lib/drivers/lwip/doc/sys_arch.txt

   1 sys_arch interface for lwIP 0.6++
   2
   3 Author: Adam Dunkels
   4
   5 The operating system emulation layer provides a common interface
   6 between the lwIP code and the underlying operating system kernel. The
   7 general idea is that porting lwIP to new architectures requires only
   8 small changes to a few header files and a new sys_arch
   9 implementation. It is also possible to do a sys_arch implementation
  10 that does not rely on any underlying operating system.
  11
  12 The sys_arch provides semaphores and mailboxes to lwIP. For the full
  13 lwIP functionality, multiple threads support can be implemented in the
  14 sys_arch, but this is not required for the basic lwIP
  15 functionality. Previous versions of lwIP required the sys_arch to
  16 implement timer scheduling as well but as of lwIP 0.5 this is
  17 implemented in a higher layer.
  18
  19 In addition to the source file providing the functionality of sys_arch,
  20 the OS emulation layer must provide several header files defining
  21 macros used throughout lwip.  The files required and the macros they
  22 must define are listed below the sys_arch description.
  23
  24 Semaphores can be either counting or binary - lwIP works with both
  25 kinds. Mailboxes are used for message passing and can be implemented
  26 either as a queue which allows multiple messages to be posted to a
  27 mailbox, or as a rendez-vous point where only one message can be
  28 posted at a time. lwIP works with both kinds, but the former type will
  29 be more efficient. A message in a mailbox is just a pointer, nothing
  30 more.
  31
  32 Semaphores are represented by the type "sys_sem_t" which is typedef'd
  33 in the sys_arch.h file. Mailboxes are equivalently represented by the
  34 type "sys_mbox_t". lwIP does not place any restrictions on how
  35 sys_sem_t or sys_mbox_t are represented internally.
  36
  37 The following functions must be implemented by the sys_arch:
  38
  39 - void sys_init(void)
  40
  41   Is called to initialize the sys_arch layer.
  42
  43 - sys_sem_t sys_sem_new(u8_t count)
  44
  45   Creates and returns a new semaphore. The "count" argument specifies
  46   the initial state of the semaphore.
  47
  48 - void sys_sem_free(sys_sem_t sem)
  49
  50   Deallocates a semaphore.
  51
  52 - void sys_sem_signal(sys_sem_t sem)
  53
  54   Signals a semaphore.
  55
  56 - u32_t sys_arch_sem_wait(sys_sem_t sem, u32_t timeout)
  57
  58   Blocks the thread while waiting for the semaphore to be
  59   signaled. If the "timeout" argument is non-zero, the thread should
  60   only be blocked for the specified time (measured in
  61   milliseconds). If the "timeout" argument is zero, the thread should be
  62   blocked until the semaphore is signalled.
  63
  64   If the timeout argument is non-zero, the return value is the number of
  65   milliseconds spent waiting for the semaphore to be signaled. If the
  66   semaphore wasn't signaled within the specified time, the return value is
  67   SYS_ARCH_TIMEOUT. If the thread didn't have to wait for the semaphore
  68   (i.e., it was already signaled), the function may return zero.
  69
  70   Notice that lwIP implements a function with a similar name,
  71   sys_sem_wait(), that uses the sys_arch_sem_wait() function.
  72
  73 - sys_mbox_t sys_mbox_new(int size)
  74
  75   Creates an empty mailbox for maximum "size" elements. Elements stored
  76   in mailboxes are pointers. You have to define macros "_MBOX_SIZE"
  77   in your lwipopts.h, or ignore this parameter in your implementation
  78   and use a default size.
  79
  80 - void sys_mbox_free(sys_mbox_t mbox)
  81
  82   Deallocates a mailbox. If there are messages still present in the
  83   mailbox when the mailbox is deallocated, it is an indication of a
  84   programming error in lwIP and the developer should be notified.
  85
  86 - void sys_mbox_post(sys_mbox_t mbox, void *msg)
  87
  88   Posts the "msg" to the mailbox. This function have to block until
  89   the "msg" is really posted.
  90
  91 - err_t sys_mbox_trypost(sys_mbox_t mbox, void *msg)
  92
  93   Try to post the "msg" to the mailbox. Returns ERR_MEM if this one
  94   is full, else, ERR_OK if the "msg" is posted.
  95
  96 - u32_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u32_t timeout)
  97
  98   Blocks the thread until a message arrives in the mailbox, but does
  99   not block the thread longer than "timeout" milliseconds (similar to
 100   the sys_arch_sem_wait() function). If "timeout" is 0, the thread should
 101   be blocked until a message arrives. The "msg" argument is a result
 102   parameter that is set by the function (i.e., by doing "*msg =
 103   ptr"). The "msg" parameter maybe NULL to indicate that the message
 104   should be dropped.
 105
 106   The return values are the same as for the sys_arch_sem_wait() function:
 107   Number of milliseconds spent waiting or SYS_ARCH_TIMEOUT if there was a
 108   timeout.
 109
 110   Note that a function with a similar name, sys_mbox_fetch(), is
 111   implemented by lwIP.
 112
 113 - u32_t sys_arch_mbox_tryfetch(sys_mbox_t mbox, void **msg)
 114
 115   This is similar to sys_arch_mbox_fetch, however if a message is not
 116   present in the mailbox, it immediately returns with the code
 117   SYS_MBOX_EMPTY. On success 0 is returned.
 118
 119   To allow for efficient implementations, this can be defined as a
 120   function-like macro in sys_arch.h instead of a normal function. For
 121   example, a naive implementation could be:
 122     #define sys_arch_mbox_tryfetch(mbox,msg) \
 123       sys_arch_mbox_fetch(mbox,msg,1)
 124   although this would introduce unnecessary delays.
 125
 126 - struct sys_timeouts *sys_arch_timeouts(void)
 127
 128   Returns a pointer to the per-thread sys_timeouts structure. In lwIP,
 129   each thread has a list of timeouts which is repressented as a linked
 130   list of sys_timeout structures. The sys_timeouts structure holds a
 131   pointer to a linked list of timeouts. This function is called by
 132   the lwIP timeout scheduler and must not return a NULL value.
 133
 134   In a single thread sys_arch implementation, this function will
 135   simply return a pointer to a global sys_timeouts variable stored in
 136   the sys_arch module.
 137
 138 If threads are supported by the underlying operating system and if
 139 such functionality is needed in lwIP, the following function will have
 140 to be implemented as well:
 141
 142 - sys_thread_t sys_thread_new(char *name, void (* thread)(void *arg), void *arg, int stacksize, int prio)
 143
 144   Starts a new thread named "name" with priority "prio" that will begin its
 145   execution in the function "thread()". The "arg" argument will be passed as an
 146   argument to the thread() function. The stack size to used for this thread is
 147   the "stacksize" parameter. The id of the new thread is returned. Both the id
 148   and the priority are system dependent.
 149
 150 - sys_prot_t sys_arch_protect(void)
 151
 152   This optional function does a "fast" critical region protection and returns
 153   the previous protection level. This function is only called during very short
 154   critical regions. An embedded system which supports ISR-based drivers might
 155   want to implement this function by disabling interrupts. Task-based systems
 156   might want to implement this by using a mutex or disabling tasking. This
 157   function should support recursive calls from the same task or interrupt. In
 158   other words, sys_arch_protect() could be called while already protected. In
 159   that case the return value indicates that it is already protected.
 160
 161   sys_arch_protect() is only required if your port is supporting an operating
 162   system.
 163
 164 - void sys_arch_unprotect(sys_prot_t pval)
 165
 166   This optional function does a "fast" set of critical region protection to the
 167   value specified by pval. See the documentation for sys_arch_protect() for
 168   more information. This function is only required if your port is supporting
 169   an operating system.
 170
 171 Note:
 172
 173 Be carefull with using mem_malloc() in sys_arch. When malloc() refers to
 174 mem_malloc() you can run into a circular function call problem. In mem.c
 175 mem_init() tries to allcate a semaphore using mem_malloc, which of course
 176 can't be performed when sys_arch uses mem_malloc.
 177
 178 -------------------------------------------------------------------------------
 179 Additional files required for the "OS support" emulation layer:
 180 -------------------------------------------------------------------------------
 181
 182 cc.h       - Architecture environment, some compiler specific, some
 183              environment specific (probably should move env stuff
 184              to sys_arch.h.)
 185
 186   Typedefs for the types used by lwip -
 187     u8_t, s8_t, u16_t, s16_t, u32_t, s32_t, mem_ptr_t
 188
 189   Compiler hints for packing lwip's structures -
 190     PACK_STRUCT_FIELD(x)
 191     PACK_STRUCT_STRUCT
 192     PACK_STRUCT_BEGIN
 193     PACK_STRUCT_END
 194
 195   Platform specific diagnostic output -
 196     LWIP_PLATFORM_DIAG(x)    - non-fatal, print a message.
 197     LWIP_PLATFORM_ASSERT(x)  - fatal, print message and abandon execution.
 198     Portability defines for printf formatters:
 199     U16_F, S16_F, X16_F, U32_F, S32_F, X32_F, SZT_F
 200
 201   "lightweight" synchronization mechanisms -
 202     SYS_ARCH_DECL_PROTECT(x) - declare a protection state variable.
 203     SYS_ARCH_PROTECT(x)      - enter protection mode.
 204     SYS_ARCH_UNPROTECT(x)    - leave protection mode.
 205
 206   If the compiler does not provide memset() this file must include a
 207   definition of it, or include a file which defines it.
 208
 209   This file must either include a system-local <errno.h> which defines
 210   the standard *nix error codes, or it should #define LWIP_PROVIDE_ERRNO
 211   to make lwip/arch.h define the codes which are used throughout.
 212
 213
 214 perf.h     - Architecture specific performance measurement.
 215   Measurement calls made throughout lwip, these can be defined to nothing.
 216     PERF_START               - start measuring something.
 217     PERF_STOP(x)             - stop measuring something, and record the result.
 218
 219 sys_arch.h - Tied to sys_arch.c
 220
 221   Arch dependent types for the following objects:
 222     sys_sem_t, sys_mbox_t, sys_thread_t,
 223   And, optionally:
 224     sys_prot_t
 225
 226   Defines to set vars of sys_mbox_t and sys_sem_t to NULL.
 227     SYS_MBOX_NULL NULL
 228     SYS_SEM_NULL NULL