typedef struct
{
    int a;
    int b;
    float c;
    /// blah blah

} SomeParams;

__constant SomeParams Parameters;

__kernel void Foo()
{
    int a = Parameters.a;
    /// do something useful...
}

__constant float pi = 3.14f;

__kernel void Foo(__constant SomeParams Parameters)
{
    int a = Parameters.a;
    /// do something useful...
}