Re: Attempting to understand odd snd-usb-audio code and behavior

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On Wednesday 23 Aug 2017 14:52:43 Clemens Ladisch wrote:
> Keith A. Milner wrote:
> >>> kernel: [ 1987.452947] usb 1-1: Unable to change format on ep #8e:
> >>> already in use> 
> > So the error messages are due to a userspace app trying to open playback
> > and capture at the same time?
> 
> For this device, playback requires a running capture stream to determine
> the rate of packets.  This error happens when the application tries to
> open the capture stream after the playback stream.  (If the format does
> not actually need to change, the error message is misleading.)
> 
> Your actual problem is that the device does not send any capture packets.

OK, I found some more time to look at this.

The background is a Roland device. The device descriptor is attached, but it 
seems to be common to a lot of Roland devices.

Specifically on this device, the Playback endpoint is 0x0d and there is a 
related Implicit feedback endpoint at 0x8e.

I have USB traces of the device operation under Windows and the (non) 
operation under Linux. The primary difference seems to be that, under Windows, 
the Implict Feedback endpoint (0x8e) is opened prior to playback commencing. 
Given previous discussions (and further research) on Implicit feedback, this 
makes sense.

Under Linux, this does not happen.

I've been studying the code in pcm.c and endpoint.c to work out why this is, 
and have some questions on the code function.

The flow of the code to deal with implict feedback setup in pcm.c is, roughly, 
as follows:
set_sync_endpoint
  set_sync_ep_implicit_fb_quirk
     search_roland_implicit_fb

set_sync_ep_implicit_fb_quirk seems to deal with quirks relating to implicit 
feedback and, within this, is a specific check for Roland which ends up 
branching to "add_sync_ep" which adds a new endpoint via endpoint.c function 
snd_usb_add_endpoint and assigns this as as the sync endpoint:

	subs->sync_endpoint = snd_usb_add_endpoint(subs->stream->chip,
						   alts, ep, !subs->direction,
						   SND_USB_ENDPOINT_TYPE_DATA);

	subs->data_endpoint->sync_master = subs->sync_endpoint;

Within endpoint.c, this new endpoint is added to chip->ep_list :

	list_add_tail(&ep->list, &chip->ep_list);

After set_sync_ep_implicit_fb_quirk has completed it returns to 
set_sync_endpoint which then checks that there are at least 2 endpoints:

	if (altsd->bNumEndpoints < 2)
		return 0;

However, in the case I'm testing, on this device, this test fails as 
bNumEndpoints is 1, even though we've just added a second endpoint for the 
Implicit feedback.

Looking at the code, endpoint.c does not alter bNumEndpoints, which seems to 
be a fundamental piece of data held against the usb_host_interface instance 
and, presumably, set from the device descriptor by underlying kernel code.

My question is, is the test for bNumEndpoints correct here? From what I can 
gather, the whole point of the set_sync_ep_implicit_fb_quirk and 
search_roland_implicit_fb code is to add an endpoint that can then be used by 
set_sync_endpoint, but this seems to be ignored by doing the test for 
bNumEndpoints.

Or, perhaps, is it that the way that this endpoint is added is incorrect.

I have, by the way, tried commenting out the bNumEndpoints test, and it fails 
at the subsequent sync-pipe check with:

invalid sync pipe. bmAttributes 00, bLength 0, bSynchAddress 00

To me this suggests the endpoint isn't being set correctly somehow.

Any pointers gratefully received.

Cheers,

Keith


Bus 001 Device 013: ID 0582:01d6 Roland Corp. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass          255 Vendor Specific Class
  bDeviceSubClass         0 
  bDeviceProtocol       255 
  bMaxPacketSize0        64
  idVendor           0x0582 Roland Corp.
  idProduct          0x01d6 
  bcdDevice            0.00
  iManufacturer           1 BOSS
  iProduct                2 GT-1
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength          188
    bNumInterfaces          4
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xc0
      Self Powered
    MaxPower                0mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           0
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol      0 
      iInterface              0 
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           0
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      2 
      bInterfaceProtocol      2 
      iInterface              0 
      ** UNRECOGNIZED:  06 24 f1 01 00 00
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       1
      bNumEndpoints           1
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      2 
      bInterfaceProtocol      2 
      iInterface              0 
      ** UNRECOGNIZED:  07 24 01 01 00 01 00
      ** UNRECOGNIZED:  0b 24 02 01 02 04 18 01 44 ac 00
      ** UNRECOGNIZED:  06 24 f1 04 16 00
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x0d  EP 13 OUT
        bmAttributes            5
          Transfer Type            Isochronous
          Synch Type               Asynchronous
          Usage Type               Data
        wMaxPacketSize     0x0038  1x 56 bytes
        bInterval               1
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       0
      bNumEndpoints           0
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      2 
      bInterfaceProtocol      1 
      iInterface              0 
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       1
      bNumEndpoints           1
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      2 
      bInterfaceProtocol      1 
      iInterface              0 
      ** UNRECOGNIZED:  07 24 01 07 00 01 00
      ** UNRECOGNIZED:  0b 24 02 01 02 04 18 01 44 ac 00
      ** UNRECOGNIZED:  06 24 f1 04 16 00
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x8e  EP 14 IN
        bmAttributes           37
          Transfer Type            Isochronous
          Synch Type               Asynchronous
          Usage Type               Implicit feedback Data
        wMaxPacketSize     0x0038  1x 56 bytes
        bInterval               1
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        3
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      3 
      bInterfaceProtocol      0 
      iInterface              0 
      ** UNRECOGNIZED:  06 24 f1 02 01 01
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x84  EP 4 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        3
      bAlternateSetting       1
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass      3 
      bInterfaceProtocol      0 
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               4
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x85  EP 5 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               4
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.00
  bDeviceClass          255 Vendor Specific Class
  bDeviceSubClass         0 
  bDeviceProtocol       255 
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0001
  Self Powered
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