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

--- Comment #3 from Johan Hovold (johan@kernel.org) ---

On Fri, Jan 08, 2021 at 11:37:13AM +0000, bugzilla-daemon@bugzilla.kernel.org
wrote:

Unfortunately, I don't have a device at hand right now. But I remember the
lsusb output was showing FT-X type. And under Windows, all three chips I
mentioned work correctly with all prescalers. I contacted FTDI tech support
with this problem. They advised me to have such a patch for ftdi_sio.c and it
solved the problem. Here's the patch:

      if ((priv-> chip_type == FT2232C) || (priv-> chip_type == FT2232H) ||

            (priv-> chip_type == FT4232H) || (priv-> chip_type == FT232H) ||

(priv-> chip_type == FTX)) {

            / * Probably the BM type needs the MSB of the encoded fractional

            * divider also moved like for the chips above. Any infos? * /

            index = (u16) ((index << 8) | priv-> interface);

      }

Ah, that's good to hear. Do you want to submit that as a proper patch
that I can apply, or do you want me to do that?

But I don't know why the most significant bit of the prescaler is masked by
something. I have no information about the chip configuration registers. It
looks like this is confidential information. But they could provide it to you
as the driver developer.

I don't have access to any details about these chips apart from the
public sources either, I'm afraid.

But here it seems the upper bits simply needs to be shifted to make room
for the interface number just like for the H-types (which can have more
than one port).

Johan

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