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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.