FreeCalypso > hg > freecalypso-hwlab
annotate doc/Linux-DTR-RTS-flaw @ 164:9336a48746d9
audio-tones: experimental tools and findings
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sat, 13 Nov 2021 04:49:06 +0000 |
parents | 95c2a67e1219 |
children | fb2f6497ba53 |
rev | line source |
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95c2a67e1219
doc, linux-patch: update for failure to mainline DUART28C support
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1 There is a fundamental flaw in the Linux kernel serial port handling subsystem |
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doc, linux-patch: update for failure to mainline DUART28C support
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2 that affects anyone who builds special hardware in which DTR and/or RTS modem |
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3 control lines are repurposed for some non-traditional functions. The flaw is |
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doc, linux-patch: update for failure to mainline DUART28C support
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4 that whenever a serial port is opened under Linux, the kernel immediately and |
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doc, linux-patch: update for failure to mainline DUART28C support
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5 unstoppably asserts DTR and RTS (their initial power-up state prior to software |
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6 action is negated on every sane serial port hardware implementation), *without* |
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doc, linux-patch: update for failure to mainline DUART28C support
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7 giving userspace applications any ability to say "no, please don't do it". |
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8 |
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9 As long as DTR and RTS modem control outputs from the DTE are used for their |
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10 original RS-232 functions, automatically raising both signals on serial port |
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11 open is harmless - and because these signals often need to be asserted in order |
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12 for serial communication to take place, having them raised automatically on |
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13 open (without requiring an explicit TIOCMBIS ioctl) is a convenience which many |
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14 applications rely on - if this standard kernel behaviour were to be changed |
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15 for the general case (outside of special quirk configurations), a lot of |
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16 applications will break. Linux implements this standard behaviour as mandated |
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17 by POSIX and other similar standards, and those standards are in turn based on |
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18 the original 1970s UNIX where this architectural design originates. |
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19 |
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20 However, this standard Linux behaviour (and going all the way back to 1970s in |
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21 the greater UNIX family) is a total killer for custom hardware designs in which |
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22 DTR and/or RTS outputs from the UART are repurposed for some totally different |
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23 functions. Suppose that the hardware is wired in such a way that asserting the |
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24 control output causes an explosive charge to be set off, or causes a radio |
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25 transmitter to turn on (perhaps operating on some tightly regulated frequency |
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26 supporting mission-critical services, where spurious out-of-protocol |
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27 transmissions are not permissible), or applies a hard reset to some other |
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28 component that may be part of a live production system that must not be casually |
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29 reset - in all listed examples having such hardware wiring would be perfectly |
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30 safe in an OS-less environment where the custom application controls the custom |
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31 hardware as desired, without any OS inserting its own mind: the hardware design |
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32 of most serial ports (both traditional and USB-serial) guarantees that the |
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33 initial power-up state of both DTR and RTS outputs prior to software action will |
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34 always be negated, and the custom application thus gets to decide if and when |
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35 each of the two signals (independent of each other) should be asserted. |
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36 |
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37 Everything works great if the application runs on the bare metal and directly |
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38 controls the hardware (or runs under something like DOS, which is the same as |
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39 running on bare metal for the present purpose of operating serial ports), but |
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40 add Linux into the equation, and things quickly begin to break. The problem is |
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41 that the moment you open a serial port under Linux (and sadly, the same thing |
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42 happens under most other current OSes too), the kernel automatically asserts |
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43 both DTR and RTS immediately on the open operation itself, without giving |
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44 userspace applications any way to say "no, please don't do it". Some people |
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45 have been proposing new termios flags that would suppress this auto-assertion |
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46 on subsequent opens, but you have to open the port first in order to do termios |
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47 or other ioctls on it, and if the auto-assertion of DTR and RTS on that initial |
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48 open causes irreparable damage, then you are screwed no matter what you do. |
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49 |
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50 The only currently possible solution to this madness is to patch the kernel to |
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51 suppress this automatic assertion of DTR & RTS upon serial port open. But one |
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52 cannot simply change the standard behaviour for all serial ports, as lots of |
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53 standard applications for classic serial communication (where DTR and RTS do |
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54 need to be asserted) will break in that case. Instead the suppression of |
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55 automatic assertion of DTR & RTS on open needs to be conditionalized in some |
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56 way, so that the modified against-standards serial port open behaviour is |
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57 applied ONLY when special modem-control-repurposed hardware is being operated |
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58 on, and not for ordinary applications operating on ordinary serial ports. |
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59 |
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60 Given the current state of Linux and what is possible in the current reality, |
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61 if a patch is to be applied to the kernel, creating the ability to exempt |
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62 certain serial port open operations from the standard POSIX requirement of |
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63 automatically asserting DTR & RTS, there are only 3 practically feasible ways |
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64 to communicate to the kernel that a given serial port (or a given individual |
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65 open operation on a serial port) should be exempt from automatic assertion of |
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66 DTR & RTS: |
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67 |
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68 1) Create a new open flag like O_NODTR, or reuse/abuse some existing open flag |
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69 like O_DIRECT which currently has no effect on serial ports. |
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70 |
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71 2) Create a sysfs attribute that is attached to every serial port, controlling |
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72 whether or not DTR & RTS should be automatically asserted on open, with the |
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73 default being standards-mandated traditional UNIX behaviour of |
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74 auto-assertion. |
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75 |
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76 3) In special cases where the custom DTR/RTS-repurposed hardware is inseparably |
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77 integrated (on the same custom PCB) with a USB-serial chip, such that the |
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78 EEPROM controlling the USB VID:PID of the USB-serial device identifies not |
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79 just the USB-serial converter part, but the entire product board as a whole, |
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80 including the circuits that repurpose DTR and RTS for non-serial purposes, |
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81 then the most sensible approach is to mark the USB-serial device as special |
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82 and disable auto-assertion of DTR & RTS on this special device when the |
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83 custom USB VID:PID is detected. |
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84 |
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85 As it happens, our own FreeCalypso hardware gadget with repurposed DTR & RTS |
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86 that requires suppression of auto-assertion of these signals (the optional boot |
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87 control feature of our DUART28 adapter) falls into the last special category |
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88 above (custom USB-serial device unambiguously distinguished by a custom USB ID), |
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89 hence this special case is the one that I (Mother Mychaela) have been focusing |
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90 on the most - as humans, we all have a natural right to put our own self- |
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91 interest first. |
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92 |
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93 I (Mother Mychaela) have no way of knowing whether or not there is even one |
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94 person alive on Earth today who has an active use case where a need exists to |
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95 suppress automatic assertion of DTR & RTS for some serial device, but that |
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96 device does not have the same quality of being inseparably integrated with a |
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97 custom USB ID as our DUART28C, i.e., an active use case where a need exists to |
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98 signal to the kernel "please don't auto-assert DTR & RTS on this serial port" |
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99 and moreover do this special signaling for "any" serial port, rather than one |
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100 identified by a custom USB VID:PID. For all I know, I may very well be the |
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101 only person alive on Earth today who has an active need for auto-DTR/RTS |
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102 suppression - but I need it ONLY for a device that has a unique distinctive USB |
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103 VID:PID, not for "any" serial port. |
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104 |
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105 I currently run Slackware Linux 14.2 as my personal OS, running Linux kernel |
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106 version 4.4.14 around which this version of Slackware was built - when I tried |
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107 running newer 4.4.x kernels, I was getting crashes which I could not debug. I |
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108 currently run this elderly Linux kernel version with my own custom patch applied |
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109 to the ftdi_sio driver, a patch that adds support for FreeCalypso DUART28C |
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110 (custom USB ID) and applies the appropriate special quirk just for this USB ID, |
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111 not affecting any other devices - a quirk that suppresses automatic DTR & RTS |
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112 assertion on FT2232D Channel B, the UART channel on which these signals are |
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113 repurposed on DUART28 hardware. Several different versions of this patch (made |
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114 to apply cleanly to several different kernel versions) can be found in the |
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115 linux-patch directory in the present source repository. |
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116 |
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117 In 2020-09 I made a good-faith, due-diligence attempt to get the hardware |
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118 support patch for DUART28C (a patch to ftdi_sio driver that recognizes the new |
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119 USB ID and applies the necessary quirk, entirely contained inside this driver) |
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120 mainlined - I submitted the patch to ftdi_sio maintainer Johan Hovold. I was |
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121 quickly met with hostility, with Johan telling me to redesign my hardware (he |
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122 was basically telling me to throw away 20 perfectly good boards) in some |
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123 different way that would be more in line with the 1970s UNIX worldview for DTR |
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124 and RTS, which is what Linux currently implements. |
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125 |
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126 Some time later I was able to kinda-somewhat-partially convince Johan that the |
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127 current handling of DTR and RTS is a serious problem for some users, and he was |
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128 a little more agreeable to my patch - but instead of merging it as-is, he |
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129 proposed an expanded patch (getting into the tty subsystem, outside of just |
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130 USB-serial) that solves a more general problem. Johan's proposed patch |
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131 introduced an internal flag telling the tty layer to suppress DTR & RTS |
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132 assertion on open, and a sysfs attribute (added to all classic serial and USB- |
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133 serial ports) that exposes this flag. A patch to ftdi_sio that recognizes my |
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134 custom USB ID and sets this flag in the quirk function was still included in |
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135 that proposed patch series, so I was happy with the proposal. |
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136 |
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137 However, Johan's sysfs proposal was quickly shot down by other kernel |
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138 maintainers who didn't like the sysfs approach, and Johan himself was not too |
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139 interested in defending his sysfs proposal either - instead he favors a termios |
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140 flag that would only affect second and subsequent repeated opens of a device, |
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141 after the initial open to set that flag. Of course this termios flag idea does |
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142 not help at all, given that the very first open of the serial port would still |
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143 unstoppably assert DTR & RTS, causing irreparable damage - if these control |
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144 signals are wired to set off explosives, for example, the user's house would be |
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145 up in flames the moment he issues that magic stty command to set the new termios |
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146 flag, and Johan's assurances that second and subsequent opens of the same |
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147 serial port would not auto-assert DTR & RTS would be of little help to the poor |
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148 guy who just lost his house. |
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149 |
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150 By the end of 2021-01 I realized that my battle against Johan and Greg K-H is |
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151 hopeless, so I give up. The only workable solution at this point is for all |
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152 affected people to stop running unpatched mainline kernels and to apply our own |
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153 local patches instead, preferably with our own coordination amongst ourselves |
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154 so we have some degree of standardization among our kind. The whole discussion |
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155 is archived here: |
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156 |
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157 https://lore.kernel.org/linux-serial/X8iuCXYhOBVMGvXv@localhost/T/ |
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158 |
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159 I shall indefinitely, for as long as I am alive, maintain my ftdi_sio driver |
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160 patch that adds support for FreeCalypso DUART28C hardware. And because I do |
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161 not know whether or not there exists even one person on Earth who would benefit |
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162 from an ability to suppress DTR & RTS assertion under Linux on "any" serial |
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163 port, outside of tightly integrated USB-based devices with custom USB IDs, I |
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164 also make the following conditional offer: *if* at least one person comes |
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165 forward to me and demonstrates that he or she has an active use case of the |
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166 kind I am talking about, *then* I will also dig up Johan's patch (the one |
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167 rejected by other maintainers) adding a sysfs attribute, providing a working |
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168 solution for "any" serial port, start actively supporting that sysfs patch, and |
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169 maybe even make another attempt at convincing kernel maintainers to mainline it. |
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170 But I will go down that path *only* if there is at least one person alive on |
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171 Earth (just one person would be enough) who would actively benefit from this |
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172 feature - otherwise there is no point. |