Distribution: Debian Lenny
Hardware Environment: Thinkpad with Sierra Wireless MC8775 UMTS-card
Software Environment: debian lenny i386 + tp_smapi module to enable / disable wan

Problem Description:

On my thinkpad x61s laptop, I get several times: 

WARNING: at drivers/usb/serial/usb-serial.c:320 serial_write_room+0x41/0x4d [usbserial]()

in dmesg after powercycling the Sierra Wireless MC8775 UMTS-card two
times.

Steps to reproduce:

[card is active after boot]

echo disable > /proc/acpi/ibm/wan

[ttyUSB0-2 disconnect]

echo enable > /proc/acpi/ibm/wan

[ttyUSB0-2 connect]
[normal ppp operation of card]

echo disable > /proc/acpi/ibm/wan

[ttyUSB0-2 disconnect]

echo enable > /proc/acpi/ibm/wan

[ttyUSB0-2 connect]

This results in warnings like this:

[  313.825743] ------------[ cut here ]------------
[  313.825743] WARNING: at drivers/usb/serial/usb-serial.c:320 serial_write_room+0x41/0x4d [usbserial]()
[  313.825743] Modules linked in: i915 drm ppdev parport_pc lp parport acpi_cpufreq cpufreq_userspace cpufreq_stats cpufreq_powersave cpufreq_ondemand freq_table cpufreq_conservative xt_state ip6table_mangle ip6table_filter ip6_tables ipv6 iptable_mangle iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack iptable_filter ip_tables x_tables ext2 fuse dm_crypt dm_mod coretemp tpm_tis tpm tpm_bios tp_smapi thinkpad_ec firewire_sbp2 uinput loop cm4040_cs snd_hda_intel snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss snd_seq_midi snd_rawmidi snd_seq_midi_event arc4 ecb snd_seq i2c_i801 i2c_core crypto_blkcipher psmouse snd_timer snd_seq_device pcmcia rtc_cmos rtc_core rtc_lib sierra serio_raw iwl3945 pcspkr usbserial snd yenta_socket firmware_class rsrc_nonstatic pcmcia_core mac80211 soundcore iTCO_wdt snd_page_alloc cfg80211 battery ac video output bay button thinkpad_acpi intel_agp agpgart led_class evdev nvram ext3 jbd mbcache ata_generic sd_mod piix ahci libata scsi_mod sdhci mmc_core ide_pci_generic firewire_ohci firewire_core crc_itu_t ide_core ehci_hcd uhci_hcd usbcore e1000e dock thermal processor fan
[  313.825743] Pid: 0, comm: swapper Tainted: G        W 2.6.26.5-pristine #1
[  313.825743]  [<c011f6ac>] warn_on_slowpath+0x40/0x79
[  313.825743]  [<c0115978>] enqueue_task+0x52/0x5d
[  313.825743]  [<c011599e>] activate_task+0x1b/0x2e
[  313.825743]  [<c0118829>] try_to_wake_up+0xe8/0xf1
[  313.825743]  [<c0128585>] signal_wake_up+0x24/0x32
[  313.825743]  [<c012890a>] send_signal+0x1f6/0x20b
[  313.825743]  [<f8ae243a>] serial_write_room+0x41/0x4d [usbserial]
[  313.825743]  [<c021f1a4>] tty_write_room+0x15/0x16
[  313.825743]  [<c021d668>] opost+0xf/0x1ae
[  313.825743]  [<c021e6e8>] n_tty_receive_buf+0x2d1/0xcb5
[  313.825743]  [<c0115978>] enqueue_task+0x52/0x5d
[  313.825743]  [<c011599e>] activate_task+0x1b/0x2e
[  313.825743]  [<c0118829>] try_to_wake_up+0xe8/0xf1
[  313.825743]  [<c012ee37>] autoremove_wake_function+0xd/0x2d
[  313.825743]  [<c0115978>] enqueue_task+0x52/0x5d
[  313.825743]  [<c021aa5c>] tty_buffer_request_room+0x108/0x112
[  313.825743]  [<c021a18a>] flush_to_ldisc+0xde/0x17c
[  313.825743]  [<f8bb0bff>] sierra_indat_callback+0x8c/0xe8 [sierra]
[  313.825743]  [<f885f11c>] usb_hcd_giveback_urb+0x60/0x8b [usbcore]
[  313.825743]  [<f8879826>] uhci_giveback_urb+0xe5/0x160 [uhci_hcd]
[  313.825743]  [<f8879e7f>] uhci_scan_schedule+0x50c/0x752 [uhci_hcd]
[  313.825743]  [<f887b8ce>] uhci_irq+0xfe/0x10b [uhci_hcd]
[  313.825743]  [<f885ee0e>] usb_hcd_irq+0x24/0x5a [usbcore]
[  313.825743]  [<c014c47c>] handle_IRQ_event+0x1a/0x3f
[  313.825743]  [<c014d281>] handle_fasteoi_irq+0x70/0xa2
[  313.825743]  [<c0105cbe>] do_IRQ+0x4f/0x65
[  313.825743]  [<c01041ab>] common_interrupt+0x23/0x28
[  313.825743]  [<c01300d8>] run_posix_cpu_timers+0x4b2/0x74e
[  313.825743]  [<f8848152>] acpi_idle_enter_simple+0x184/0x1f2 [processor]
[  313.825743]  [<c02035a1>] acpi_set_register+0x144/0x14d
[  313.825743]  [<f8847d4a>] acpi_idle_enter_bm+0xc0/0x344 [processor]
[  313.825743]  [<c0132871>] pm_qos_requirement+0x23/0x27
[  313.825743]  [<c023f752>] cpuidle_idle_call+0x56/0x85
[  313.825743]  [<c023f6fc>] cpuidle_idle_call+0x0/0x85
[  313.825743]  [<c01029ae>] cpu_idle+0x8d/0xab
[  313.825743]  =======================
[  313.825743] ---[ end trace f77bdb068b6acaf2 ]---