• Michał Pecio's avatar
    USB: pl2303: fix baud-rate divisor calculations · 49bda212
    Michał Pecio authored
    This commit fixes the following issues:
    1. The 9th bit of buf was believed to be the LSB of divisor's
    exponent, but the hardware interprets it as MSB (9th bit) of the
    mantissa. The exponent is actually one bit shorter and applies
    to base 4, not 2 as previously believed.
    2. Loop iterations doubled the exponent instead of incrementing.
    3. The exponent wasn't checked for overflow.
    4. The function returned requested rate instead of actual rate.
    Due to issue #2, the old code deviated from the wrong formula
    described in #1 and actually yielded correct rates when divisor
    was lower than 4096 by using exponents of 0, 2 or 4 base-2,
    interpreted as 0, 1, 2 base-4 with the 9th mantissa bit clear.
    However, at 93.75 kbaud or less the rate turned out too slow
    due to #2 or too fast due to #2 and #3.
    I tested this patch by sending and validating 0x00,0x01,..,0xff
    to an FTDI dongle at 234, 987, 2401, 9601, 31415, 115199, 250k,
    500k, 750k, 1M, 1.5M, 3M+1 baud. All rates passed.
    I also used pv to check speed at some rates unsupported by FTDI:
    45 (the lowest possible), 2M, 4M, 5M and 6M-1. Looked sane.
    Signed-off-by: default avatarMichal Pecio <michal.pecio@gmail.com>
    Fixes: 399aa9a7
     ("USB: pl2303: use divisors for unsupported baud
    Cc: stable <stable@vger.kernel.org> # v3.18
    [johan: update summary ]
    Signed-off-by: default avatarJohan Hovold <johan@kernel.org>