227 lines
9.3 KiB
Plaintext
227 lines
9.3 KiB
Plaintext
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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). If the "timeout" argument is zero, the thread should be
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blocked until the semaphore is signalled.
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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(int size)
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Creates an empty mailbox for maximum "size" elements. Elements stored
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in mailboxes are pointers. You have to define macros "_MBOX_SIZE"
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in your lwipopts.h, or ignore this parameter in your implementation
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and use a default size.
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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. This function have to block until
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the "msg" is really posted.
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- err_t sys_mbox_trypost(sys_mbox_t mbox, void *msg)
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Try to post the "msg" to the mailbox. Returns ERR_MEM if this one
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is full, else, ERR_OK if the "msg" is posted.
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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). If "timeout" is 0, the thread should
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be blocked until a message arrives. 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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- u32_t sys_arch_mbox_tryfetch(sys_mbox_t mbox, void **msg)
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This is similar to sys_arch_mbox_fetch, however if a message is not
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present in the mailbox, it immediately returns with the code
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SYS_MBOX_EMPTY. On success 0 is returned.
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To allow for efficient implementations, this can be defined as a
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function-like macro in sys_arch.h instead of a normal function. For
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example, a naive implementation could be:
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#define sys_arch_mbox_tryfetch(mbox,msg) \
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sys_arch_mbox_fetch(mbox,msg,1)
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although this would introduce unnecessary delays.
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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 thread 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(char *name, void (* thread)(void *arg), void *arg, int stacksize, int prio)
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Starts a new thread named "name" with priority "prio" that will begin its
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execution in the function "thread()". The "arg" argument will be passed as an
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argument to the thread() function. The stack size to used for this thread is
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the "stacksize" parameter. The id of the new thread is returned. Both the id
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and 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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Note:
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Be carefull with using mem_malloc() in sys_arch. When malloc() refers to
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mem_malloc() you can run into a circular function call problem. In mem.c
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mem_init() tries to allcate a semaphore using mem_malloc, which of course
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can't be performed when sys_arch uses mem_malloc.
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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 <errno.h> 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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