--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Linux-DTR-RTS-flaw	Mon Sep 11 06:22:47 2023 +0000
@@ -0,0 +1,172 @@
+There is a fundamental flaw in the Linux kernel serial port handling subsystem
+that affects anyone who builds special hardware in which DTR and/or RTS modem
+control lines are repurposed for some non-traditional functions.  The flaw is
+that whenever a serial port is opened under Linux, the kernel immediately and
+unstoppably asserts DTR and RTS (their initial power-up state prior to software
+action is negated on every sane serial port hardware implementation), *without*
+giving userspace applications any ability to say "no, please don't do it".
+
+As long as DTR and RTS modem control outputs from the DTE are used for their
+original RS-232 functions, automatically raising both signals on serial port
+open is harmless - and because these signals often need to be asserted in order
+for serial communication to take place, having them raised automatically on
+open (without requiring an explicit TIOCMBIS ioctl) is a convenience which many
+applications rely on - if this standard kernel behaviour were to be changed
+for the general case (outside of special quirk configurations), a lot of
+applications will break.  Linux implements this standard behaviour as mandated
+by POSIX and other similar standards, and those standards are in turn based on
+the original 1970s UNIX where this architectural design originates.
+
+However, this standard Linux behaviour (and going all the way back to 1970s in
+the greater UNIX family) is a total killer for custom hardware designs in which
+DTR and/or RTS outputs from the UART are repurposed for some totally different
+functions.  Suppose that the hardware is wired in such a way that asserting the
+control output causes an explosive charge to be set off, or causes a radio
+transmitter to turn on (perhaps operating on some tightly regulated frequency
+supporting mission-critical services, where spurious out-of-protocol
+transmissions are not permissible), or applies a hard reset to some other
+component that may be part of a live production system that must not be casually
+reset - in all listed examples having such hardware wiring would be perfectly
+safe in an OS-less environment where the custom application controls the custom
+hardware as desired, without any OS inserting its own mind: the hardware design
+of most serial ports (both traditional and USB-serial) guarantees that the
+initial power-up state of both DTR and RTS outputs prior to software action will
+always be negated, and the custom application thus gets to decide if and when
+each of the two signals (independent of each other) should be asserted.
+
+Everything works great if the application runs on the bare metal and directly
+controls the hardware (or runs under something like DOS, which is the same as
+running on bare metal for the present purpose of operating serial ports), but
+add Linux into the equation, and things quickly begin to break.  The problem is
+that the moment you open a serial port under Linux (and sadly, the same thing
+happens under most other current OSes too), the kernel automatically asserts
+both DTR and RTS immediately on the open operation itself, without giving
+userspace applications any way to say "no, please don't do it".  Some people
+have been proposing new termios flags that would suppress this auto-assertion
+on subsequent opens, but you have to open the port first in order to do termios
+or other ioctls on it, and if the auto-assertion of DTR and RTS on that initial
+open causes irreparable damage, then you are screwed no matter what you do.
+
+The only currently possible solution to this madness is to patch the kernel to
+suppress this automatic assertion of DTR & RTS upon serial port open.  But one
+cannot simply change the standard behaviour for all serial ports, as lots of
+standard applications for classic serial communication (where DTR and RTS do
+need to be asserted) will break in that case.  Instead the suppression of
+automatic assertion of DTR & RTS on open needs to be conditionalized in some
+way, so that the modified against-standards serial port open behaviour is
+applied ONLY when special modem-control-repurposed hardware is being operated
+on, and not for ordinary applications operating on ordinary serial ports.
+
+Given the current state of Linux and what is possible in the current reality,
+if a patch is to be applied to the kernel, creating the ability to exempt
+certain serial port open operations from the standard POSIX requirement of
+automatically asserting DTR & RTS, there are only 3 practically feasible ways
+to communicate to the kernel that a given serial port (or a given individual
+open operation on a serial port) should be exempt from automatic assertion of
+DTR & RTS:
+
+1) Create a new open flag like O_NODTR, or reuse/abuse some existing open flag
+   like O_DIRECT which currently has no effect on serial ports.
+
+2) Create a sysfs attribute that is attached to every serial port, controlling
+   whether or not DTR & RTS should be automatically asserted on open, with the
+   default being standards-mandated traditional UNIX behaviour of
+   auto-assertion.
+
+3) In special cases where the custom DTR/RTS-repurposed hardware is inseparably
+   integrated (on the same custom PCB) with a USB-serial chip, such that the
+   EEPROM controlling the USB VID:PID of the USB-serial device identifies not
+   just the USB-serial converter part, but the entire product board as a whole,
+   including the circuits that repurpose DTR and RTS for non-serial purposes,
+   then the most sensible approach is to mark the USB-serial device as special
+   and disable auto-assertion of DTR & RTS on this special device when the
+   custom USB VID:PID is detected.
+
+As it happens, our own FreeCalypso hardware gadget with repurposed DTR & RTS
+that requires suppression of auto-assertion of these signals (the optional boot
+control feature of our DUART28 adapter) falls into the last special category
+above (custom USB-serial device unambiguously distinguished by a custom USB ID),
+hence this special case is the one that I (Mother Mychaela) have been focusing
+on the most - as humans, we all have a natural right to put our own self-
+interest first.
+
+I (Mother Mychaela) have no way of knowing whether or not there is even one
+person alive on Earth today who has an active use case where a need exists to
+suppress automatic assertion of DTR & RTS for some serial device, but that
+device does not have the same quality of being inseparably integrated with a
+custom USB ID as our DUART28C, i.e., an active use case where a need exists to
+signal to the kernel "please don't auto-assert DTR & RTS on this serial port"
+and moreover do this special signaling for "any" serial port, rather than one
+identified by a custom USB VID:PID.  For all I know, I may very well be the
+only person alive on Earth today who has an active need for auto-DTR/RTS
+suppression - but I need it ONLY for a device that has a unique distinctive USB
+VID:PID, not for "any" serial port.
+
+I currently run Slackware Linux 14.2 as my personal OS, running Linux kernel
+version 4.4.14 around which this version of Slackware was built - when I tried
+running newer 4.4.x kernels, I was getting crashes which I could not debug.  I
+currently run this elderly Linux kernel version with my own custom patch applied
+to the ftdi_sio driver, a patch that adds support for FreeCalypso DUART28C
+(custom USB ID) and applies the appropriate special quirk just for this USB ID,
+not affecting any other devices - a quirk that suppresses automatic DTR & RTS
+assertion on FT2232D Channel B, the UART channel on which these signals are
+repurposed on DUART28 hardware.  Several different versions of this patch (made
+to apply cleanly to several different kernel versions) can be found in the
+linux-patch directory in the present source repository.
+
+In 2020-09 I made a good-faith, due-diligence attempt to get the hardware
+support patch for DUART28C (a patch to ftdi_sio driver that recognizes the new
+USB ID and applies the necessary quirk, entirely contained inside this driver)
+mainlined - I submitted the patch to ftdi_sio maintainer Johan Hovold.  I was
+quickly met with hostility, with Johan telling me to redesign my hardware (he
+was basically telling me to throw away 20 perfectly good boards) in some
+different way that would be more in line with the 1970s UNIX worldview for DTR
+and RTS, which is what Linux currently implements.
+
+Some time later I was able to kinda-somewhat-partially convince Johan that the
+current handling of DTR and RTS is a serious problem for some users, and he was
+a little more agreeable to my patch - but instead of merging it as-is, he
+proposed an expanded patch (getting into the tty subsystem, outside of just
+USB-serial) that solves a more general problem.  Johan's proposed patch
+introduced an internal flag telling the tty layer to suppress DTR & RTS
+assertion on open, and a sysfs attribute (added to all classic serial and USB-
+serial ports) that exposes this flag.  A patch to ftdi_sio that recognizes my
+custom USB ID and sets this flag in the quirk function was still included in
+that proposed patch series, so I was happy with the proposal.
+
+However, Johan's sysfs proposal was quickly shot down by other kernel
+maintainers who didn't like the sysfs approach, and Johan himself was not too
+interested in defending his sysfs proposal either - instead he favors a termios
+flag that would only affect second and subsequent repeated opens of a device,
+after the initial open to set that flag.  Of course this termios flag idea does
+not help at all, given that the very first open of the serial port would still
+unstoppably assert DTR & RTS, causing irreparable damage - if these control
+signals are wired to set off explosives, for example, the user's house would be
+up in flames the moment he issues that magic stty command to set the new termios
+flag, and Johan's assurances that second and subsequent opens of the same
+serial port would not auto-assert DTR & RTS would be of little help to the poor
+guy who just lost his house.
+
+By the end of 2021-01 I realized that my battle against Johan and Greg K-H is
+hopeless, so I give up.  The only workable solution at this point is for all
+affected people to stop running unpatched mainline kernels and to apply our own
+local patches instead, preferably with our own coordination amongst ourselves
+so we have some degree of standardization among our kind.  The whole discussion
+is archived here:
+
+https://lore.kernel.org/linux-serial/X8iuCXYhOBVMGvXv@localhost/T/
+
+I shall indefinitely, for as long as I am alive, maintain my ftdi_sio driver
+patch that adds support for FreeCalypso DUART28C hardware.  And because I do
+not know whether or not there exists even one person on Earth who would benefit
+from an ability to suppress DTR & RTS assertion under Linux on "any" serial
+port, outside of tightly integrated USB-based devices with custom USB IDs, I
+also make the following conditional offer: *if* at least one person comes
+forward to me and demonstrates that he or she has an active use case of the
+kind I am talking about, *then* I will also dig up Johan's patch (the one
+rejected by other maintainers) adding a sysfs attribute, providing a working
+solution for "any" serial port, start actively supporting that sysfs patch, and
+maybe even make another attempt at convincing kernel maintainers to mainline it.
+But I will go down that path *only* if there is at least one person alive on
+Earth (just one person would be enough) who would actively benefit from this
+feature - otherwise there is no point.