Hi there,

since my propblem is with usb to serial adapters I was unsure if this is
the right place. If not, please let me know.

After equipping a server with a lot of ftdi singleport usb2serial
devices, I ran into a serial device limit that is included in
include/linux/usb/serial.h .

The limit of devices seems to be 255, resulting in ttyUSB254 to be the
last supported device.

The codesnippet in question is:

#define SERIAL_TTY_MINORS 254 /* loads of devices :) */
#define SERIAL_TTY_NO_MINOR 255 /* No minor was assigned */

I successfully modified it to support 256 devices and fail gracefully
afterward:

#define SERIAL_TTY_MINORS 256 /* loads of devices :) */
#define SERIAL_TTY_NO_MINOR 255 /* No minor was assigned */

That results in the following dmesg output:

[..]
usb 1-8.4.4.4.3: Detected FT232BM
usb 1-8.4.4.4.3: Number of endpoints 2
usb 1-8.4.4.4.3: Endpoint 1 MaxPacketSize 64
usb 1-8.4.4.4.3: Endpoint 2 MaxPacketSize 64
usb 1-8.4.4.4.3: Setting MaxPacketSize 64
usb 1-8.4.4.4.3: FTDI USB Serial Device converter now attached to ttyUSB254
hub 1-8.4.4.4:1.0: port 4, status 0101, change 0000, 12 Mb/s
usb 1-8.4.4.4.4: new full-speed USB device number 91 using ehci-pci
usb 1-8.4.4.4.4: ep0 maxpacket = 8
usb 1-8.4.4.4.4: default language 0x0409
usb 1-8.4.4.4.4: udev 91, busnum 1, minor = 90
usb 1-8.4.4.4.4: New USB device found, idVendor=0403, idProduct=6001
usb 1-8.4.4.4.4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 1-8.4.4.4.4: Product: USB <-> Serial
usb 1-8.4.4.4.4: Manufacturer: FTDI
usb 1-8.4.4.4.4: usb_probe_device
usb 1-8.4.4.4.4: configuration #1 chosen from 1 choice
usb 1-8.4.4.4.4: adding 1-8.4.4.4.4:1.0 (config #1, interface 0)
ftdi_sio 1-8.4.4.4.4:1.0: usb_probe_interface
ftdi_sio 1-8.4.4.4.4:1.0: usb_probe_interface - got id
ftdi_sio 1-8.4.4.4.4:1.0: FTDI USB Serial Device converter detected
usb 1-8.4.4.4.4: Detected FT232BM
usb 1-8.4.4.4.4: Number of endpoints 2
usb 1-8.4.4.4.4: Endpoint 1 MaxPacketSize 64
usb 1-8.4.4.4.4: Endpoint 2 MaxPacketSize 64
usb 1-8.4.4.4.4: Setting MaxPacketSize 64
usb 1-8.4.4.4.4: FTDI USB Serial Device converter now attached to ttyUSB255

If any further devices are connected, I get the expected error that no
more free sedrial devices are there.

hub 4-1.1:1.0: state 7 ports 4 chg 001e evt 0000
hub 4-1.1:1.0: port 1, status 0101, change 0000, 12 Mb/s
usb 4-1.1.1: new full-speed USB device number 89 using uhci_hcd
usb 4-1.1.1: ep0 maxpacket = 8
usb 4-1.1.1: default language 0x0409
usb 4-1.1.1: udev 89, busnum 4, minor = 472
usb 4-1.1.1: New USB device found, idVendor=0403, idProduct=6001
usb 4-1.1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 4-1.1.1: Product: FT232R USB UART
usb 4-1.1.1: Manufacturer: FTDI
usb 4-1.1.1: SerialNumber: A500EG03
usb 4-1.1.1: usb_probe_device
usb 4-1.1.1: configuration #1 chosen from 1 choice
usb 4-1.1.1: adding 4-1.1.1:1.0 (config #1, interface 0)
ftdi_sio 4-1.1.1:1.0: usb_probe_interface
ftdi_sio 4-1.1.1:1.0: usb_probe_interface - got id
ftdi_sio 4-1.1.1:1.0: FTDI USB Serial Device converter detected
ftdi_sio 4-1.1.1:1.0: No more free serial devices
ftdi_sio: probe of 4-1.1.1:1.0 failed with error -5
hub 4-1.1:1.0: port 2, status 0101, change 0000, 12 Mb/s
usb 4-1.1.2: new full-speed USB device number 90 using uhci_hcd
usb 4-1.1.2: ep0 maxpacket = 8
usb 4-1.1.2: default language 0x0409
usb 4-1.1.2: udev 90, busnum 4, minor = 473
usb 4-1.1.2: New USB device found, idVendor=0403, idProduct=6001
usb 4-1.1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 4-1.1.2: Product: FT232R USB UART
usb 4-1.1.2: Manufacturer: FTDI
usb 4-1.1.2: SerialNumber: A500EG07
usb 4-1.1.2: usb_probe_device
usb 4-1.1.2: configuration #1 chosen from 1 choice
usb 4-1.1.2: adding 4-1.1.2:1.0 (config #1, interface 0)
ftdi_sio 4-1.1.2:1.0: usb_probe_interface
ftdi_sio 4-1.1.2:1.0: usb_probe_interface - got id
ftdi_sio 4-1.1.2:1.0: FTDI USB Serial Device converter detected
ftdi_sio 4-1.1.2:1.0: No more free serial devices
ftdi_sio: probe of 4-1.1.2:1.0 failed with error -5

and so on.

Output of `lsusb | sort` with 256 usb2serial devices connected:
http://pastebin.com/wqrYUbwZ

In case you end up agreeing with me that there are no reasons to limit
the devices to 255, would you be inclined to fix this in the main kernel
tree? Do I have to open up a bug somewhere?

I also tried increasing the numbers further but I run into rollover
issues as the kernel tries to reassign ttyUSB0 ttyUSB1 which is already
taken. It seems without some other modification 256 devices is the limit?

Thanks for your efforts!

Tobias