On Sun, Jun 21, 2020, at 9:09 AM, Alan Stern wrote:
> On Sat, Jun 20, 2020 at 10:38:33PM -0500, Sid Spry wrote:
>
> > When I was using a repurposed audio device I had to set an alternate mode. Is
> > that related to the issue here? The alternate mode seems to be a relic of the
> > descriptor layout before I dropped the device class and substituted my own
> > driver. The current descriptors specify no alternate modes.
>
> I don't know much about FunctionFS, so I can't help with your main
> question about isochronous packets not being dropped. But I can explain
> this.
>
Your responses have been quite helpful; I appreciate them.
For those reading along I have gotten a set of descriptors working (detailed
below) however write(2) still blocks meaning at least one stale data packet
will be received. However, I realize now that typically a writer thread will be
sending an asynchronously updated buffer and the single stale packet was
probably never noticed, especially for AV applications.
Unfortunately for my application the stale packet *does* matter, and I am still
seeking a way to mitigate it.
> In the USB-2.0 spec, at the end of section 5.6.3 the text says:
>
> All device default interface settings must not include any
> isochronous endpoints with non-zero data payload sizes (specified
> via wMaxPacketSize in the endpoint descriptor). Alternate
> interface settings may specify non-zero data payload sizes for
> isochronous endpoints.
>
> That's why you had to select an alternate setting before transferring
> any isochronous data. Any isochronous endpoint in altsetting 0 must have
> its maxpacket size set to 0.
>
This was the issue. I rely too heavily on the USB audio class spec, I should go
through all of the spec.
I now must ask the list: What is the relation of the isochronous endpoint setup
to the allocated bandwidth on the bus? I understand the limit of 3 1024 byte
transfers per frame, but this says nothing about how it will be allocated or
how a device is refused bandwidth. Do I need to look for link degradation on
the application layer? It seems like having a single non-spec device means
the OS can't arbitrate link bandwidth.
Also!
For the list's consideration I have included an accepted but nonworking
configuration that perplexes me. The application note for the original device
I used specified a set of descriptors which was like so (device and
configuration omitted):
(Vendor Microphone Class Descriptors)
```
INTERFACE 0: Vendor Specific ===========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x0
bAlternateSetting : 0x0
bNumEndpoints : 0x0
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x1
bInterfaceProtocol : 0x0
iInterface : 0x0
INTERFACE 1: Vendor Specific ===========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x1
bAlternateSetting : 0x0
bNumEndpoints : 0x0
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x2
bInterfaceProtocol : 0x0
iInterface : 0x0
INTERFACE 1, 1: Vendor Specific ========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x1
bAlternateSetting : 0x1
bNumEndpoints : 0x1
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x2
bInterfaceProtocol : 0x0
iInterface : 0x0
ENDPOINT 0x81: Isochronous IN ========================
bLength : 0x9 (7 bytes)
bDescriptorType : 0x5 Endpoint
bEndpointAddress : 0x81 IN
bmAttributes : 0x5 Isochronous
wMaxPacketSize : 0xc8 (200 bytes)
bInterval : 0x1
```
If I (almost) match that:
(FunctionFS Device Descriptors)
```
INTERFACE 0: Vendor Specific ===========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x0
bAlternateSetting : 0x0
bNumEndpoints : 0x0
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x0
bInterfaceProtocol : 0x0
iInterface : 0x5 Source/Sink
INTERFACE 1: Vendor Specific ===========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x1
bAlternateSetting : 0x0
bNumEndpoints : 0x0
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x0
bInterfaceProtocol : 0x0
iInterface : 0x6 Isoc Source/Sink
INTERFACE 1, 1: Vendor Specific ========================
bLength : 0x9 (9 bytes)
bDescriptorType : 0x4 Interface
bInterfaceNumber : 0x1
bAlternateSetting : 0x1
bNumEndpoints : 0x2
bInterfaceClass : 0xff Vendor Specific
bInterfaceSubClass : 0x0
bInterfaceProtocol : 0x0
iInterface : 0x6 Isoc Source/Sink
ENDPOINT 0x81: Isochronous IN ========================
bLength : 0x9 (7 bytes)
bDescriptorType : 0x5 Endpoint
bEndpointAddress : 0x81 IN
bmAttributes : 0x5 Isochronous
wMaxPacketSize : 0x200 (512 bytes)
bInterval : 0x1
ENDPOINT 0x1: Isochronous OUT ========================
bLength : 0x9 (7 bytes)
bDescriptorType : 0x5 Endpoint
bEndpointAddress : 0x1 OUT
bmAttributes : 0x5 Isochronous
wMaxPacketSize : 0x200 (512 bytes)
bInterval : 0x1
```
libusb seems to encounter an error:
(pyusb error output)
```
>>> import usb
>>> ds = [d for d in usb.core.find(find_all=True, idVendor=0x1d6b, idProduct=0x0104)]
>>> d = ds[0]
>>> d.set_interface_altsetting(interface=1, alternate_setting=1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.7/site-packages/usb/core.py", line 902, in set_interface_altsetting
self._ctx.managed_set_interface(self, interface, alternate_setting)
File "/usr/lib/python3.7/site-packages/usb/core.py", line 102, in wrapper
return f(self, *args, **kwargs)
File "/usr/lib/python3.7/site-packages/usb/core.py", line 204, in managed_set_interface
self.backend.set_interface_altsetting(self.handle, i.bInterfaceNumber, alt)
File "/usr/lib/python3.7/site-packages/usb/backend/libusb1.py", line 807, in set_interface_altsetting
altsetting))
File "/usr/lib/python3.7/site-packages/usb/backend/libusb1.py", line 595, in _check
raise USBError(_strerror(ret), ret, _libusb_errno[ret])
usb.core.USBError: [Errno None] Other error
```
(libusb error from C code)
```
libusb: error [op_set_interface] setintf failed error -1 errno 32
```
But, if interface 1 alternate setting 0 is dropped, and interface 1 alternate
setting 1 is kept, both invocations work and my C code spits out data very
fast, although I must inspect it further as I seem to be duplicating data in my
reads.
