Re: [PATCH] Alternate setting 1 must be selected for interface 0 on the model that I received. Else the rest is identical.

[Hans Petter Selasky <hselasky@c2i.net>]

On Monday 23 May 2011 21:06:32 Mauro Carvalho Chehab wrote:
> Em 23-05-2011 15:48, Hans Petter Selasky escreveu:
> > On Monday 23 May 2011 20:14:45 Mauro Carvalho Chehab wrote:
> >> Em 23-05-2011 11:37, Hans Petter Selasky escreveu:
> >> 
> >> I don't have any ttusb device here, but I doubt that this would work.
> > 
> > Hi,
> > 
> > It is already tested and works fine.
> 
> This will work for you, but it will likely break for the others. Your patch
> is assuming that returning an error if selecting alt 1 is enough to know
> that alt 0 should be used.
> 
> > What I see is that interface 1 does not have an alternate setting like
> > the driver code expects, while interface 0 does. So it is the opposite
> > of what the driver expects. Maybe the manufacturer changed something.
> > Endpoints are still the same.
> 
> That sometimes happen. Or maybe you just need a different size.
> 
> > Please find attached an USB descriptor dump from this device.
> 
> Int 0, endpoint 0:
> 
>     Interface 0
>       bLength = 0x0009
>       bDescriptorType = 0x0004
>       bInterfaceNumber = 0x0000
>       bAlternateSetting = 0x0000
>       bNumEndpoints = 0x0003
>       bInterfaceClass = 0x0000
>       bInterfaceSubClass = 0x0000
>       bInterfaceProtocol = 0x0000
>       iInterface = 0x0000  <no string>
> 
> ...
> 
>      Endpoint 2
>         bLength = 0x0007
>         bDescriptorType = 0x0005
>         bEndpointAddress = 0x0082  <IN>
>         bmAttributes = 0x0001  <ISOCHRONOUS>
>         wMaxPacketSize = 0x0000
>         bInterval = 0x0001
>         bRefresh = 0x0000
>         bSynchAddress = 0x0000
> 
> ...
> 
>     Interface 0 Alt 1
>       bLength = 0x0009
>       bDescriptorType = 0x0004
>       bInterfaceNumber = 0x0000
>       bAlternateSetting = 0x0001
>       bNumEndpoints = 0x0003
>       bInterfaceClass = 0x0000
>       bInterfaceSubClass = 0x0000
>       bInterfaceProtocol = 0x0000
>       iInterface = 0x0000  <no string>
> 
> ...
>      Endpoint 2
>         bLength = 0x0007
>         bDescriptorType = 0x0005
>         bEndpointAddress = 0x0082  <IN>
>         bmAttributes = 0x0001  <ISOCHRONOUS>
>         wMaxPacketSize = 0x0390
>         bInterval = 0x0001
>         bRefresh = 0x0000
>         bSynchAddress = 0x0000
> 

Hi,

> Hmm... assuming that the driver is using ISOC transfers, the difference
> between alt 0 and alt 1 is that, on alt0, the mwMaxPacketSize is 0 (so,
> you can't use it for isoc transfers), while, on alt 1, wMaxPacketSize is
> 0x390.
> 
> What the driver should be doing is to select an alt mode where the
> wMaxPacketSize is big enough to handle the transfer.

I can write the code to do that. Summed up:

1) Search interface 0, for alternate settings that have an ISOC-IN and 
wMaxPacket != 0. Select this alternate setting.

2) Search interface 1, for alternate settings that have an ISOC-IN and 
wMaxPacket != 0. Select this alternate setting.
 
3) Done.

Do you think this will work better?

> Calculating what "big enough"   is device-dependent, but, basically, a 480
> Mbps USB bus is capable of providing 800 isoc slots per interval. If the
> packets are bigger, the max bandwidth is bigger.

This is a FULL speed device, max 10MBit/second.

> You're able to see the amount of packets per interval by doing a cat
> /proc/bus/usb/devices:
> 
> T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480  MxCh= 8
> B:  Alloc=  0/800 us ( 0%), #Int=  0, #Iso=  0
> 
> The "B:" line above shows the USB bandwidth usage.

Do you need this information to proceed with the patch?

--HPS