sys_arch interface for lwIP 0.6++

Author: Adam Dunkels

The operating system emulation layer provides a common interface
between the lwIP code and the underlying operating system kernel. The
general idea is that porting lwIP to new architectures requires only
small changes to a few header files and a new sys_arch
implementation. It is also possible to do a sys_arch implementation
that does not rely on any underlying operating system.

The sys_arch provides semaphores and mailboxes to lwIP. For the full
lwIP functionality, multiple threads support can be implemented in the
sys_arch, but this is not required for the basic lwIP
functionality. Previous versions of lwIP required the sys_arch to
implement timer scheduling as well but as of lwIP 0.5 this is
implemented in a higher layer.

In addition to the source file providing the functionality of sys_arch,
the OS emulation layer must provide several header files defining
macros used throughout lwip.  The files required and the macros they
must define are listed below the sys_arch description.

Semaphores can be either counting or binary - lwIP works with both
kinds. Mailboxes are used for message passing and can be implemented
either as a queue which allows multiple messages to be posted to a
mailbox, or as a rendez-vous point where only one message can be
posted at a time. lwIP works with both kinds, but the former type will
be more efficient. A message in a mailbox is just a pointer, nothing
more. 

Semaphores are represented by the type "sys_sem_t" which is typedef'd
in the sys_arch.h file. Mailboxes are equivalently represented by the
type "sys_mbox_t". lwIP does not place any restrictions on how
sys_sem_t or sys_mbox_t are represented internally.

The following functions must be implemented by the sys_arch:

- void sys_init(void)

  Is called to initialize the sys_arch layer.

- sys_sem_t sys_sem_new(u8_t count)

  Creates and returns a new semaphore. The "count" argument specifies
  the initial state of the semaphore.

- void sys_sem_free(sys_sem_t sem)

  Deallocates a semaphore.

- void sys_sem_signal(sys_sem_t sem)

  Signals a semaphore.

- u32_t sys_arch_sem_wait(sys_sem_t sem, u32_t timeout)

  Blocks the thread while waiting for the semaphore to be
  signaled. If the "timeout" argument is non-zero, the thread should
  only be blocked for the specified time (measured in
  milliseconds).

  If the timeout argument is non-zero, the return value is the number of
  milliseconds spent waiting for the semaphore to be signaled. If the
  semaphore wasn't signaled within the specified time, the return value is
  SYS_ARCH_TIMEOUT. If the thread didn't have to wait for the semaphore
  (i.e., it was already signaled), the function may return zero.

  Notice that lwIP implements a function with a similar name,
  sys_sem_wait(), that uses the sys_arch_sem_wait() function.

- sys_mbox_t sys_mbox_new(void)

  Creates an empty mailbox.

- void sys_mbox_free(sys_mbox_t mbox)

  Deallocates a mailbox. If there are messages still present in the
  mailbox when the mailbox is deallocated, it is an indication of a
  programming error in lwIP and the developer should be notified.

- void sys_mbox_post(sys_mbox_t mbox, void *msg)

  Posts the "msg" to the mailbox.

- u32_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u32_t timeout)

  Blocks the thread until a message arrives in the mailbox, but does
  not block the thread longer than "timeout" milliseconds (similar to
  the sys_arch_sem_wait() function). The "msg" argument is a result
  parameter that is set by the function (i.e., by doing "*msg =
  ptr"). The "msg" parameter maybe NULL to indicate that the message
  should be dropped.

  The return values are the same as for the sys_arch_sem_wait() function:
  Number of milliseconds spent waiting or SYS_ARCH_TIMEOUT if there was a
  timeout.

  Note that a function with a similar name, sys_mbox_fetch(), is
  implemented by lwIP. 
  
- struct sys_timeouts *sys_arch_timeouts(void)

  Returns a pointer to the per-thread sys_timeouts structure. In lwIP,
  each thread has a list of timeouts which is repressented as a linked
  list of sys_timeout structures. The sys_timeouts structure holds a
  pointer to a linked list of timeouts. This function is called by
  the lwIP timeout scheduler and must not return a NULL value. 

  In a single threadd sys_arch implementation, this function will
  simply return a pointer to a global sys_timeouts variable stored in
  the sys_arch module.
  
If threads are supported by the underlying operating system and if
such functionality is needed in lwIP, the following function will have
to be implemented as well:

- sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg, int prio)

  Starts a new thread with priority "prio" that will begin its execution in the
  function "thread()". The "arg" argument will be passed as an argument to the
  thread() function. The id of the new thread is returned. Both the id and
  the priority are system dependent.

- sys_prot_t sys_arch_protect(void)

  This optional function does a "fast" critical region protection and returns
  the previous protection level. This function is only called during very short
  critical regions. An embedded system which supports ISR-based drivers might
  want to implement this function by disabling interrupts. Task-based systems
  might want to implement this by using a mutex or disabling tasking. This
  function should support recursive calls from the same task or interrupt. In
  other words, sys_arch_protect() could be called while already protected. In
  that case the return value indicates that it is already protected.

  sys_arch_protect() is only required if your port is supporting an operating
  system.

- void sys_arch_unprotect(sys_prot_t pval)

  This optional function does a "fast" set of critical region protection to the
  value specified by pval. See the documentation for sys_arch_protect() for
  more information. This function is only required if your port is supporting
  an operating system.

-------------------------------------------------------------------------------
Additional files required for the "OS support" emulation layer:
-------------------------------------------------------------------------------

cc.h       - Architecture environment, some compiler specific, some
             environment specific (probably should move env stuff 
             to sys_arch.h.)

  Typedefs for the types used by lwip -
    u8_t, s8_t, u16_t, s16_t, u32_t, s32_t, mem_ptr_t

  Platform specific diagnostic output -
    LWIP_PLATFORM_DIAG(x)    - non-fatal, print a message.
    LWIP_PLATFORM_ASSERT(x)  - fatal, print message and abandon execution.

  "lightweight" synchronization mechanisms -
    SYS_ARCH_DECL_PROTECT(x) - declare a protection state variable.
    SYS_ARCH_PROTECT(x)      - enter protection mode.
    SYS_ARCH_UNPROTECT(x)    - leave protection mode.

  If the compiler does not provide memset() this file must include a
  definition of it, or include a file which defines it.

  This file must either include a system-local <errno.h> which defines
  the standard *nix error codes, or it should #define LWIP_PROVIDE_ERRNO
  to make lwip/arch.h define the codes which are used throughout.


perf.h     - Architecture specific performance measurement.
  Measurement calls made throughout lwip, these can be defined to nothing.
    PERF_START               - start measuring something.
    PERF_STOP(x)             - stop measuring something, and record the result.

sys_arch.h - Tied to sys_arch.c

  Arch dependent types for the following objects:
    sys_sem_t, sys_mbox_t, sys_thread_t,
  And, optionally:
    sys_prot_t

  Defines to set vars of sys_mbox_t and sys_sem_t to NULL.
    SYS_MBOX_NULL NULL
    SYS_SEM_NULL NULL