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annotate doc/Replug-after-EEPROM-write @ 69:065f68a94b6b

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doc/FTDI-EEPROM-format: beginning of article
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 14 Sep 2023 23:56:25 +0000
parents b6a5ee85c6a3
children
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1 USB replug after EEPROM modification
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2 ====================================
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4 With the current state of software, if you use any of the tools provided in
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5 this package to modify the configuration EEPROM content of your USB-serial
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6 adapter device (FTDI, CP2102, possibly others to be supported in the future),
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7 you then have to physically unplug and replug the USB device. This unplug-
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8 replug sequence is needed because:
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10 * Most USB-serial chips require a deep reset in order to reread their EEPROM
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11 and fully apply the new configuration, which may include a new USB VID:PID
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12 or other USB descriptor changes.
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14 * With some chips safe and correct EEPROM programming requires unbinding the
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15 kernel's usb-serial driver, causing the ttyUSB device for the USB-serial chip
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16 (or multiple ttyUSB devices for multichannel chips) to disappear. With the
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17 current state of Linux, we can command the kernel to unbind its drivers from
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18 a specific interface on a specific chip (*without* rmmod of the responsible
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19 driver, killing all other devices of the same tyoe), but we have no way to
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20 tell the kernel to rebind everything when we are done.
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22 Why is physical plug/unplug manipulation needed, why can't we command the needed
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23 effect via software? As it turns out, there are no *valid* reasons why it can't
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24 be done - but we are not currently able to do so because Linux kernel USB
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25 maintainers are being pricks and won't support functionality that doesn't fit
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26 into their worldview. Given that Harald Welte discovered this problem back in
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27 2017:
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29 https://laforge.gnumonks.org/blog/20170524-usb-port-powercycle/
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30 https://marc.info/?l=linux-usb&m=149557709602259&w=2
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32 (the thread shows Harald's good-faith attempt to reason with those pricks and
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33 their dismissive responses) and given my own (Mother Mychaela's) sour experience
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34 with trying to get a simple patch into ftdi_sio (failed attempt to mainline the
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35 patch adding support for DUART28C), the situation with Linux currently looks
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36 hopeless.