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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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