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    SubjectRe: [BUG] serial base baud misdetected in 2.6.27
    Alan Cox wrote:
    > On Tue, 04 Nov 2008 12:24:45 -0600
    > "Chris Friesen" <cfriesen@nortel.com> wrote:
    >
    >> I'm running an ATCA7101 board. This board has two 16550 ports that get
    >> detected as ttyS4 and ttyS5.
    >>
    >> With a much earlier kernel (2.6.14) the serial port base speed is
    >> correctly detected as 230400, which gives the desired 115200 when I
    >> specify that on the kernel commandline.
    >>
    >> With 2.6.16 and 2.6.27 (not sure about the intervening kernels) the
    >> serial port base speed is detected as 921600, which means that I need to
    >> specify 460800 on the commandline to get a real speed of 115200 (as
    >> verified by an external terminal server).
    >>
    >> Any ideas what could be causing this misdetection? I'm having lab
    >> issues right now so I can't track it down, but I'll have a go at it
    >> eventually.
    >
    > If this is a PCI card it may be matching against a PCI entry for a
    > similar card with different crystals and need the right svid/sdid adding.

    This is an ATCA board, so it's not a separate PCI card, but it does look
    like it's hanging off the PCI bus. The vendor/device matches to an Exar
    Corp. XR17C152, which is given a base_baud of 921600 in
    drivers/serial/8250_pci.c.

    According to the chip specs, 921600 is correct with the default crystal
    and control register values. However, other crystals can be used and
    there's also a control register bit to enable a prescalar which cuts the
    baud rate by a factor of four. I suspect that one of these has been
    done on our boards.

    The confusing thing is that the 2.6.14 code seems to also use a value of
    921600, but somehow it works. I wonder if our kernel vendor has tweaked
    something in the code to adjust this value later on.

    Chris




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