Ubuntu

Unwanted serial port I/O during boot

Asked by krix

When my PC boots it seems to send the character

0x7f

on the serial port. Unfortunately I have a serial device attached that is quite sensible to commands it does not understand, so this device will go into an error state, when my PC boots.

Does anyone know how I can avoid Ubuntu sending anything on the serial port during boot?
Does Ubuntu do any hardware probing, that I should disable?

The output to the serial device is sent right after I see this message on the screen:
Uncompressing linux .. OK, booting kernel

I run Ubuntu 6.04 server editiion.

Thanks.

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Launchpad Janitor (janitor) said : 		#1

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

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krix (kristian-noergaard) said : 		#2

Launchpad Janitor wrote:
> Your question #14745 on Ubuntu changed:
> https://answers.launchpad.net/ubuntu/+question/14745
>
> Status: Open => Expired
>
> Launchpad Janitor expired the question:
> This question was expired because it remained in the 'Open' state
> without activity for the last 15 days.
>
>

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Launchpad Janitor (janitor) said : 		#3

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

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krix (kristian-noergaard) said : 		#4

Launchpad Janitor wrote:
> Your question #14745 on Ubuntu changed:
> https://answers.launchpad.net/ubuntu/+question/14745
>
> Status: Open => Expired
>
> Launchpad Janitor expired the question:
> This question was expired because it remained in the 'Open' state
> without activity for the last 15 days.
>
>

Revision history for this message
Launchpad Janitor (janitor) said : 		#5

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

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krix (kristian-noergaard) said : 		#6

I still need an answer.

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Launchpad Janitor (janitor) said : 		#7

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

Revision history for this message
krix (kristian-noergaard) said : 		#8

Launchpad Janitor wrote:
> Your question #14745 on Ubuntu changed:
> https://answers.launchpad.net/ubuntu/+question/14745
>
> Status: Open => Expired
>
> Launchpad Janitor expired the question:
> This question was expired because it remained in the 'Open' state
> without activity for the last 15 days.
>
>

Revision history for this message
Ralph Janke (txwikinger) said : 		#9

Thanks for the question.

Does this only happen when you boot ubuntu? Do you know when during the boot process this is sent?

Thanks

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krix (kristian-noergaard) said : 		#10

Thanks for responding to this!
I thought all hope was gone...

Yes, it only happens on boot.
And it happens right after the screen says:
Uncompressing linux .. OK, booting kernel

That is: before any other kernel messages appears, or services are being started.

I wonder if it is related to some hardware probing like kudzu or discover or whatever Ubuntu uses.

BTW: The problem is still present on Ubuntu 7.10.

Revision history for this message
Ralph Janke (txwikinger) said : 		#11

What happens if you boot a different linux version (you might try some live-CD, i.e. Knoppix). Did you ever run windows on this computer? what happens then?

It is very possible that it has something to do with kudzu. Can you work around this problem? How often do you reboot this computer? Can you reset the device after the boot?

Revision history for this message
krix (kristian-noergaard) said : 		#12

This has happened with 4 or 5 computers.
I have not tried to run windows on these computers.

The problem is seen with
Ubuntu 6.06
Ubuntu 7.10
Knoppix or debian ( I don't remember which one of them )

No, I can't reset the device after the boot, without doing af poweron/poweroff of the device.
And that is unfortunately very inconvenient, this system needs to work immediately after booting it.

Any suggestions on how to work around this problem?
I don't know kudzu, does Ubuntu use kudzu?
Can I disable kudzu or parts of kudzu?

Revision history for this message
Ralph Janke (txwikinger) said : 		#13

I believe the detection of the serial ports happens inside the kernel. In one of the kernel modules.

I need to do some research myself to give you further answers.

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Ralph Janke (txwikinger) said : 		#14

It could also be that overlapping IRQs create this problem. Have you tried to exclusively reserve the IRQ/DMA etc for the serial port in the bios?

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Ralph Janke (txwikinger) said : 		#15

Maybe you want to go to the IRC-channel #ubuntu-kernel. Someone there might be able to help you.

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krix (kristian-noergaard) said : 		#16

Hmmm, I did some further investigation on this.
Unfortunaltely I don't have the original system ( Ubuntu 6.06 ) at hand, so my investigations was done with a different system and Ubuntu 7.10.

This led me to believe that some hardware-related electrical noise is generating the whole issue, since the problem is very depending on the order and timing with which I turn on the PC and the device attached to the serial port.
So right now I can make it work if I let the PC boot automatically when it detects power-on (BIOS setting) in stead of waiting for the user to push the PC-boot button.
So my conclusion is that when the PC detected power, it was doing some secret stuff maybe to setup for network booting etc, and then it went to a sleeping state, awaiting the user to press the boot button. And this secret stuff and the following go-to-sleep feature, was somehow generating noise on the serial port.

Anyway, I am still convinced that the Ubuntu 6.06 actually did send a byte on the serial port when it was booting the kernel - or maybe it just flushed some buffer, and this buffer happened to be filled with some garbage...?

Yes, I am somewhat confused now, however I am not convinced I can blame Ubuntu anymore, and I am considering this question closed.