bitread.py: fully parse type 1 and config packet length

[cfib]

Setting a USR_ACCESS value that starts with the same bit pattern as a type-2 packet (0b010[...]) causes bitread.py to generate an incorrect .bits file and thus incorrect fault analysis data.

The reason for this is that the check for the type-2 configuration packet is incorrectly triggered by this user-defined number.

To address this issue and tries to prevent incorrect type-2 packets from being parsed, this PR:

• fully parses and skips type-1 packets,
• checks if a type-1 FDRI write of length 0 immediately precedes the type-2 packet
• tracks the number of frames parsed from the type-2 packet in parse_config_packet() and checks if this matches the length of the type-2 packet supplied in the bitstream.

---

To reproduce using the original bitread.py, generate multiple variants of bitstreams of the bfat_demo_1 design using Vivado:

open_checkpoint demo_1.dcp
write_bitstream demo_1.mwe.original.bit
set_property BITSTREAM.CONFIG.USR_ACCESS 0x00000000 [current_design]
write_bitstream demo_1.mwe.edited.00000000.bit
set_property BITSTREAM.CONFIG.USR_ACCESS 0x40000000 [current_design]
write_bitstream demo_1.mwe.edited.40000000.bit

Now, running the original bitread.py

python3 bitread.py demo_1.mwe.original.bit
python3 bitread.py demo_1.mwe.edited.00000000.bit
python3 bitread.py demo_1.mwe.edited.40000000.bit

gives the following .bits files:

$ ls -l *.bits
-rw-rw-r-- 1 user user 57310500 Sep 27 13:59 demo_1.mwe.edited.00000000.bits
-rw-rw-r-- 1 user user 58114060 Sep 27 13:59 demo_1.mwe.edited.40000000.bits
-rw-rw-r-- 1 user user 57310500 Sep 27 13:59 demo_1.mwe.original.bits

$ head -3 *.bits
==> demo_1.mwe.edited.00000000.bits <==
bit_0000009b_050_21
bit_00000e00_000_11
bit_00000e00_000_17

==> demo_1.mwe.edited.40000000.bits <==
bit_00000000_000_00
bit_00000000_000_13
bit_00000000_000_14

==> demo_1.mwe.original.bits <==
bit_0000009b_050_21
bit_00000e00_000_11
bit_00000e00_000_17

$ sha1sum *.bits
6218857cb299ffe3e90e15c650f5b6a8e7e68d8d  demo_1.mwe.edited.00000000.bits
3957d261789e01d9c7b1c4b226b804903c68a4b6  demo_1.mwe.edited.40000000.bits
6218857cb299ffe3e90e15c650f5b6a8e7e68d8d  demo_1.mwe.original.bits

---

Running the version of bitread.py from this PR gives:

$ ls -l *.bits
-rw-rw-r-- 1 user user 57310500 Sep 27 14:09 demo_1.mwe.edited.00000000.bits
-rw-rw-r-- 1 user user 57310500 Sep 27 14:09 demo_1.mwe.edited.40000000.bits
-rw-rw-r-- 1 user user 57310500 Sep 27 14:09 demo_1.mwe.original.bits

$ head -3 *.bits
==> demo_1.mwe.edited.00000000.bits <==
bit_0000009b_050_21
bit_00000e00_000_11
bit_00000e00_000_17

==> demo_1.mwe.edited.40000000.bits <==
bit_0000009b_050_21
bit_00000e00_000_11
bit_00000e00_000_17

==> demo_1.mwe.original.bits <==
bit_0000009b_050_21
bit_00000e00_000_11
bit_00000e00_000_17

$ sha1sum *.bits
6218857cb299ffe3e90e15c650f5b6a8e7e68d8d  demo_1.mwe.edited.00000000.bits
6218857cb299ffe3e90e15c650f5b6a8e7e68d8d  demo_1.mwe.edited.40000000.bits
6218857cb299ffe3e90e15c650f5b6a8e7e68d8d  demo_1.mwe.original.bits