Limit on Serial ports on a Redhat Linux?

[Robert Walker_8]

Hi,

We are looking at RHEL5 (beta 2) and we have tried to install 3 multiport serial cards (4 porters). These cards are fairly basic just emulate 16550. This has caused problems so I switched to RHEL4U1 and on a deskpro as this has been running the conman software source kit for some time on a single 4 porter board + 2 on board serial ports - thus providing 7 serial ports.

However on the deskpro after about 7 ports it would say:

"Couldn't register serial port 0000:00:0e.0: -28"

The log is as follows:

Serial: 8250/16550 driver $Revision: 1.90 $ 8 ports, IRQ sharing enabled
ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
ttyS1 at I/O 0x2f8 (irq = 3) is a NS16550A
PCI: Found IRQ 9 for device 0000:00:0d.0
ttyS4 at I/O 0x2060 (irq = 9) is a 16550A
ttyS5 at I/O 0x2068 (irq = 9) is a 16550A
ttyS6 at I/O 0x2070 (irq = 9) is a 16550A
ttyS7 at I/O 0x2078 (irq = 9) is a 16550A
PCI: Found IRQ 11 for device 0000:00:0e.0
ttyS2 at I/O 0x2088 (irq = 11) is a 16550A
ttyS3 at I/O 0x20a0 (irq = 11) is a 16550A
Couldn't register serial port 0000:00:0e.0: -28

Another server (Proliant ML570 and RHEL5B2) coughs up after 4 (including the on board serial port) with a similar message although the remaining 9 generate a similar "couldnt register serial port " error message.

Any ideas?

[Peter Leddy_1]

Hi Robert,

Have you had a look at the Serial HOWTO - http://www.tldp.org/HOWTO/Serial-HOWTO.html

Chapter 6 deals with Multiport Serial Cards and configuring the kernel and chapter 17 deals with troubleshooting which may help you.

Rgds,

Peter

[Robert Walker_8]

Hi Peter,

I found the howto later on and have been trying variations on grub of 8250.nr_uarts=12 or nr_uarts=12 however this doesnt seem to work still get the Couldnt register serial port error -28.

Have also tried mknod and MAKEDEV with no luck.

Robert.

[Heironimus]

On a newer system serial support could be a kernel module instead of built-in. If that's the case you'd need to find the proper options to use to load the module (modinfo would probably tell you) instead of using kernel arguments in the boot loader.

[Robert Walker_8]

Hi,

Some further updates. I have received some 16550 8 port serial cards. If I install them and dont have on grub line 8250.nr_uarts=25 (3x8 + 1 on board) I only get 4 serial ports on RHEL5B2 or 8 on RHEL4.

If I put the grub line I get on RHEL5 25 serial ports.

The other card is a NetMos Technology PCI 9845 Multi-I/O Controller and placing 8250.nr_uarts results in "Unknown boot option `8250.nr_uarts=15': ignoring" and then "Serial: 8250/16550 driver $Revision: 1.90 $ 8 ports, IRQ sharing enabled"

Where as the other card (8 porter) is a "Serial controller: Timedia Technology Co Ltd PCI2S550 (Dual 16550 UART)" which allows for 8250.nr_uarts=n on the grub boot line.

So far lsmod and modinfo dont provide much details - as I cannot find a module which relates to either of these - originally thinking it may have been parport_serial however this is not loaded?

Robert.

[Mike Stroyan]

The 8250.nr_uarts kernel parameter is part of the SERIAL_8250 driver. You can see if that driver is a module or is linked into the kernel by looking at /boot/config$(uname -r) . Look for
CONFIG_SERIAL_8250=y or CONFIG_SERIAL_8250=m .
While you are looking at the config file also notice the settings of CONFIG_SERIAL_8250_NR_UARTS and CONFIG_SERIAL_8250_RUNTIME_UARTS which set the maximum number of serial ports and the maximum number to register at boot time. The CONFIG_SERIAL_8250_NR_UARTS value is a hard limit which cannot be exceeded by the boot parameter.

[Robert Walker_8]

Thanks Mike.

From what you said it would seem that its not a module, but linked into the kernel. Also the maximum UARTS that I should be able to run is 4 - but I see 8.

CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_SERIAL_8250_CS=m
# CONFIG_SERIAL_8250_ACPI is not set
CONFIG_SERIAL_8250_NR_UARTS=4
CONFIG_SERIAL_8250_EXTENDED=y
# CONFIG_SERIAL_8250_MANY_PORTS is not set
CONFIG_SERIAL_8250_SHARE_IRQ=y
CONFIG_SERIAL_8250_DETECT_IRQ=y
CONFIG_SERIAL_8250_MULTIPORT=y
CONFIG_SERIAL_8250_RSA=y

I logged a call wih Redhat at the same time. They have the config setting on RHEL4 systems set with CONFIG_SERIAL_8250_MANY_PORTS=n which wont allow more than the default limit. Still if the default was 4 and MANY_PORTS are set to no then why can I get 8 serial adapters?

Robert.

[Rob Leadbeater]

Hi Robert,

I know this doesn't necessarily solve your problem as you've already got your cards, but you could look at using a multiport serial card from Comtrol.

http://www.comtrol.com/products/RocketPort.asp

These aren't the cheapest cards however they do come with good Linux drivers. I've not had experience with them on recent releases, but I've had an 8 port card running in an old RedHat 6.2 box for several years, and it has been faultless...

Hope this helps,

Regards,

Rob

[Robert Walker_8]

Hi,

Thanks to all. My logged call with Redhat has come through. Apparently someone in Redhat decided that CONFIG_SERIAL_8250_MANY_PORTS should be set to N, and thus doesnt allow users of RHEL4 to set the 8250.NR_UARTS=nn (where nn is the number of ports) - I have been informed however by Redhat support that this will be fixed in RHEL4 Update 5 - which is supposed to be out in the next month or so.

I have been provided with an unsupported new kernel which provides 64 serial ports to get me out of my jam. If anyone else has the problem then mention about the internal bugzilla number 189859 as this should speed up your requirements to get the test kernel if needed.

Otherwise thanks to all for advice and suggestions. I am a tiny weeny bit wiser about the kernel than I was before.

Regards,

Robert.

[Robert Walker_8]

Redhat provided a solution as it was a mod made in the kernel config - contact Redhat mention internal bugzilla number 189859 to speed your process up.