How to get data from the device using IOBufferMemoryDescriptor in driverKit - usb

Question

I'm trying to create a driver for my usb device, using iOS and DriverKit.
I'm basing my code in the example used in WWDC: https://github.com/knightsc/USBApp
My driver starts fine when the device is connected and the readCompleted method is called fine, but the action->GetReference() gets only \0 characteres.
Also in order to know that the usb device is actually working I've connected it to my mac first and using PyUSB I can see that it's returning data in chunks of 1024 bytes in the interface 0.
This is the data I get in PyUSB:
array('B', [6, 0, 1, 0, 2, 0, 3, 0, 4, 0, 5, 0, 6, 0, 7, 0, 8, 0, 9, 0, 10, 0, 11, 0, 12, 0, 13, 0, 14, 0, 15, 0, 16, 0, 17, 0, 18, 0, 19, 0, 20, 0, 21, 0, 22, 0, 23, 0, 24, 0, 25, 0, 26, 0, 27, 0, 28, 0, 29, 0, 30, 0, 31, 0, 32, 0, 33, 0, 34, 0, 35, 0, 36, 0, 37, 0, 38, 0, 39, 0, 40, 0, 41, 0, 42, 0, 43, 0, 44, 0, 45, 0, 46, 0, 47, 0, 48, 0, 49, 0, 50, 0, 51, 0, 52, 0, 53, 0, 54, 0, 55, 0, 56, 0, 57, 0, 58, 0, 59, 0, 60, 0, 61, 0, 62, 0, 63, 0, 64, 0, 65, 0, 66, 0, 67, 0, 68, 0, 69, 0, 70, 0, 71, 0, 72, 0, 73, 0, 74, 0, 75, 0, 76, 0, 77, 0, 78, 0, 79, 0, 80, 0, 81, 0, 82, 0, 83, 0, 84, 0, 85, 0, 86, 0, 87, 0, 88, 0, 89, 0, 90, 0, 91, 0, 92, 0, 93, 0, 94, 0, 95, 0, 96, 0, 97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102, 0, 103, 0, 104, 0, 105, 0, 106, 0, 107, 0, 108, 0, 109, 0, 110, 0, 111, 0, 112, 0, 113, 0, 114, 0, 115, 0, 116, 0, 117, 0, 118, 0, 119, 0, 120, 0, 121, 0, 122, 0, 123, 0, 124, 0, 125, 0, 126, 0, 127, 0, 128, 0, 129, 0, 130, 0, 131, 0, 132, 0, 133, 0, 134, 0, 135, 0, 136, 0, 137, 0, 138, 0, 139, 0, 140, 0, 141, 0, 142, 0, 143, 0, 144, 0, 145, 0, 146, 0, 147, 0, 148, 0, 149, 0, 150, 0, 151, 0, 152, 0, 153, 0, 154, 0, 155, 0, 156, 0, 157, 0, 158, 0, 159, 0, 160, 0, 161, 0, 162, 0, 163, 0, 164, 0, 165, 0, 166, 0, 167, 0, 168, 0, 169, 0, 170, 0, 171, 0, 172, 0, 173, 0, 174, 0, 175, 0, 176, 0, 177, 0, 178, 0, 179, 0, 180, 0, 181, 0, 182, 0, 183, 0, 184, 0, 185, 0, 186, 0, 187, 0, 188, 0, 189, 0, 190, 0, 191, 0, 192, 0, 193, 0, 194, 0, 195, 0, 196, 0, 197, 0, 198, 0, 199, 0, 200, 0, 201, 0, 202, 0, 203, 0, 204, 0, 205, 0, 206, 0, 207, 0, 208, 0, 209, 0, 210, 0, 211, 0, 212, 0, 213, 0, 214, 0, 215, 0, 216, 0, 217, 0, 218, 0, 219, 0, 220, 0, 221, 0, 222, 0, 223, 0, 224, 0, 225, 0, 226, 0, 227, 0, 228, 0, 229, 0, 230, 0, 231, 0, 232, 0, 233, 0, 234, 0, 235, 0, 236, 0, 237, 0, 238, 0, 239, 0, 240, 0, 241, 0, 242, 0, 243, 0, 244, 0, 245, 0, 246, 0, 247, 0, 248, 0, 249, 0, 250, 0, 251, 0, 252, 0, 253, 0, 254, 0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
This is the Ivars:
struct Mk1dDriver_IVars
{
IOUSBHostInterface *interface;
IOUSBHostPipe *inPipe;
OSAction *ioCompleteCallback;
IOBufferMemoryDescriptor *inData;
uint16_t maxPacketSize;
};
This is the Start method:
kern_return_t
IMPL(Mk1dDriver, Start)
{
kern_return_t ret;
IOUSBStandardEndpointDescriptors descriptors;
ret = Start(provider, SUPERDISPATCH);
__Require(kIOReturnSuccess == ret, Exit);
ret = RegisterService();
if (ret != kIOReturnSuccess)
{
Log("Start() - Failed to register service with error: 0x%08x.", ret);
goto Exit;
}
ivars->interface = OSDynamicCast(IOUSBHostInterface, provider);
__Require_Action(NULL != ivars->interface, Exit, ret = kIOReturnNoDevice);
ret = ivars->interface->Open(this, 0, NULL);
__Require(kIOReturnSuccess == ret, Exit);
ret = ivars->interface->CopyPipe(kMyEndpointAddress, &ivars->inPipe);
__Require(kIOReturnSuccess == ret, Exit);
ret = ivars->interface->CreateIOBuffer(kIOMemoryDirectionIn,
1024,
&ivars->inData);
__Require(kIOReturnSuccess == ret, Exit);
ret = OSAction::Create(this,
Mk1dDriver_ReadComplete_ID,
IOUSBHostPipe_CompleteAsyncIO_ID,
0,
&ivars->ioCompleteCallback);
__Require(kIOReturnSuccess == ret, Exit);
ret = ivars->inPipe->AsyncIO(ivars->inData,
ivars->maxPacketSize,
ivars->ioCompleteCallback,
0);
__Require(kIOReturnSuccess == ret, Exit);
os_log(OS_LOG_DEFAULT,"Finish");
// WWDC slides don't show the full function
// i.e. this is still unfinished
Exit:
return ret;
}
The only difference in this compared with the code from Apple is that I set capacity in the method CreateIOBuffer to 1024. This is because if I leave it to 0 it will return an error that memory could not be allocated: kIOReturnNoMemory
And the ReadComplete method:
void
IMPL(Mk1dDriver, ReadComplete)
{
char output[1024];
memcpy(action->GetReference(), &output, 1024);
os_log(OS_LOG_DEFAULT,"ReadComplete");
If I put a breakpoint in the log, I can see all the positions in output will be \0
Any idea what I'm doing wrong?
Thanks

Answer 1

You need to store some reference to the IOBufferMemoryDescriptor* that you asked AsyncIO to write to when the data from the device is received (ivars->inData) so that you can access it when the completion callback ReadComplete is called. You can store this in the memory that you can access with GetReference().
You should set the size of the custom memory that should be allocated for you. Currently you are allocating 0 bytes. See OSAction::Create.
In ReadComplete you can then call GetReference() to access the memory. Since you know that this memory contains a reference to the IOBufferMemoryDescriptor that data has been written to, you can then use it with memcpy.
Something like this:
ret = OSAction::Create(this,
Mk1dDriver_ReadComplete_ID,
IOUSBHostPipe_CompleteAsyncIO_ID,
sizeof(IOBufferMemoryDescriptor*),
&ivars->ioCompleteCallback);
memcpy(ivars->ioCompleteCallback->GetReference(),
ivars->inData, sizeof(IOBufferMemoryDescriptor*));
First parameter to memcpy is the destination.
void IMPL(Mk1dDriver, ReadComplete)
{
IOBufferMemoryDescriptor* ptr;
memcpy(ptr, action->GetReference(), sizeof(IOBufferMemoryDescriptor*));
IOAddressSegment addressSegement{};
ptr->GetAddressRange(&addressSegement);
char output[1024];
memcpy(output, addressSegement.address, addressSegement.length);
}