4. Miscellaneous Support Files

Warning

This chapter contains outdated and confusing information.

4.1. GCC Compiler Specifications File

The file bsp_specs defines the start files and libraries that are always used with this BSP. The format of this file is admittedly cryptic and this document will make no attempt to explain it completely. Below is the bsp_specs file from the PowerPC psim BSP:

%rename endfile old_endfile
%rename startfile old_startfile
%rename link old_link
*startfile:
%{!qrtems: %(old_startfile)} \
%{!nostdlib: %{qrtems: ecrti%O%s rtems_crti%O%s crtbegin.o%s start.o%s}}
*link:
%{!qrtems: %(old_link)} %{qrtems: -Qy -dp -Bstatic -e _start -u __vectors}
*endfile:
%{!qrtems: %(old_endfile)} %{qrtems: crtend.o%s ecrtn.o%s}

The first section of this file renames the built-in definition of some specification variables so they can be augmented without embedded their original definition. The subsequent sections specify what behavior is expected when the -qrtems option is specified.

The *startfile section specifies that the BSP specific file start.o will be used instead of crt0.o. In addition, various EABI support files (ecrti.o etc.) will be linked in with the executable.

The *link section adds some arguments to the linker when it is invoked by GCC to link an application for this BSP.

The format of this file is specific to the GNU Compiler Suite. The argument used to override and extend the compiler built-in specifications is available in all recent GCC versions. The -specs option is present in all egcs distributions and gcc distributions starting with version 2.8.0.

4.2. README Files

Most BSPs provide one or more README files. Generally, there is a README file at the top of the BSP source. This file describes the board and its hardware configuration, provides vendor information, local configuration information, information on downloading code to the board, debugging, etc.. The intent of this file is to help someone begin to use the BSP faster.

A README file in a BSP subdirectory typically explains something about the contents of that subdirectory in greater detail. For example, it may list the documentation available for a particular peripheral controller and how to obtain that documentation. It may also explain some particularly cryptic part of the software in that directory or provide rationale on the implementation.

4.3. Times

This file contains the results of the RTEMS Timing Test Suite. It is in a standard format so that results from one BSP can be easily compared with those of another target board.

If a BSP supports multiple variants, then there may be multiple times files. Usually these are named times.VARIANTn.

4.4. Tools Subdirectory

Some BSPs provide additional tools that aid in using the target board. These tools run on the development host and are built as part of building the BSP. Most common is a script to automate running the RTEMS Test Suites on the BSP. Examples of this include:

  • powerpc/psim includes scripts to ease use of the simulator
  • m68k/mvme162 includes a utility to download across the VMEbus into target memory if the host is a VMEbus board in the same chasis.

4.5. bsp.h Include File

The file include/bsp.h contains prototypes and definitions specific to this board. Every BSP is required to provide a bsp.h. The best approach to writing a bsp.h is copying an existing one as a starting point.

Many bsp.h files provide prototypes of variables defined in the linker script (linkcmds).

4.6. tm27.h Include File

The tm27 test from the RTEMS Timing Test Suite is designed to measure the length of time required to vector to and return from an interrupt handler. This test requires some help from the BSP to know how to cause and manipulate the interrupt source used for this measurement. The following is a list of these:

  • MUST_WAIT_FOR_INTERRUPT - modifies behavior of tm27.
  • Install_tm27_vector - installs the interrupt service routine for the Interrupt Benchmark Test (tm27).
  • Cause_tm27_intr - generates the interrupt source used in the Interrupt Benchmark Test (tm27).
  • Clear_tm27_intr - clears the interrupt source used in the Interrupt Benchmark Test (tm27).
  • Lower_tm27_intr - lowers the interrupt mask so the interrupt source used in the Interrupt Benchmark Test (tm27) can generate a nested interrupt.

All members of the Timing Test Suite are designed to run WITHOUT the Clock Device Driver installed. This increases the predictability of the tests’ execution as well as avoids occassionally including the overhead of a clock tick interrupt in the time reported. Because of this it is sometimes possible to use the clock tick interrupt source as the source of this test interrupt. On other architectures, it is possible to directly force an interrupt to occur.

4.7. sbrk() Implementation

Although nearly all BSPs give all possible memory to the C Program Heap at initialization, it is possible for a BSP to configure the initial size of the heap small and let it grow on demand. If the BSP wants to dynamically extend the heap used by the C Library memory allocation routines (i.e. malloc family), then the``sbrk`` routine must be functional. The following is the prototype for this routine:

void * sbrk(ptrdiff_t increment)

The increment amount is based upon the sbrk_amount parameter passed to the bsp_libc_init during system initialization.

If your BSP does not want to support dynamic heap extension, then you do not have to do anything special. However, if you want to support sbrk, you must provide an implementation of this method and define CONFIGURE_MALLOC_BSP_SUPPORTS_SBRK in bsp.h. This informs rtems/confdefs.h to configure the Malloc Family Extensions which support sbrk.

4.8. bsp_fatal_extension() - Cleanup the Hardware

The bsp_fatal_extension() is an optional BSP specific initial extension invoked once a fatal system state is reached. Most of the BSPs use the same shared version of bsp_fatal_extension() that does nothing or performs a system reset. This implementation is located in the bsps/shared/start/bspfatal-default.c file.

The bsp_fatal_extension() routine can be used to return to a ROM monitor, insure that interrupt sources are disabled, etc.. This routine is the last place to ensure a clean shutdown of the hardware. The fatal source, internal error indicator, and the fatal code arguments are available to evaluate the fatal condition. All of the non-fatal shutdown sequences ultimately pass their exit status to rtems_shutdown_executive and this is what is passed to this routine in case the fatal source is RTEMS_FATAL_SOURCE_EXIT.

On some BSPs, it prints a message indicating that the application completed execution and waits for the user to press a key before resetting the board. The PowerPC/gen83xx and PowerPC/gen5200 BSPs do this when they are built to support the FreeScale evaluation boards. This is convenient when using the boards in a development environment and may be disabled for production use.

4.9. Configuration Macros

Each BSP can define macros in bsp.h which alter some of the the default configuration parameters in rtems/confdefs.h. This section describes those macros:

  • CONFIGURE_MALLOC_BSP_SUPPORTS_SBRK must be defined if the BSP has proper support for sbrk. This is discussed in more detail in the previous section.
  • BSP_IDLE_TASK_BODY may be defined to the entry point of a BSP specific IDLE thread implementation. This may be overridden if the application provides its own IDLE task implementation.
  • BSP_IDLE_TASK_STACK_SIZE may be defined to the desired default stack size for the IDLE task as recommended when using this BSP.
  • BSP_INTERRUPT_STACK_SIZE may be defined to the desired default interrupt stack size as recommended when using this BSP. This is sometimes required when the BSP developer has knowledge of stack intensive interrupt handlers.
  • BSP_ZERO_WORKSPACE_AUTOMATICALLY is defined when the BSP requires that RTEMS zero out the RTEMS C Program Heap at initialization. If the memory is already zeroed out by a test sequence or boot ROM, then the boot time can be reduced by not zeroing memory twice.
  • BSP_DEFAULT_UNIFIED_WORK_AREAS is defined when the BSP recommends that the unified work areas configuration should always be used. This is desirable when the BSP is known to always have very little RAM and thus saving memory by any means is desirable.

4.10. set_vector() - Install an Interrupt Vector

On targets with Simple Vectored Interrupts, the BSP must provide an implementation of the set_vector routine. This routine is responsible for installing an interrupt vector. It invokes the support routines necessary to install an interrupt handler as either a “raw” or an RTEMS interrupt handler. Raw handlers bypass the RTEMS interrupt structure and are responsible for saving and restoring all their own registers. Raw handlers are useful for handling traps, debug vectors, etc.

The set_vector routine is a central place to perform interrupt controller manipulation and encapsulate that information. It is usually implemented as follows:

rtems_isr_entry set_vector(                 /* returns old vector */
  rtems_isr_entry handler,                  /* isr routine        */
  rtems_vector_number vector,               /* vector number      */
  int                 type                  /* RTEMS or RAW intr  */
)
{
  if the type is RAW
    install the raw vector
  else
    use rtems_interrupt_catch to install the vector
  perform any interrupt controller necessary to unmask the interrupt source
  return the previous handler
}

Note

The i386, PowerPC and ARM ports use a Programmable Interrupt Controller model which does not require the BSP to implement set_vector. BSPs for these architectures must provide a different set of support routines.

4.11. Interrupt Delay Profiling

The RTEMS profiling needs support by the BSP for the interrupt delay times. In case profiling is enabled via the RTEMS build configuration option --enable-profiling (in this case the pre-processor symbol RTEMS_PROFILING is defined) a BSP may provide data for the interrupt delay times. The BSP can feed interrupt delay times with the _Profiling_Update_max_interrupt_delay() function (#include <rtems/score/profiling.h>). For an example please have a look at bsps/sparc/leon3/clock/ckinit.c.

4.12. Programmable Interrupt Controller API

A BSP can use the PIC API to install Interrupt Service Routines through a set of generic methods. In order to do so, the header files <bsp/irq-generic.h> and <bsp/irq-info.h> must be included by the bsp specific irq.h file present in the include/ directory. The irq.h acts as a BSP interrupt support configuration file which is used to define some important MACROS. It contains the declarations for any required global functions like bsp_interrupt_dispatch(). Thus later on, every call to the PIC interface requires including <bsp/irq.h>

The generic interrupt handler table is intitalized by invoking the bsp_interrupt_initialize() method from bsp_start() in the bspstart.c file which sets up this table to store the ISR addresses, whose size is based on the definition of macros, BSP_INTERRUPT_VECTOR_MIN and BSP_INTERRUPT_VECTOR_MAX in include/bsp.h

For the generic handler table to properly function, some bsp specific code is required, that should be present in irq/irq.c. The bsp-specific functions required to be writen by the BSP developer are :

  • bsp_interrupt_facility_initialize() contains bsp specific interrupt initialization code(Clear Pending interrupts by modifying registers, etc.). This method is called from bsp_interrupt_initialize() internally while setting up the table.
  • bsp_interrupt_handler_default() acts as a fallback handler when no ISR address has been provided corresponding to a vector in the table.
  • bsp_interrupt_dispatch() service the ISR by handling any bsp specific code & calling the generic method bsp_interrupt_handler_dispatch() which in turn services the interrupt by running the ISR after looking it up in the table. It acts as an entry to the interrupt switchboard, since the bsp branches to this function at the time of occurrence of an interrupt.
  • bsp_interrupt_vector_enable() enables interrupts and is called in irq-generic.c while setting up the table.
  • bsp_interrupt_vector_disable() disables interrupts and is called in irq-generic.c while setting up the table & during other important parts.

An interrupt handler is installed or removed with the help of the following functions :

rtems_status_code rtems_interrupt_handler_install(   /* returns status code */
  rtems_vector_number     vector,                    /* interrupt vector */
  const char             *info,                      /* custom identification text */
  rtems_option            options,                   /* Type of Interrupt */
  rtems_interrupt_handler handler,                   /* interrupt handler */
  void                   *arg                        /* parameter to be passed
                                                        to handler at the time of
                                                        invocation */
)
rtems_status_code rtems_interrupt_handler_remove(   /* returns status code */
  rtems_vector_number     vector,                   /* interrupt vector */
  rtems_interrupt_handler handler,                  /* interrupt handler */
  void                   *arg                       /* parameter to be passed to handler */
)