[Bug 210351] New: Wrong setting baudrate for FTDI chip FT230X

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https://bugzilla.kernel.org/show_bug.cgi?id=210351

            Bug ID: 210351
           Summary: Wrong setting baudrate for FTDI chip FT230X
           Product: Drivers
           Version: 2.5
    Kernel Version: all
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: USB
          Assignee: drivers_usb@kernel-bugs.kernel.org
          Reporter: svv75@mail.ru
        Regression: No

Hello!

I have three chips FT232BL, FT232R and FT230X. I am using them as a VCP on
Linux (Ubuntu 18.04). According to Application Note AN_120 from FTDI, the baud
rate generators of all these chips have 8 sub-integer prescalers. Divisor =
3000000 / baudrate = n + 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875. Where
n is an integer between 2 and16384.
So, I measured real baud rates, for n = 2 in combination with all possible
sub-integer prescalers.
And it turned out that all sub-integer prescalers worked for FT232BL, FT232R
chips.

And for the FT230X, only a few prescalers work correctly. The following shows
exactly how prescalers behave:
0          - correct
0.125  - correct
0.25     - correct
0.375   - WRONG! (works like 0.125)
0.5        - correct
0.625    - WRONG! (works like 0.5)
0.75      - WRONG! (works like 0.25)
0.875    - WRONG! (works like 0.125)

The same for the other n.

I think it is a driver problem, it initializes the sub-integer prescaler
incorrectly. Since under Windows FT230X works fine with all sub-integer
prescalers.

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