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NAME
mbpool — buffer pools for network interfaces
SYNOPSIS
#include <sys/types.h>
#include <machine/bus.h>
#include <sys/mbpool.h>
struct mbpool;
int
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);
void
mbp_destroy(struct mbpool *mbp);
void *
mbp_alloc(struct mbpool *mbp, bus_addr_t *pa, uint32_t *hp);
void
mbp_free(struct mbpool *mbp, void *p);
void
mbp_ext_free(void *, void *);
void
mbp_card_free(struct mbpool *mbp);
void
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);
void
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
DESCRIPTION
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 handles.
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 buffer.
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.
SEE ALSO
mbuf(9)
AUTHORS
Harti Brandt <harti@FreeBSD.org>
CAVEATS
The function mbp_sync() is currently a no-op because bus_dmamap_sync(9) is missing the offset and length
parameters.
Debian July 15, 2003 MBPOOL(9)