FreeCalypso > hg > fc-tourmaline
annotate doc/Nucleus-change @ 63:7c7fe9d0ea7c
mfw_mme.[ch]: white space cleanup (bogotabs and trailing ws)
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Thu, 22 Oct 2020 23:57:49 +0000 |
parents | a1799f6d6aa7 |
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rev | line source |
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a1799f6d6aa7
doc/Nucleus-change article written
Mychaela Falconia <falcon@freecalypso.org>
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1 The specific integration of ATI Nucleus PLUS RTOS in TI's stable TCS211 fw |
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2 (which served as the baseline for several vendors' production fw) exhibits one |
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3 hair-raising bug. While we don't know for sure where and how they maintained |
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4 Nucleus library sources for compilation (the version we got has them censored |
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5 out), we do see that Nucleus header files (nucleus.h and ??_defs.h) exist in |
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6 two different locations in the source tree in two different versions: |
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7 |
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8 * One version exists under chipsetsw/os/nucleus |
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9 * The other version exists under gpf/inc/nuc & gpf/inc/nuc/arm7 |
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10 |
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11 The two versions of these header files under these two paths in TCS211 are not |
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12 the same! The main nucleus.h header file is the same in both places, cs_defs.h |
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13 and tm_defs.h versions differ only in comments, but tc_defs.h is the real |
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14 kicker: the version under gpf/inc/nuc has an extra field added to the TC_HCB |
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15 aka NU_HISR structure, making this structure one word longer than in the other |
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16 version! More specifically, in ATI's original Nucleus this structure is 22 |
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17 words long with 4 unused dummy words at the end; TI's GPF version adds a fifth |
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18 dummy word (thankfully toward the end, not shifting any actually-used members |
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19 of the struct), putting the total struct size at 23 words. |
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20 |
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21 It would be one thing if TI had made this change consistently, but they didn't: |
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22 some modules were compiled with one version of the headers and got the 22-word |
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23 version of the struct, while other modules were compiled with the other header |
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24 file version and got the 23-word version of the struct. How can their fw work |
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25 with this bug in it? Answer: TCS211 fw works despite this Nucleus integration |
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26 bug because: |
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27 |
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28 * None of the actually-used members of the struct change offsets between the |
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29 two versions; |
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30 |
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31 * Some places in the code have 22-word structs allocated in memory while other |
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32 places have 23-word structs, but when they pass pointers to these structs to |
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33 Nucleus API functions, those functions don't access past the actually-used |
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34 part at the beginning (the part before dummy words), and they never do |
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35 anything like zeroing out the full size of the expected struct. |
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36 |
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37 * The only place in TCS211 fw where the total size of the struct matters is |
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38 where NU_HISR is embedded in another structure, and there is one such place |
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39 in GPF. Here breakage would result if different modules using these structs |
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40 and arrays were compiled with different header file versions, but all modules |
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41 that touch this part are compiled with the GPF version of nucleus.h, NU_DEBUG |
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42 and tc_defs.h. |
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43 |
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44 Needless to say, resolving this bogosity has been an important part of |
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45 FreeCalypso firmware deblobbing. Naturally the most ideal solution would have |
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46 been to remove the bogus extra word added by TI and consistently use the |
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47 original 22-word struct everywhere, but there is one further complication: I |
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48 (Mother Mychaela) don't feel comfortable with moving away from the original blob |
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49 version of the OSL component of GPF, and these COFF objects have been compiled |
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50 with the 23-word version of TC_HCB aka NU_HISR. |
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51 |
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52 The following alternative approach has been implemented in FC Tourmaline: |
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53 |
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54 * The new source version of Nucleus by Comrade XVilka has been checked in under |
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55 src/nucleus, and this new source version is the one we are using instead of |
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56 TI's binary object version. |
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57 |
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58 * The new Nucleus header files src/nucleus/nucleus.h and src/nucleus/??_defs.h |
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59 are the only ones used in Tourmaline - both old versions have been removed |
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60 from active -I include paths. |
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61 |
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62 * The new src/nucleus/tc_defs.h header file has been patched to replicate TI's |
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63 23-word version of TC_HCB aka NU_HISR, and the NU_HISR_SIZE definition in |
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64 src/nucleus/nucleus.h has also been adjusted to match. |
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65 |
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66 Thus we are using the 23-word version of TC_HCB aka NU_HISR everywhere, with 5 |
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67 dummy words at the end rather than 4, adding 4 extra bytes of wasted RAM space |
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68 to every instance of this struct throughout the firmware - but there are only a |
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69 small number of these instances, thus the waste is negligible. In return we |
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70 gain 100% consistency (the same version of the struct is used everywhere in our |
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71 fw), and we retain the ability to keep the original OSL blobs which I am not |
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72 ready to give up. |