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1 583 jeremybenn
sys_arch interface for lwIP 0.6++
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Author: Adam Dunkels
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The operating system emulation layer provides a common interface
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between the lwIP code and the underlying operating system kernel. The
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general idea is that porting lwIP to new architectures requires only
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small changes to a few header files and a new sys_arch
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implementation. It is also possible to do a sys_arch implementation
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that does not rely on any underlying operating system.
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The sys_arch provides semaphores and mailboxes to lwIP. For the full
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lwIP functionality, multiple threads support can be implemented in the
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sys_arch, but this is not required for the basic lwIP
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functionality. Previous versions of lwIP required the sys_arch to
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implement timer scheduling as well but as of lwIP 0.5 this is
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implemented in a higher layer.
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In addition to the source file providing the functionality of sys_arch,
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the OS emulation layer must provide several header files defining
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macros used throughout lwip.  The files required and the macros they
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must define are listed below the sys_arch description.
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Semaphores can be either counting or binary - lwIP works with both
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kinds. Mailboxes are used for message passing and can be implemented
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either as a queue which allows multiple messages to be posted to a
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mailbox, or as a rendez-vous point where only one message can be
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posted at a time. lwIP works with both kinds, but the former type will
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be more efficient. A message in a mailbox is just a pointer, nothing
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more.
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Semaphores are represented by the type "sys_sem_t" which is typedef'd
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in the sys_arch.h file. Mailboxes are equivalently represented by the
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type "sys_mbox_t". lwIP does not place any restrictions on how
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sys_sem_t or sys_mbox_t are represented internally.
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The following functions must be implemented by the sys_arch:
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- void sys_init(void)
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  Is called to initialize the sys_arch layer.
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- sys_sem_t sys_sem_new(u8_t count)
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  Creates and returns a new semaphore. The "count" argument specifies
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  the initial state of the semaphore.
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- void sys_sem_free(sys_sem_t sem)
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  Deallocates a semaphore.
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- void sys_sem_signal(sys_sem_t sem)
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  Signals a semaphore.
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- u32_t sys_arch_sem_wait(sys_sem_t sem, u32_t timeout)
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  Blocks the thread while waiting for the semaphore to be
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  signaled. If the "timeout" argument is non-zero, the thread should
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  only be blocked for the specified time (measured in
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  milliseconds).
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  If the timeout argument is non-zero, the return value is the number of
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  milliseconds spent waiting for the semaphore to be signaled. If the
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  semaphore wasn't signaled within the specified time, the return value is
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  SYS_ARCH_TIMEOUT. If the thread didn't have to wait for the semaphore
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  (i.e., it was already signaled), the function may return zero.
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  Notice that lwIP implements a function with a similar name,
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  sys_sem_wait(), that uses the sys_arch_sem_wait() function.
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- sys_mbox_t sys_mbox_new(void)
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  Creates an empty mailbox.
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- void sys_mbox_free(sys_mbox_t mbox)
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  Deallocates a mailbox. If there are messages still present in the
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  mailbox when the mailbox is deallocated, it is an indication of a
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  programming error in lwIP and the developer should be notified.
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- void sys_mbox_post(sys_mbox_t mbox, void *msg)
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  Posts the "msg" to the mailbox.
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- u32_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u32_t timeout)
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  Blocks the thread until a message arrives in the mailbox, but does
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  not block the thread longer than "timeout" milliseconds (similar to
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  the sys_arch_sem_wait() function). The "msg" argument is a result
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  parameter that is set by the function (i.e., by doing "*msg =
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  ptr"). The "msg" parameter maybe NULL to indicate that the message
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  should be dropped.
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  The return values are the same as for the sys_arch_sem_wait() function:
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  Number of milliseconds spent waiting or SYS_ARCH_TIMEOUT if there was a
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  timeout.
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  Note that a function with a similar name, sys_mbox_fetch(), is
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  implemented by lwIP.
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- struct sys_timeouts *sys_arch_timeouts(void)
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  Returns a pointer to the per-thread sys_timeouts structure. In lwIP,
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  each thread has a list of timeouts which is repressented as a linked
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  list of sys_timeout structures. The sys_timeouts structure holds a
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  pointer to a linked list of timeouts. This function is called by
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  the lwIP timeout scheduler and must not return a NULL value.
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  In a single threadd sys_arch implementation, this function will
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  simply return a pointer to a global sys_timeouts variable stored in
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  the sys_arch module.
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If threads are supported by the underlying operating system and if
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such functionality is needed in lwIP, the following function will have
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to be implemented as well:
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- sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg, int prio)
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  Starts a new thread with priority "prio" that will begin its execution in the
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  function "thread()". The "arg" argument will be passed as an argument to the
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  thread() function. The id of the new thread is returned. Both the id and
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  the priority are system dependent.
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- sys_prot_t sys_arch_protect(void)
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  This optional function does a "fast" critical region protection and returns
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  the previous protection level. This function is only called during very short
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  critical regions. An embedded system which supports ISR-based drivers might
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  want to implement this function by disabling interrupts. Task-based systems
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  might want to implement this by using a mutex or disabling tasking. This
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  function should support recursive calls from the same task or interrupt. In
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  other words, sys_arch_protect() could be called while already protected. In
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  that case the return value indicates that it is already protected.
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  sys_arch_protect() is only required if your port is supporting an operating
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  system.
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- void sys_arch_unprotect(sys_prot_t pval)
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  This optional function does a "fast" set of critical region protection to the
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  value specified by pval. See the documentation for sys_arch_protect() for
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  more information. This function is only required if your port is supporting
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  an operating system.
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-------------------------------------------------------------------------------
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Additional files required for the "OS support" emulation layer:
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-------------------------------------------------------------------------------
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cc.h       - Architecture environment, some compiler specific, some
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             environment specific (probably should move env stuff
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             to sys_arch.h.)
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  Typedefs for the types used by lwip -
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    u8_t, s8_t, u16_t, s16_t, u32_t, s32_t, mem_ptr_t
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  Compiler hints for packing lwip's structures -
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    PACK_STRUCT_FIELD(x)
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    PACK_STRUCT_STRUCT
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    PACK_STRUCT_BEGIN
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    PACK_STRUCT_END
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  Platform specific diagnostic output -
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    LWIP_PLATFORM_DIAG(x)    - non-fatal, print a message.
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    LWIP_PLATFORM_ASSERT(x)  - fatal, print message and abandon execution.
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  "lightweight" synchronization mechanisms -
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    SYS_ARCH_DECL_PROTECT(x) - declare a protection state variable.
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    SYS_ARCH_PROTECT(x)      - enter protection mode.
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    SYS_ARCH_UNPROTECT(x)    - leave protection mode.
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  If the compiler does not provide memset() this file must include a
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  definition of it, or include a file which defines it.
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  This file must either include a system-local  which defines
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  the standard *nix error codes, or it should #define LWIP_PROVIDE_ERRNO
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  to make lwip/arch.h define the codes which are used throughout.
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perf.h     - Architecture specific performance measurement.
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  Measurement calls made throughout lwip, these can be defined to nothing.
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    PERF_START               - start measuring something.
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    PERF_STOP(x)             - stop measuring something, and record the result.
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sys_arch.h - Tied to sys_arch.c
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  Arch dependent types for the following objects:
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    sys_sem_t, sys_mbox_t, sys_thread_t,
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  And, optionally:
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    sys_prot_t
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  Defines to set vars of sys_mbox_t and sys_sem_t to NULL.
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    SYS_MBOX_NULL NULL
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    SYS_SEM_NULL NULL

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