seL4 Level-1 Tutorials - Exercise #7: I/O Timer

Objective

Use I/O APIs to allocate and initialize a timer object. 

Background

Talking to I/O devices is very dependent on the target platform of the system largely because different platforms use different DMAs and I/O ports for their devices. Some I/O devices have standardized I/O ports which makes them easier to access than others. 

seL4 projects provide platform support libraries which define an interface to handle I/O devices in a generic way. The interface can be specialized to create device drivers when needed. The timer is an example of such device. 

The platsupport library provides a very important C structure:

struct ps_io_ops {
     ps_io_mapper_t                                               io_mapper;
     ps_io_port_ops_t                                            io_port_ops;
     ps_dma_man_t                                                dma_manager;
     ps_io_fdt_t                                                        io_fdt;
#ifdef CONFIG_ARCH_ARM
     clock_sys_t                                                       clock_sys;
     mux_sys_t                                                         mux_sys;
#endif
     ps_interface_registration_ops_t                interface_registration_ops;
     ps_malloc_ops_t                                            malloc_ops;
     ps_irq_ops_t                                                    irq_ops;
};

This structure provides a generic interface to any I/O device. In other words, it allows you to define drivers for different I/O devices all in the userland environment. For this exercise, we will use this structure to initialize an ltimer. The driver is already provided by the utilities library. 

Steps

    1.  Initialize build environment:

         The output should be: 

    2.  Create I/O memory allocation operations structure (TODO #1 in main.c). This corresponds to the malloc_ops member shown in the background section. We initialize it using the driver framework provided by seL4:

         Recompile and simulate. The output should be: 

    3.  Create a new I/O mapper (TODO #2 in main.c). The mapper will allow the driver to communicate with the necessary IO and ports. It can be initialized using: 

         Recompile and simulate. The output should be: 

    4.  Create new FDT operations (TODO #3 in main.c). This step is crucial and it can be customized if needed. The FDT operations structure is part of the seL4 driver framework and it uses a default cookie structure that is defined by the specific driver. For default usage, you do not customize it. Anyway, the initialization can be done using: 

         Recompile and simulate. The output should be: 

    5.  Initialize architecture specific operations (TODO #4 in main.c). This is the last step in the process to initialize the driver properly. Everything we’ve done until now did not target a specific architecture or platform. This step will look for the code corresponding to the current target build environment – if there is an available implementation, it will be used. If there is not, it will throw a build error. The call can be done using: 

         Recompile and simulate. The output should be: 

    6.  Destroy the timer (TODO #5 in main.c). After using the timer, make sure to destroy it when it is unnecessary:

         Recompile and simulate. The output should be: 

Learned APIs

sel4platsupport_new_malloc_ops
sel4platsupport_new_io_mapper
sel4platsupport_new_fdt_ops
sel4platsupport_new_arch_ops

⟵ Exercise-6

table of contents

Exercise-8 ⟶