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From: Rich Townsend <rhdt@bartol.udel.edu>
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Subject: [Bluez-devel] SCO MTU problem on CSR-based Belkin F8T020/F8T007
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Date: Mon, 15 May 2006 22:27:10 -0400

Dear all,

I've recently acquired a Belkin F8T020/F8T007 PCMCIA bluetooth interface 
(basically, a compact flash card in an adapter sleeve), and I'm trying to get 
this linked up to a Digiwave BH-2000 headset.

Mostly, everything is functioning correctly; the interface provides a UART on 
/dev/ttyS3, which Bluez has no problem working with using the hci_uart driver. 
The problem, however, comes in getting the ALSA module, snd_bt_sco, to behave 
properly. I can get the headset to connect to my computer, and do things such as 
change the volume; but I've had no luck in getting any sound to play on the headset.

After a little digging around, it turns out that no SCO packets are being 
transmitted by the interface. Closer investigation shows that the problem is 
with the outgoing MTU; the len value passed to sco_send_frame is invariably 
longer than the default SCO MTU of the interface (64), with the result that 
sco_send_frame aborts early with an EINVAL return value (which in turn is 
silently ignored).

As a test fix, I upped the SCO MTU on the interface to 255, and now stuff 
actually gets transmitted -- and I can hear sound on the headset, albeit very 
distorted.

So, it seems snd_bt_sco is disregarding the MTU settings of the interface. Why, 
I'm not sure; a quick look through the snd_bt_sco source code doesn't reveal 
anything obvious (in fact, it's not clear how the module picks up the MTU). I'd 
appreciate hearing suggestions about how I might go about fixing this.

For the record:

Kernel 2.6.16 (x86)

hciconfig -a (after upping MTU so TX works):

hci0:   Type: UART
         BD Address: 00:02:72:81:23:27 ACL MTU: 192:8 SCO MTU: 255:8
         UP RUNNING PSCAN ISCAN AUTH ENCRYPT
         RX bytes:5063515 acl:52 sco:30471 events:94 errors:0
         TX bytes:4612824 acl:42 sco:0 commands:27604 errors:0
         Features: 0xff 0xff 0x0f 0x00 0x00 0x00 0x00 0x00
         Packet type: DM1 DM3 DM5 DH1 DH3 DH5 HV1 HV2 HV3
         Link policy: RSWITCH HOLD SNIFF PARK
         Link mode: SLAVE ACCEPT
         Name: 'BlueZ (0)'
         Class: 0x000100
         Service Classes: Unspecified
         Device Class: Computer, Uncategorized
         HCI Ver: 1.1 (0x1) HCI Rev: 0x110 LMP Ver: 1.1 (0x1) LMP Subver: 0x110
         Manufacturer: Cambridge Silicon Radio (10)

hcitool info 00:13:8A:06:31:0E (the headset):

Requesting information ...
         BD Address:  00:13:8A:06:31:0E
         Device Name: BH2000
         LMP Version: 1.2 (0x2) LMP Subversion: 0x611
         Manufacturer: Cambridge Silicon Radio (10)
         Features: 0xfc 0xfe 0x0f 0x00 0x08 0x08 0x00 0x00
                 <encryption> <slot offset> <timing accuracy> <role switch>
                 <hold mode> <sniff mode> <RSSI> <channel quality> <SCO link>
                 <HV2 packets> <HV3 packets> <u-law log> <A-law log> <CVSD>
                 <paging scheme> <power control> <transparent SCO>
                 <AFH cap. slave> <AFH cap. master>

Big note: both devices are CSR, so this *ISN'T* the well-known MTU problem 
experienced by Broadcom chipsets. Smaller note: although the hciconfig output is 
after I upped the MTU and got (distorted) sound on the headset, the SCO TX count 
is still at zero. Hmmmm....

Let me know if you need more info...

cheers,

Rich





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