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first round of documentation for DUART28C boot control addition
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 16 Sep 2020 06:10:39 +0000
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children 26daa2720bda
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6d97866bad79 first round of documentation for DUART28C boot control addition
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1 FreeCalypso DUART28 adapter is a new piece of hardware that became available in
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2 2020-09. In principal terms it is a drop-in replacement for the PLDkit FT2232D
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3 adapter we've been using previously, but made specifically for working with
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4 Calypso targets, rather than generic. However, in addition to providing this
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5 drop-in replacement for our previous solution, our DUART28 also provides one
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6 new optional feature: target boot control.
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7
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8 The target boot control feature of FreeCalypso DUART28 consists of CTL1 and CTL2
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9 open drain outputs driven by FT2232D Channel B RTS and DTR, respectively, and
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10 this feature is optional in that you don't have to connect these signals: if you
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11 leave them unconnected, they won't do anything, and you can completely ignore
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12 the existence of this additional feature. However, if you find this new feature
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13 useful, you can make the necessary wire connections, apply the necessary patch
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14 to your Linux kernel ftdi_sio driver and reprogram the EEPROM to the DUART28C
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15 configuration - and then you will gain the ability to control your Calypso
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16 board's PWON and RESET from your host computer.
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17
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18 Hardware connections
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19 ====================
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20
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21 The official FreeCalypso convention is that DUART28 CTL1 output should be
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22 connected to PWON on the Calypso board, and CTL2 should be connected to RESET.
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23 However, these wire connections can be made ONLY if you are going to also apply
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24 the necessary patch to your Linux kernel ftdi_sio driver and reprogram the
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25 EEPROM to the DUART28C configuration - if you connect CTL1 and CTL2 to Calypso
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26 PWON and RESET without doing the other necessary steps, your setup won't work
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27 at all: the Calypso+Iota chipset will be held down in test reset and thus
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28 inoperable whenever you open Channel B ttyUSB device for regular serial
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29 communication.
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30
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31 Linux kernel driver patch
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32 =========================
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33
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34 FreeCalypso DUART28 adapter is based on the FT2232D chip from FTDI, and all
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35 FTDI-based USB-serial adapters are supported by the ftdi_sio kernel driver
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36 under Linux. This driver contains a table of all supported USB IDs, and this
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37 table needs to be extended (meaning that the driver needs to be patched)
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38 whenever a new (previously unknown) USB ID needs to be supported. The internal
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39 architecture of this driver and its USB ID table also allows various quirks to
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40 be associated with specific USB IDs, meaning that the standard behaviour of the
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41 driver can be modified in special ways for specific ID-distinguishable USB
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42 devices.
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43
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44 The way in which we have creatively repurposed otherwise unused Channel B RTS
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45 and DTR outputs into PWON and RESET controls is not compatible with the standard
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46 unpatched ftdi_sio driver, or rather with the standard unpatched Linux tty
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47 serial port subsystem in general: the standard Linux tty behaviour (apparently
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48 stipulated by POSIX) is that whenever any given ttyUSB device is opened, its
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49 DTR and RTS outputs are immediately asserted, and userspace is not given any
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50 opportunity to intervene prior to this automatic assertion of DTR & RTS. On our
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51 non-standard hardware this standard Linux behaviour would cause an unstoppable
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52 superdeep reset to be triggered whenever the second serial channel is opened for
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53 regular communication, which is clearly not acceptable.
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54
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55 Our solution to this problem consists of a custom USB ID that makes the standard
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56 unpatched ftdi_sio driver not recognize our DUART28C adapter (DUART28 hardware
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57 with DUART28C EEPROM config) at all, and a custom patch to the ftdi_sio driver
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58 that teaches it to recognize our custom USB ID *and* apply a special quirk for
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59 it. The quirk suppresses automatic assertion of DTR & RTS on ttyUSB device
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60 open, thereby giving userspace applications full control of these UART outputs
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61 via TIOCMBIS and TIOCMBIC ioctls.
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62
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63 The patch has been submitted to the ftdi_sio maintainer for inclusion in
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64 mainline Linux, but we should not set our hopes too high: while the patch should
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65 be non-controversial given that it adds support for an entirely new USB ID (one
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66 that was officially allocated to Falconia Partners LLC by FTDI, mind you) and
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67 does not disrupt anything pre-existing in any way whatsoever, getting ANY kind
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68 of patch accepted into mainline is generally tantamount to the Seven Circles of
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69 Hell. And even if we do get our patch accepted into bleeding-edge mainline
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70 Linux, it does not help Slackware users running older kernels without updates -
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71 thus serious FreeCalypso users should be prepared to apply our ftdi_sio driver
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72 patch locally on their own systems.
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73
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74 The official Slackware-applicable version of the needed ftdi_sio driver patch
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75 can be found in our freecalypso-hwlab Hg repository.
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76
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77 EEPROM configuration
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78 ====================
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79
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80 In order to make our DUART28 adapter an acceptable replacement for generic "raw"
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81 FT2232x adapters (or CP2105-based Sysmocom mv-uart) which are supported out of
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82 the box by standard unpatched ftdi_sio and cp210x drivers, we ship our DUART28
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83 boards with DUART28S configuration in the EEPROM, rather than DUART28C. The S
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84 configuration sets the USB ID to FT2232x default of 0x0403:0x6010, which the
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85 standard unpatched ftdi_sio driver recognizes as a completely vanilla USB to
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86 dual UART adapter - but CTL outputs must be left unconnected in this
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87 configuration, as they cannot function correctly with this unpatched driver.
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88
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89 If you have applied our DUART28C support patch to your ftdi_sio driver, then
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90 you can reprogram the EEPROM on your board into the C configuration and start
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91 using CTL outputs. The necessary EEPROM config files and tools can be found in
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92 our freecalypso-hwlab Hg repository.
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93
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94 Support in FC host tools
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95 ========================
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96
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97 If you have gone through all of the above, you get the following Calypso target
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98 boot control capabilities with FC host tools:
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99
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100 * If you run any of our loadtools programs (fc-loadtool, fc-iram, fc-xram etc)
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101 or rvinterf on the ttyUSB device corresponding to DUART28 Channel B, which
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102 will normally be connected to Calypso IrDA UART, adding a -Prts option will
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103 make the tool pulse RTS, triggering PWON, or adding a -Pdtr option will make
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104 the tool pulse DTR, triggering RESET.
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105
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106 * Standalone fc-pulse-dtr and fc-pulse-rts utilities are also provided, doing
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107 the obvious.
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108
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109 Note the -P option: prior to this DUART28C support addition, the string argument
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110 to the -P option (boot control name) was an arbitrary user-defined short name
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111 to be looked up in /opt/freecalypso/bootctrl.conf, a user-created configuration
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112 file directing our tools to run a user-provided external boot control command.
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113 The new logic is that -Pdtr and -Prts are special reserved names for the
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114 built-in DUART28C boot control function, whereas any other name will be looked
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115 up in the configuration file and the associated user-provided external command
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116 will be executed.