System Information

Presence of PDD

Some drivers may only require an LDD, and not provide a PDD. However, if the driver has a PDD, then the driver must inform the framework of this. The LDD should set the DLogicalDevice::iParseMask bitmask with the KDeviceAllowPhysicalDevice flag. If this flag is not set, then the Kernel assumes that the driver does not have a PDD.

// Logical Channel Second stage constructor
DExDriverLogicalDevice:: DExDriverLogicalDevice ()
    {
    iParseMask = KDeviceAllowPhysicalDevice | KDeviceAllowUnit;
    ...
    }

Capabilities

The device driver framework provides a method to set and provide general information to the user on the capabilities of the device driver. Typically, implementations simply return the version number of the LDD. The device driver framework allows both the LDD and PDD to set the device capabilities.

The user typically calls the RDevice::GetCaps() function to retrieve the device capabilities.

RDevice device;
r = device.Open(KDriverName);
if (r==KErrNone)
    {
    TPckgBuf<RExDriver::TUartCaps> caps;
    device.GetCaps(caps);         
    ...
    device.Close();
    }

Note: The device capabilities in this context refer to the services that the driver can offer. Do not confuse this with the idea of platform security capabilities, which are completely different.

HAL handlers

Symbian platform provides a Hardware Abstraction Layer (HAL) interface that can be used by device drivers and kernel extensions to provide information on device specific attributes to user code. It comprises set and get interfaces used by the Kernel and by user code. The attributes are divided into groups of similar functionality, each of which is managed by a function called a HAL handler.

Drivers must register any HAL handlers they provide, and de-register them when unloading. Kernel extensions do not remove their HAL handlers.

// Adds a HAL entry handling function for the specified group of 
// HAL entries.
Kern::AddHalEntry(EHalGroupDisplay, &handler, this, 1);

// Removes the HAL handler
TInt Kern::RemoveHalEntry(EHalGroupDisplay);

The arguments to AddHalEntry() are the ID of a HAL group, a pointer to the handler function and a pointer to a data structure that will be passed to the handler function. The HAL handler function prototype is defined by THalFunc.

// Implementation of the kernel extension's exported function
EXPORT_C void TExClientInterface::MyExAPI()
    {
    …
    ExController->MyExAPI();
    …
    }

LOCAL_C TInt halFunction(TAny* aPtr, TInt aFunction, TAny* a1, 
        TAny* a2)
    {
    DLcdPowerHandler* pH=(DLcdPowerHandler*)aPtr;
    return pH->HalFunction(aFunction,a1,a2);
    }

TInt DLcdPowerHandler::HalFunction(TInt aFunction, TAny* a1, 
TAny* a2)
    {
    ...
    switch(aFunction)
        {
        ...
        }
    }

When the user calls HAL::Set() or HAL::Get(), it invokes the corresponding halFunction() of the HAL function group.