I am working on a DriverKit system extension that handles some kind of USB Ethernet adapter.
I'm trying to receive data from a Bulk endpoint. Each USB transfer should be up to 512kB, but the actual size of the transfer is not known in advance.
What I do is the following:
- Allocate an IOBufferMemoryDescriptor with IOUSBHostInterface::CreateIOBuffer:
ret = ivars->interface->CreateIOBuffer(kIOMemoryDirectionIn, 524288, &b->buffer);
- Create an OSAction to be used as transfer complete callback:
ret = CreateActionReadComplete(0, &b->action);
- Start the I/O request:
ret = ivars->pipe->AsyncIO(b->buffer, 524288, b->action, 0);
At that point, I see that instead of starting a single 524288 bytes transfer, it starts 14 36864 bytes transfers and a 8192 bytes one. Which will absolutely not work with the USB device. Is there a transfer size limit I am not aware of?
