karlkiste
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uart: "Uncompressing Linux..."

Sat Feb 09, 2013 5:27 pm

" done, booting the kernel."

This string is being sent through the uart at power-up. I've removed all the ttyAMA0 -stuff from cmdline, but this message is still there. I would prefer it to go away. I'm going to connect the TX lines of several Pi's together, and if they output data at boot they would be probably out of sync and the pin drivers will collide. I'm not sure if the drivers will be fried within those few milliseconds, but I'd better like not to try ;-)

Any idea anyone?

I've tried different values for init_uart_baud, but the speed seems only to depend on init_uart_clock.

M33P
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Re: uart: "Uncompressing Linux..."

Sat Feb 09, 2013 11:02 pm

The message is most likely posted by the bootloader as it does its startup routine. Therefore the GPIOs will be configured as output and driven likewise before Linux even gets a look-in.

If you're going to multiplex Pis together, perhaps you should consider a 3-wire RS485-style bus arrangement - but unfortunately the PL011 UART on the Pi doesn't have any flow control pins mapped out...

A kludge fix would be to insert series resistors in the Tx line - high enough to limit the current to safe levels should one Pi drive Tx high, and another drive it low.

dom
Raspberry Pi Engineer & Forum Moderator
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Re: uart: "Uncompressing Linux..."

Sat Feb 09, 2013 11:59 pm

That message comes from kernel:
https://github.com/raspberrypi/linux/bl ... isc.c#L147

The GPU doesn't output data to uart, but it does configure those pins to use the UART alt function, so TX will be configured as an output.

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jojopi
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 12:21 am

Simply using an uncompressed kernel image should avoid that message, since it is from the decompressing stub and not the kernel proper. (Which is why cmdline has not yet been parsed, and there is no console driver, and nowhere but the UART to send the message to.)
M33P wrote:If you're going to multiplex Pis together, perhaps you should consider a 3-wire RS485-style bus arrangement - but unfortunately the PL011 UART on the Pi doesn't have any flow control pins mapped out...
If you mean CTS/RTS, they are available on P5. Though I do not see what that has to do with using differential signalling.

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bgreat
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 1:28 am

jojopi wrote:If you mean CTS/RTS, they are available on P5. Though I do not see what that has to do with using differential signalling.
I believe the intended reference was for RS-485 multidrop signalling where the RTS line may be used to for controlling the transmitter state. RTS0 is also available on P1 (alternate function for GPIO 17).

Enjoy!
Bill

karlkiste
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 7:59 am

Thanks for all the input :-)
The GPU [...] does configure those pins to use the UART alt function, so TX will be configured as an output.
Yes, this is OK with me. It's a good feature, and shouldn't be changed. I could live with the outputs connected to each other, if I was sure all the outputs are driving the same level. It is the PL011 uart which is enabled, no? Its baud rate can be set to, according to the datasheet "dc to uartclock/16". So I tried setting "init_uart_baud=0" in config.txt, which should extend the start bit to last forever, but transmission speed is still 115200.

RS485 drivers are made for collision robustness, so I wouldn't have to fear hardware damage. But my intention was really to connect the Pi's through a simple ribbon cable. If I used extra hardware, a wired-and with pullup or so would do to protect the outputs. The mentioned series resistors would do as well. But it would all be more than just a cable ;-)
Simply using an uncompressed kernel image should avoid that message
I'll give that a try, only thing is, if I forget it after an update, I have a whole bunch of Pi's to sell on ebay with defective TX drivers :-D

karlkiste
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 10:03 am

karlkiste wrote:
Simply using an uncompressed kernel image should avoid that message
I'll give that a try, [...]
Hah, simply ;-) I thought it was simple, but I can't find out how to decompress the kernel. Seems to be a bit different than on intel platform :-D

karlkiste
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 10:45 am

Are there specs for the output drivers? I can't find them anywhere. The pins are rated, I'm sure. And I can't imagine why the output specs shouldn't be made public. Just like maximum source/sink current, hi/low voltage, allowable short-circuit duration, clamping diode currents and things like that.

If I was claiming a warranty for a Pi, and I was told "The GPIOs are damaged from overloading" I would like to know what overloading means.

Can anyone help me there?

karlkiste
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Re: uart: "Uncompressing Linux..."

Sun Feb 10, 2013 1:36 pm

karlkiste wrote: Hah, simply ;-) I thought it was simple, but I can't find out how to decompress the kernel.
As the usual scripts did not work, I had a little script try to gunzip the file after cutting away (n=n+1) bytes from the beginning. So this line worked:

dd if=kernel.img skip=1 bs=17836 | gzip -d >kernel_uncompressed.img

And indeed, the message is gone! About one second after power-up the TX line goes high, and stays there until told differently :-)

Thank you all!

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ThoManPi
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Re: uart: "Uncompressing Linux..."

Tue Feb 09, 2016 3:18 pm

dom wrote: The GPU doesn't output data to uart, but it does configure those pins to use the UART alt function, so TX will be configured as an output.
Where in the source code could i see whats happen to the UART? My problem is verry similar: viewtopic.php?f=66&t=135534

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PeterO
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Re: uart: "Uncompressing Linux..."

Tue Feb 09, 2016 3:35 pm

Reopening a thread that had been dead for 3 years hardly likely to help when you already have a current thread open.
PeterO
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ThoManPi
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Re: uart: "Uncompressing Linux..."

Wed Feb 10, 2016 8:57 am

It seems to me also in this thread the problem has not been solved. I had announced myself in the forum to be able to solve this problem and I do not know how to reach the really knowing ones.

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rpdom
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Re: uart: "Uncompressing Linux..."

Wed Feb 10, 2016 9:27 am

ThoManPi wrote:It seems to me also in this thread the problem has not been solved. I had announced myself in the forum to be able to solve this problem and I do not know how to reach the really knowing ones.
You have already posted to a new thread. That is all that is needed. You also have many replies on that one, so posting here is not needed.

General advice to everyone: If a thread is over six months old it is probably not directly relevant to you. Start a new one. You will get more help that way.

BTW, dom mentioned the GPU regarding the UART. The GPU source code is not available.

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