Identify true error value for kernel API's like kmalloc() and ioremap()

Hi How to check for the error values for kernel API's like kmalloc() and ioremap(). If the call fails, the return value is NULL. So, how do I know what caused the failure? I have looked at examples in the kernel code. They are returning a hard-coded value of -ENOMEM if kmalloc() fails. Is there a way? Regards, Manty