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.