Using z/VM the 3270 terminal emulator also emulates an IBM 3215 console
which outputs line by line. When the screen is full, the console enters
the MORE... state and waits for the operator to confirm the data
on the screen by pressing a clear key. If this does not happen in the
default time frame (currently 50 seconds) the console enters the HOLDING
state.
It then waits another time frame (currently 10 seconds) before the output
continues on the next screen. When the operator presses the clear key
during these wait times, the output continues immediately.

This may lead to a very long boot time when the console
has to print many messages, also the system may hang because of the
console's limited buffer space and the system waits for the console
output to drain and finally to finish. This problem can only occur
when a terminal emulator is actually connected to the 3215 console
driver. If not z/VM simply drops console output.

Remedy this rare situation and add a kernel boot command line parameter
con3215_drop. It can be set to 0 (do not drop) or 1 (do drop) which is
the default. This instructs the kernel drop console data when the
console buffer is full. This speeds up the boot time considerable and
also does not hang the system anymore.

Add a sysfs attribute file for console IBM 3215 named con_drop.
This allows for changing the behavior after the boot, for example when
during interactive debugging a panic/crash is expected.

Here is a test of the new behavior using the following test program:
 #/bin/bash
 declare -i cnt=4

 mode=$(cat /sys/bus/ccw/drivers/3215/con_drop)
 [ $mode = yes ] && cnt=25

 echo "cons_drop $(cat /sys/bus/ccw/drivers/3215/con_drop)"
 echo "vmcp term more 5 2"
 vmcp term more 5 2
 echo "Run $cnt iterations of "'echo t > /proc/sysrq-trigger'

 for i in $(seq $cnt)
 do
	echo "$i. command 'echo t > /proc/sysrq-trigger' at $(date +%F,%T)"
	echo t > /proc/sysrq-trigger
	sleep 1
 done
 echo "droptest done" > /dev/kmsg
 #

Output with sysfs attribute con_drop set to 1:
 # ./droptest.sh
 cons_drop yes
 vmcp term more 5 2
 Run 25 iterations of echo t > /proc/sysrq-trigger
 1. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:09
 2. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:10
 3. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:11
 4. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:12
 5. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:13
 6. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:14
 7. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:15
 8. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:16
 9. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:17
 10. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:18
 11. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:19
 12. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:20
 13. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:21
 14. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:22
 15. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:23
 16. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:24
 17. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:25
 18. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:26
 19. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:27
 20. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:28
 21. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:29
 22. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:30
 23. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:31
 24. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:32
 25. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:15:33
 #

There are no hangs anymore.

Output with sysfs attribute con_drop set to 0 and identical
setting for z/VM console 'term more 5 2'. Sometimes hitting the
clear key at the x3270 console to progress output.

 # ./droptest.sh
 cons_drop no
 vmcp term more 5 2
 Run 4 iterations of echo t > /proc/sysrq-trigger
 1. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:20:58
 2. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:24:32
 3. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:28:04
 4. command 'echo t > /proc/sysrq-trigger' at 2022-09-02,10:31:37
 #

Details:
Enable function raw3215_write() to handle tab expansion and newlines
and feed it with input not larger than the console buffer of 65536
bytes. Function raw3125_putchar() just forwards its character for
output to raw3215_write().

This moves tab to blank conversion to one function raw3215_write()
which also does call raw3215_make_room() to wait for enough free
buffer space.

Function handle_write() loops over all its input and segments input
into chunks of console buffer size (should the input be larger).

Rework tab expansion handling logic to avoid code duplication.
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>