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doc/Nucleus-change article written
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 20 Oct 2020 04:11:55 +0000
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53:a8c8a5521073 54:a1799f6d6aa7
1 The specific integration of ATI Nucleus PLUS RTOS in TI's stable TCS211 fw
2 (which served as the baseline for several vendors' production fw) exhibits one
3 hair-raising bug. While we don't know for sure where and how they maintained
4 Nucleus library sources for compilation (the version we got has them censored
5 out), we do see that Nucleus header files (nucleus.h and ??_defs.h) exist in
6 two different locations in the source tree in two different versions:
7
8 * One version exists under chipsetsw/os/nucleus
9 * The other version exists under gpf/inc/nuc & gpf/inc/nuc/arm7
10
11 The two versions of these header files under these two paths in TCS211 are not
12 the same! The main nucleus.h header file is the same in both places, cs_defs.h
13 and tm_defs.h versions differ only in comments, but tc_defs.h is the real
14 kicker: the version under gpf/inc/nuc has an extra field added to the TC_HCB
15 aka NU_HISR structure, making this structure one word longer than in the other
16 version! More specifically, in ATI's original Nucleus this structure is 22
17 words long with 4 unused dummy words at the end; TI's GPF version adds a fifth
18 dummy word (thankfully toward the end, not shifting any actually-used members
19 of the struct), putting the total struct size at 23 words.
20
21 It would be one thing if TI had made this change consistently, but they didn't:
22 some modules were compiled with one version of the headers and got the 22-word
23 version of the struct, while other modules were compiled with the other header
24 file version and got the 23-word version of the struct. How can their fw work
25 with this bug in it? Answer: TCS211 fw works despite this Nucleus integration
26 bug because:
27
28 * None of the actually-used members of the struct change offsets between the
29 two versions;
30
31 * Some places in the code have 22-word structs allocated in memory while other
32 places have 23-word structs, but when they pass pointers to these structs to
33 Nucleus API functions, those functions don't access past the actually-used
34 part at the beginning (the part before dummy words), and they never do
35 anything like zeroing out the full size of the expected struct.
36
37 * The only place in TCS211 fw where the total size of the struct matters is
38 where NU_HISR is embedded in another structure, and there is one such place
39 in GPF. Here breakage would result if different modules using these structs
40 and arrays were compiled with different header file versions, but all modules
41 that touch this part are compiled with the GPF version of nucleus.h, NU_DEBUG
42 and tc_defs.h.
43
44 Needless to say, resolving this bogosity has been an important part of
45 FreeCalypso firmware deblobbing. Naturally the most ideal solution would have
46 been to remove the bogus extra word added by TI and consistently use the
47 original 22-word struct everywhere, but there is one further complication: I
48 (Mother Mychaela) don't feel comfortable with moving away from the original blob
49 version of the OSL component of GPF, and these COFF objects have been compiled
50 with the 23-word version of TC_HCB aka NU_HISR.
51
52 The following alternative approach has been implemented in FC Tourmaline:
53
54 * The new source version of Nucleus by Comrade XVilka has been checked in under
55 src/nucleus, and this new source version is the one we are using instead of
56 TI's binary object version.
57
58 * The new Nucleus header files src/nucleus/nucleus.h and src/nucleus/??_defs.h
59 are the only ones used in Tourmaline - both old versions have been removed
60 from active -I include paths.
61
62 * The new src/nucleus/tc_defs.h header file has been patched to replicate TI's
63 23-word version of TC_HCB aka NU_HISR, and the NU_HISR_SIZE definition in
64 src/nucleus/nucleus.h has also been adjusted to match.
65
66 Thus we are using the 23-word version of TC_HCB aka NU_HISR everywhere, with 5
67 dummy words at the end rather than 4, adding 4 extra bytes of wasted RAM space
68 to every instance of this struct throughout the firmware - but there are only a
69 small number of these instances, thus the waste is negligible. In return we
70 gain 100% consistency (the same version of the struct is used everywhere in our
71 fw), and we retain the ability to keep the original OSL blobs which I am not
72 ready to give up.