[Techtalk] ttyS1: LSR safety check engaged!

[agoats]

> Simple question- wot the heck does 'ttyS1: LSR safety check engaged!' mean? 
> It's dmesg output. I have Googled, and found only other puzzled souls.
> 
> thanks!
> -- 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Carla Schroder

This one's obscure enough I'm going to jump in early... no offense!

All of the LSR instructions are buried in the serial driver source code
for the kernel source. 

/usr/src/linux/drivers/char/serial.c

Just browsing through the source code, it looks like they're referencing
the FIFO shift registers. Why the term LSR? I don't know unless it's
buried in one of the datasheets or application notes for one of the
supported UART's as part of a description or block diagram. 

The UART has a register that transmits and receives single line serial
data and converts it to or from parallel data. If you have 8 data lines
going into the UART, it has a shift register that gets all 8 bits and
when it is flagged, will lock out any more inputs to the register and
shift them in sequence out to a single data line. After all of the bits
are shifted out, it flags that it is finished and unlocks the parallel
lines. (There's more to this, but this takes the simple way out that
doesn't require a EE degree). 

For serial data in, serial data goes into the shift register until it is
full, locks the serial line and the parallel lines then read whatever is
in each of the register's bits. 

Now, your message 'ttyS1: LSR safety check engaged!' is buried in the
source code as flagging a buggy UART. During some of the testing to see
what kind of UART you have and whether it has certain flaws or not, it
found that yours is buggy. It isn't setting itself correctly so a work
around is being applied. 

If you want better info and all the gory details about UART's then
google for:

UART datasheet Intel

If you can get one, find the "Intel Peripheral Design Handbook", it goes
into excruciating detail of each command, timing cycles, expected
results, what it does when the result is wrong, etc. It has the
datasheets AND the application notes in one book!

I know, it's more than you wanted to know... but I thought this much
detail might help de-mystify things a little bit...


Alvin