view doc/Replug-after-EEPROM-write @ 70:09da7db45dce

doc/FTDI-EEPROM-format: document FT232R specifics
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 24 Sep 2023 21:31:37 +0000
parents b6a5ee85c6a3
children
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USB replug after EEPROM modification
====================================

With the current state of software, if you use any of the tools provided in
this package to modify the configuration EEPROM content of your USB-serial
adapter device (FTDI, CP2102, possibly others to be supported in the future),
you then have to physically unplug and replug the USB device.  This unplug-
replug sequence is needed because:

* Most USB-serial chips require a deep reset in order to reread their EEPROM
  and fully apply the new configuration, which may include a new USB VID:PID
  or other USB descriptor changes.

* With some chips safe and correct EEPROM programming requires unbinding the
  kernel's usb-serial driver, causing the ttyUSB device for the USB-serial chip
  (or multiple ttyUSB devices for multichannel chips) to disappear.  With the
  current state of Linux, we can command the kernel to unbind its drivers from
  a specific interface on a specific chip (*without* rmmod of the responsible
  driver, killing all other devices of the same tyoe), but we have no way to
  tell the kernel to rebind everything when we are done.

Why is physical plug/unplug manipulation needed, why can't we command the needed
effect via software?  As it turns out, there are no *valid* reasons why it can't
be done - but we are not currently able to do so because Linux kernel USB
maintainers are being pricks and won't support functionality that doesn't fit
into their worldview.  Given that Harald Welte discovered this problem back in
2017:

https://laforge.gnumonks.org/blog/20170524-usb-port-powercycle/
https://marc.info/?l=linux-usb&m=149557709602259&w=2

(the thread shows Harald's good-faith attempt to reason with those pricks and
their dismissive responses) and given my own (Mother Mychaela's) sour experience
with trying to get a simple patch into ftdi_sio (failed attempt to mainline the
patch adding support for DUART28C), the situation with Linux currently looks
hopeless.