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Revert "Switch to an xor for the checksum" #81

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manish-pandey-arm merged 1 commit into from
Dec 1, 2025
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Revert "Switch to an xor for the checksum" #81

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45 changes: 23 additions & 22 deletions source/transfer_list.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -71,7 +71,7 @@ changes will be backwards-compatible to older readers.
* - checksum
- 0x1
- 0x4
- If enabled by the flags, the checksum is used to provide basic protection against something overwriting the TL in memory. The checksum is set to a value such that the xor over every byte in the {`tl_base_pa`, …, `tl_base_pa + used_size - 1`} address range, is equal to `0`. For the purposes of this calculation, the value of this checksum field in the TL header must be assumed as `0`. Note that the checksum must include the TL header, all TEs and the inter-TE padding, but must not include the range reserved for future TE additions up to total_size. The values of inter-TE padding bytes are not defined by this specification and may be uninitialized memory. (This means that multiple TLs with exactly the same size and contents may still have different checksum values.). If checksums are not used, this must be `0`.
- If enabled by the flags, the checksum is used to provide basic protection against something overwriting the TL in memory. The checksum is set to a value such that the sum over every byte in the {`tl_base_pa`, …, `tl_base_pa + size - 1`} address range, modulo 256, is equal to 0. For the purposes of this calculation, the value of this checksum field in the TL header must be assumed as 0. Note that the checksum includes the TL header, all TEs and the inter-TE padding, but not the range reserved for future TE additions up to max_size. The values of inter-TE padding bytes are not defined by this specification and may be uninitialized memory. (This means that multiple TLs with exactly the same size and contents may still have different checksum values.). If checksums are not used, this must be 0.

* - version
- 0x1
Expand Down Expand Up @@ -318,8 +318,9 @@ Inputs:
equal to the size of the total area reserved for the TL (if known). If not,
abort (TL is corrupted).

#. *(optional)* If `has_checksum`, check that the xor of `tl.used_size` bytes
starting at `tl_base_addr` is 0x0. If not, abort (TL is corrupted).
#. *(optional)* If `has_checksum`, check that the sum of `tl.size` bytes
starting at `tl_base_addr` minus `tl.checksum` is equal to `tl.checksum`. If
not, abort (TL is corrupted).

Reading a TL
^^^^^^^^^^^^
Expand Down Expand Up @@ -361,8 +362,8 @@ Inputs:

#. Use the `te_base_addr` of this tag for the rest of the operation.

#. If `has_checksum`, xor the `align8(new_data_size + 0x8)` bytes starting at
`te_base_addr` with `tl.checksum`.
#. If `has_checksum`, Subtract the sum of `align8(new_data_size + 0x8)` bytes
starting at `te_base_addr` from `tl.checksum`.

#. Skip the next step (step 2) with all its substeps.

Expand All @@ -371,13 +372,13 @@ Inputs:
#. If `tl.total_size - tl.used_size` is smaller than `align8(new_data_size + 0x8)`,
abort (not enough room to add TE).

#. If `has_checksum`, xor the 4 bytes from `tl_base_addr + 0x8` with
`tl_base_addr + 0xc` from `tl.checksum`.
#. If `has_checksum`, subtract the sum of the 4 bytes from
`tl_base_addr + 0x8` to `tl_base_addr + 0xc` from `tl.checksum`.

#. Add `align8(new_data_size + 0x8)` to `tl.used_size`.

#. If `has_checksum`, xor the 4 bytes from `tl_base_addr + 0x8` to
`tl_base_addr + 0xc` with `tl.checksum`.
#. If `has_checksum`, add the sum of the 4 bytes from `tl_base_addr + 0x8` to
`tl_base_addr + 0xc` to `tl.checksum`.

#. Set `te.tag_id` (`te_base_addr + 0x0`) to `new_tag_id`.

Expand All @@ -387,17 +388,17 @@ Inputs:

#. Copy or generate the TE data into `te_base_addr + 0x8`.

#. If `has_checksum`, xor the `align8(new_data_size + 0x8)` bytes starting at
`te_base_addr` with `tl.checksum`.
#. If `has_checksum`, add the sum of `align8(new_data_size + 0x8)` bytes
starting at `te_base_addr` to `tl.checksum`.

#. If an existing XFERLIST_VOID TE was chosen to be overwritten in step 1, and
`old_void_data_size - new_data_size` is greater or equal to `0x8`:

#. Use `te_base_addr + align8(new_data_size + 0x8)` as the new `te_base_addr`
for a new XFERLIST_VOID tag.

#. If `has_checksum`, xor the 8 bytes from `te_base_addr` to
`te_base_addr + 0x8` with `tl.checksum`.
#. If `has_checksum`, subtract the sum of the 8 bytes from `te_base_addr` to
`te_base_addr + 0x8` from `tl.checksum`.

#. Set `te.tag_id` (`te_base_addr + 0x0`) to `0x0` (XFERLIST_VOID).

Expand All @@ -406,8 +407,8 @@ Inputs:
#. Set `te.data_size` (`te_base_addr + 0x4`) to
`old_void_data_size - align8(new_data_size) - 0x8`.

#. If `has_checksum`, xor the 8 bytes from `te_base_addr` to
`te_base_addr + 0x8` with `tl.checksum`.
#. If `has_checksum`, add the sum of the 8 bytes from `te_base_addr` to
`te_base_addr + 0x8` to `tl.checksum`.

Adding a new TE with special data alignment requirement
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand Down Expand Up @@ -439,11 +440,11 @@ Inputs:

#. If `new_alignment` is larger than `tl.alignment`:

#. If `has_checksum`, xor `tl.alignment` with `tl.checksum`.
#. If `has_checksum`, subtract `tl.alignment` from `tl.checksum`.

#. Set `tl.alignment` to `new_alignment`.

#. If `has_checksum`, xor `tl.alignment` with `tl.checksum`.
#. If `has_checksum`, add `tl.alignment` to `tl.checksum`.

Creating a TL
^^^^^^^^^^^^^
Expand Down Expand Up @@ -472,7 +473,7 @@ Inputs:
#. If checksums are to be used, set `tl.flags` (`tl_base_addr + 0x10`) to `1`,
else `0`. This is the value of `has_checksum`.

#. If `has_checksum`, calculate the checksum as the xor of all bytes from
#. If `has_checksum`, calculate the checksum as the sum of all bytes from
`tl_base_addr` to `tl_base_addr + tl.hdr_size`, and write the result to
`tl.checksum`.

Expand All @@ -498,13 +499,13 @@ Inputs:

#. Copy `tl.used_size` bytes from `tl_base_addr` to `new_tl_base`.

#. If `has_checksum`, xor the 4 bytes from `new_tl_base + 0xc`
to `new_tl_base + 0x10` with `tl.checksum` (`new_tl_base + 0x4`).
#. If `has_checksum`, subtract the sum of the 4 bytes from `new_tl_base + 0xc`
to `new_tl_base + 0x10` from `tl.checksum` (`new_tl_base + 0x4`).

#. Set `tl.total_size` (`new_tl_base + 0xc`) to `target_size - (new_tl_base - target_base)`.

#. If `has_checksum`, xor the 4 bytes from `new_tl_base + 0xc` to
`new_tl_base + 0x10` with `tl.checksum` (`new_tl_base + 0x4`).
#. If `has_checksum`, add the sum of the 4 bytes from `new_tl_base + 0xc` to
`new_tl_base + 0x10` to `tl.checksum` (`new_tl_base + 0x4`).


.. _sec_std_entries:
Expand Down
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