 bloblist in preparation for the next stage. This involves adding a checksum so 67 that the next stage can make sure that the data arrived safely. While the 68 bloblist is in use, changes can be made which will affect the checksum, so it 69 is easier to calculate the checksum at the end after all changes are made. 70 71 72 Future work 73 ----------- 74 75 Bootstage has a mechanism to 'stash' its records for passing to the next part. 76 This should move to using bloblist, to avoid having its own mechanism for 77 passing information between U-Boot parts. 78 79 80 Simon Glass 81 sjg@chromium.org 82 12-Aug-2018 83