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     mbpool — buffer pools for network interfaces


     #include <sys/types.h>
     #include <machine/bus.h>
     #include <sys/mbpool.h>

     struct mbpool;

     mbp_create(struct mbpool **mbp, const char *name, bus_dma_tag_t dmat, u_int max_pages,
         size_t page_size, size_t chunk_size);

     mbp_destroy(struct mbpool *mbp);

     void *
     mbp_alloc(struct mbpool *mbp, bus_addr_t *pa, uint32_t *hp);

     mbp_free(struct mbpool *mbp, void *p);

     mbp_ext_free(void *, void *);

     mbp_card_free(struct mbpool *mbp);

     mbp_count(struct mbpool *mbp, u_int *used, u_int *card, u_int *free);

     void *
     mbp_get(struct mbpool *mbp, uint32_t h);

     void *
     mbp_get_keep(struct mbpool *mbp, uint32_t h);

     mbp_sync(struct mbpool *mbp, uint32_t h, bus_addr_t off, bus_size_t len, u_int op);

     MODULE_DEPEND(your_module, libmbpool, 1, 1, 1);

     options LIBMBPOOL


     Mbuf pools are intended to help drivers for interface cards that need huge amounts of
     receive buffers, and additionally provides a mapping between these buffers and 32-bit

     An example of these cards are the Fore/Marconi ForeRunnerHE cards.  These employ up to 8
     receive groups, each with two buffer pools, each of which can contain up to 8192.  This
     gives a total maximum number of more than 100000 buffers.  Even with a more moderate
     configuration the card eats several thousand buffers.  Each of these buffers must be mapped
     for DMA.  While for machines without an IOMMU and with lesser than 4GByte memory this is not
     a problem, for other machines this may quickly eat up all available IOMMU address space
     and/or bounce buffers.  On sparc64, the default I/O page size is 16k, so mapping a simple
     mbuf wastes 31/32 of the address space.

     Another problem with most of these cards is that they support putting a 32-bit handle into
     the buffer descriptor together with the physical address.  This handle is reflected back to
     the driver when the buffer is filled, and assists the driver in finding the buffer in host
     memory.  For 32-bit machines, the virtual address of the buffer is usually used as the
     handle.  This does not work for 64-bit machines for obvious reasons, so a mapping is needed
     between these handles and the buffers.  This mapping should be possible without searching
     lists and the like.

     An mbuf pool overcomes both problems by allocating DMA-able memory page wise with a per-pool
     configurable page size.  Each page is divided into a number of equally-sized chunks, the
     last MBPOOL_TRAILER_SIZE of which are used by the pool code (4 bytes).  The rest of each
     chunk is usable as a buffer.  There is a per-pool limit on pages that will be allocated.

     Additionally, the code manages two flags for each buffer: “on-card” and “used”.  A buffer
     may be in one of three states:

     free     None of the flags is set.

     on-card  Both flags are set.  The buffer is assumed to be handed over to the card and
              waiting to be filled.

     used     The buffer was returned by the card and is now travelling through the system.

     A pool is created with mbp_create().  This call specifies a DMA tag dmat to be used to
     create and map the memory pages via bus_dmamem_alloc(9).  The chunk_size includes the pool
     overhead.  It means that to get buffers for 5 ATM cells (240 bytes), a chunk size of 256
     should be specified.  This results in 12 unused bytes between the buffer, and the pool
     overhead of four byte.  The total maximum number of buffers in a pool is max_pages *
     (page_size / chunk_size).  The maximum value for max_pages is 2^14-1 (16383) and the maximum
     of page_size / chunk_size is 2^9 (512).  If the call is successful, a pointer to a newly
     allocated struct mbpool is set into the variable pointed to by mpb.

     A pool is destroyed with mbp_destroy().  This frees all pages and the pool structure itself.
     If compiled with DIAGNOSTICS, the code checks that all buffers are free.  If not, a warning
     message is issued to the console.

     A buffer is allocated with mbp_alloc().  This returns the virtual address of the buffer and
     stores the physical address into the variable pointed to by pa.  The handle is stored into
     the variable pointed to by hp.  The two most significant bits and the 7 least significant
     bits of the handle are unused by the pool code and may be used by the caller.  These are
     automatically stripped when passing a handle to one of the other functions.  If a buffer
     cannot be allocated (either because the maximum number of pages is reached, no memory is
     available or the memory cannot be mapped), NULL is returned.  If a buffer could be
     allocated, it is in the “on-card” state.

     When the buffer is returned by the card, the driver calls mbp_get() with the handle.  This
     function returns the virtual address of the buffer and clears the “on-card” bit.  The buffer
     is now in the “used” state.  The function mbp_get_keep() differs from mbp_get() in that it
     does not clear the “on-card” bit.  This can be used for buffers that are returned
     “partially” by the card.

     A buffer is freed by calling mbp_free() with the virtual address of the buffer.  This clears
     the “used” bit, and puts the buffer on the free list of the pool.  Note that free buffers
     are NOT returned to the system.  The function mbp_ext_free() can be given to m_extadd() as
     the free function.  The user argument must be the pointer to the pool.

     Before using the contents of a buffer returned by the card, the driver must call mbp_sync()
     with the appropriate parameters.  This results in a call to bus_dmamap_sync(9) for the

     All buffers in the pool that are currently in the “on-card” state can be freed with a call
     to mbp_card_free().  This may be called by the driver when it stops the interface.  Buffers
     in the “used” state are not freed by this call.

     For debugging it is possible to call mbp_count().  This returns the number of buffers in the
     “used” and “on-card” states and the number of buffers on the free list.




     Harti Brandt <>


     The function mbp_sync() is currently a no-op because bus_dmamap_sync(9) is missing the
     offset and length parameters.