[BUG] serial base baud misdetected in 2.6.27

[BUG] serial base baud misdetected in 2.6.27

[Chris Friesen]

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.

Thanks,

Chris

Re: [BUG] serial base baud misdetected in 2.6.27

[Alan Cox]

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.

Re: [BUG] serial base baud misdetected in 2.6.27

[Chris Friesen]

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

Re: [BUG] serial base baud misdetected in 2.6.27

[Alan Cox]

> 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.

Probably. Did your vendor have the sense to set subvendor/subdevice
identifiers that can be used to identify that specific unit ?

Re: [BUG] serial base baud misdetected in 2.6.27

[Chris Friesen]

Alan Cox wrote:
>> 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.
> 
> Probably. Did your vendor have the sense to set subvendor/subdevice
> identifiers that can be used to identify that specific unit ?

No idea, but I'll look into it.  Thanks for the help.

Chris