FreeCalypso > hg > freecalypso-hwlab
comparison doc/Linux-DTR-RTS-flaw @ 128:95c2a67e1219
doc, linux-patch: update for failure to mainline DUART28C support
author | Mychaela Falconia <falcon@freecalypso.org> |
---|---|
date | Tue, 02 Feb 2021 04:32:42 +0000 |
parents | |
children | fb2f6497ba53 |
comparison
equal
deleted
inserted
replaced
127:141489d31667 | 128:95c2a67e1219 |
---|---|
1 There is a fundamental flaw in the Linux kernel serial port handling subsystem | |
2 that affects anyone who builds special hardware in which DTR and/or RTS modem | |
3 control lines are repurposed for some non-traditional functions. The flaw is | |
4 that whenever a serial port is opened under Linux, the kernel immediately and | |
5 unstoppably asserts DTR and RTS (their initial power-up state prior to software | |
6 action is negated on every sane serial port hardware implementation), *without* | |
7 giving userspace applications any ability to say "no, please don't do it". | |
8 | |
9 As long as DTR and RTS modem control outputs from the DTE are used for their | |
10 original RS-232 functions, automatically raising both signals on serial port | |
11 open is harmless - and because these signals often need to be asserted in order | |
12 for serial communication to take place, having them raised automatically on | |
13 open (without requiring an explicit TIOCMBIS ioctl) is a convenience which many | |
14 applications rely on - if this standard kernel behaviour were to be changed | |
15 for the general case (outside of special quirk configurations), a lot of | |
16 applications will break. Linux implements this standard behaviour as mandated | |
17 by POSIX and other similar standards, and those standards are in turn based on | |
18 the original 1970s UNIX where this architectural design originates. | |
19 | |
20 However, this standard Linux behaviour (and going all the way back to 1970s in | |
21 the greater UNIX family) is a total killer for custom hardware designs in which | |
22 DTR and/or RTS outputs from the UART are repurposed for some totally different | |
23 functions. Suppose that the hardware is wired in such a way that asserting the | |
24 control output causes an explosive charge to be set off, or causes a radio | |
25 transmitter to turn on (perhaps operating on some tightly regulated frequency | |
26 supporting mission-critical services, where spurious out-of-protocol | |
27 transmissions are not permissible), or applies a hard reset to some other | |
28 component that may be part of a live production system that must not be casually | |
29 reset - in all listed examples having such hardware wiring would be perfectly | |
30 safe in an OS-less environment where the custom application controls the custom | |
31 hardware as desired, without any OS inserting its own mind: the hardware design | |
32 of most serial ports (both traditional and USB-serial) guarantees that the | |
33 initial power-up state of both DTR and RTS outputs prior to software action will | |
34 always be negated, and the custom application thus gets to decide if and when | |
35 each of the two signals (independent of each other) should be asserted. | |
36 | |
37 Everything works great if the application runs on the bare metal and directly | |
38 controls the hardware (or runs under something like DOS, which is the same as | |
39 running on bare metal for the present purpose of operating serial ports), but | |
40 add Linux into the equation, and things quickly begin to break. The problem is | |
41 that the moment you open a serial port under Linux (and sadly, the same thing | |
42 happens under most other current OSes too), the kernel automatically asserts | |
43 both DTR and RTS immediately on the open operation itself, without giving | |
44 userspace applications any way to say "no, please don't do it". Some people | |
45 have been proposing new termios flags that would suppress this auto-assertion | |
46 on subsequent opens, but you have to open the port first in order to do termios | |
47 or other ioctls on it, and if the auto-assertion of DTR and RTS on that initial | |
48 open causes irreparable damage, then you are screwed no matter what you do. | |
49 | |
50 The only currently possible solution to this madness is to patch the kernel to | |
51 suppress this automatic assertion of DTR & RTS upon serial port open. But one | |
52 cannot simply change the standard behaviour for all serial ports, as lots of | |
53 standard applications for classic serial communication (where DTR and RTS do | |
54 need to be asserted) will break in that case. Instead the suppression of | |
55 automatic assertion of DTR & RTS on open needs to be conditionalized in some | |
56 way, so that the modified against-standards serial port open behaviour is | |
57 applied ONLY when special modem-control-repurposed hardware is being operated | |
58 on, and not for ordinary applications operating on ordinary serial ports. | |
59 | |
60 Given the current state of Linux and what is possible in the current reality, | |
61 if a patch is to be applied to the kernel, creating the ability to exempt | |
62 certain serial port open operations from the standard POSIX requirement of | |
63 automatically asserting DTR & RTS, there are only 3 practically feasible ways | |
64 to communicate to the kernel that a given serial port (or a given individual | |
65 open operation on a serial port) should be exempt from automatic assertion of | |
66 DTR & RTS: | |
67 | |
68 1) Create a new open flag like O_NODTR, or reuse/abuse some existing open flag | |
69 like O_DIRECT which currently has no effect on serial ports. | |
70 | |
71 2) Create a sysfs attribute that is attached to every serial port, controlling | |
72 whether or not DTR & RTS should be automatically asserted on open, with the | |
73 default being standards-mandated traditional UNIX behaviour of | |
74 auto-assertion. | |
75 | |
76 3) In special cases where the custom DTR/RTS-repurposed hardware is inseparably | |
77 integrated (on the same custom PCB) with a USB-serial chip, such that the | |
78 EEPROM controlling the USB VID:PID of the USB-serial device identifies not | |
79 just the USB-serial converter part, but the entire product board as a whole, | |
80 including the circuits that repurpose DTR and RTS for non-serial purposes, | |
81 then the most sensible approach is to mark the USB-serial device as special | |
82 and disable auto-assertion of DTR & RTS on this special device when the | |
83 custom USB VID:PID is detected. | |
84 | |
85 As it happens, our own FreeCalypso hardware gadget with repurposed DTR & RTS | |
86 that requires suppression of auto-assertion of these signals (the optional boot | |
87 control feature of our DUART28 adapter) falls into the last special category | |
88 above (custom USB-serial device unambiguously distinguished by a custom USB ID), | |
89 hence this special case is the one that I (Mother Mychaela) have been focusing | |
90 on the most - as humans, we all have a natural right to put our own self- | |
91 interest first. | |
92 | |
93 I (Mother Mychaela) have no way of knowing whether or not there is even one | |
94 person alive on Earth today who has an active use case where a need exists to | |
95 suppress automatic assertion of DTR & RTS for some serial device, but that | |
96 device does not have the same quality of being inseparably integrated with a | |
97 custom USB ID as our DUART28C, i.e., an active use case where a need exists to | |
98 signal to the kernel "please don't auto-assert DTR & RTS on this serial port" | |
99 and moreover do this special signaling for "any" serial port, rather than one | |
100 identified by a custom USB VID:PID. For all I know, I may very well be the | |
101 only person alive on Earth today who has an active need for auto-DTR/RTS | |
102 suppression - but I need it ONLY for a device that has a unique distinctive USB | |
103 VID:PID, not for "any" serial port. | |
104 | |
105 I currently run Slackware Linux 14.2 as my personal OS, running Linux kernel | |
106 version 4.4.14 around which this version of Slackware was built - when I tried | |
107 running newer 4.4.x kernels, I was getting crashes which I could not debug. I | |
108 currently run this elderly Linux kernel version with my own custom patch applied | |
109 to the ftdi_sio driver, a patch that adds support for FreeCalypso DUART28C | |
110 (custom USB ID) and applies the appropriate special quirk just for this USB ID, | |
111 not affecting any other devices - a quirk that suppresses automatic DTR & RTS | |
112 assertion on FT2232D Channel B, the UART channel on which these signals are | |
113 repurposed on DUART28 hardware. Several different versions of this patch (made | |
114 to apply cleanly to several different kernel versions) can be found in the | |
115 linux-patch directory in the present source repository. | |
116 | |
117 In 2020-09 I made a good-faith, due-diligence attempt to get the hardware | |
118 support patch for DUART28C (a patch to ftdi_sio driver that recognizes the new | |
119 USB ID and applies the necessary quirk, entirely contained inside this driver) | |
120 mainlined - I submitted the patch to ftdi_sio maintainer Johan Hovold. I was | |
121 quickly met with hostility, with Johan telling me to redesign my hardware (he | |
122 was basically telling me to throw away 20 perfectly good boards) in some | |
123 different way that would be more in line with the 1970s UNIX worldview for DTR | |
124 and RTS, which is what Linux currently implements. | |
125 | |
126 Some time later I was able to kinda-somewhat-partially convince Johan that the | |
127 current handling of DTR and RTS is a serious problem for some users, and he was | |
128 a little more agreeable to my patch - but instead of merging it as-is, he | |
129 proposed an expanded patch (getting into the tty subsystem, outside of just | |
130 USB-serial) that solves a more general problem. Johan's proposed patch | |
131 introduced an internal flag telling the tty layer to suppress DTR & RTS | |
132 assertion on open, and a sysfs attribute (added to all classic serial and USB- | |
133 serial ports) that exposes this flag. A patch to ftdi_sio that recognizes my | |
134 custom USB ID and sets this flag in the quirk function was still included in | |
135 that proposed patch series, so I was happy with the proposal. | |
136 | |
137 However, Johan's sysfs proposal was quickly shot down by other kernel | |
138 maintainers who didn't like the sysfs approach, and Johan himself was not too | |
139 interested in defending his sysfs proposal either - instead he favors a termios | |
140 flag that would only affect second and subsequent repeated opens of a device, | |
141 after the initial open to set that flag. Of course this termios flag idea does | |
142 not help at all, given that the very first open of the serial port would still | |
143 unstoppably assert DTR & RTS, causing irreparable damage - if these control | |
144 signals are wired to set off explosives, for example, the user's house would be | |
145 up in flames the moment he issues that magic stty command to set the new termios | |
146 flag, and Johan's assurances that second and subsequent opens of the same | |
147 serial port would not auto-assert DTR & RTS would be of little help to the poor | |
148 guy who just lost his house. | |
149 | |
150 By the end of 2021-01 I realized that my battle against Johan and Greg K-H is | |
151 hopeless, so I give up. The only workable solution at this point is for all | |
152 affected people to stop running unpatched mainline kernels and to apply our own | |
153 local patches instead, preferably with our own coordination amongst ourselves | |
154 so we have some degree of standardization among our kind. The whole discussion | |
155 is archived here: | |
156 | |
157 https://lore.kernel.org/linux-serial/X8iuCXYhOBVMGvXv@localhost/T/ | |
158 | |
159 I shall indefinitely, for as long as I am alive, maintain my ftdi_sio driver | |
160 patch that adds support for FreeCalypso DUART28C hardware. And because I do | |
161 not know whether or not there exists even one person on Earth who would benefit | |
162 from an ability to suppress DTR & RTS assertion under Linux on "any" serial | |
163 port, outside of tightly integrated USB-based devices with custom USB IDs, I | |
164 also make the following conditional offer: *if* at least one person comes | |
165 forward to me and demonstrates that he or she has an active use case of the | |
166 kind I am talking about, *then* I will also dig up Johan's patch (the one | |
167 rejected by other maintainers) adding a sysfs attribute, providing a working | |
168 solution for "any" serial port, start actively supporting that sysfs patch, and | |
169 maybe even make another attempt at convincing kernel maintainers to mainline it. | |
170 But I will go down that path *only* if there is at least one person alive on | |
171 Earth (just one person would be enough) who would actively benefit from this | |
172 feature - otherwise there is no point. |