/share/man/man9/mbpool.9

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 26.\" Author: Hartmut Brandt <harti@FreeBSD.org>
 27.\"
 28.\" $FreeBSD$
 29.\"
 30.Dd July 15, 2003
 31.Dt MBPOOL 9
 32.Os
 33.Sh NAME
 34.Nm mbpool
 35.Nd "buffer pools for network interfaces"
 36.Sh SYNOPSIS
 37.In sys/types.h
 38.In machine/bus.h
 39.In sys/mbpool.h
 40.Vt struct mbpool ;
 41.Ft int
 42.Fo mbp_create
 43.Fa "struct mbpool **mbp" "const char *name" "bus_dma_tag_t dmat"
 44.Fa "u_int max_pages" "size_t page_size" "size_t chunk_size"
 45.Fc
 46.Ft void
 47.Fn mbp_destroy "struct mbpool *mbp"
 48.Ft "void *"
 49.Fn mbp_alloc "struct mbpool *mbp" "bus_addr_t *pa" "uint32_t *hp"
 50.Ft void
 51.Fn mbp_free "struct mbpool *mbp" "void *p"
 52.Ft void
 53.Fn mbp_ext_free "void *" "void *"
 54.Ft void
 55.Fn mbp_card_free "struct mbpool *mbp"
 56.Ft void
 57.Fn mbp_count "struct mbpool *mbp" "u_int *used" "u_int *card" "u_int *free"
 58.Ft "void *"
 59.Fn mbp_get "struct mbpool *mbp" "uint32_t h"
 60.Ft "void *"
 61.Fn mbp_get_keep "struct mbpool *mbp" "uint32_t h"
 62.Ft void
 63.Fo mbp_sync
 64.Fa "struct mbpool *mbp" "uint32_t h" "bus_addr_t off" "bus_size_t len"
 65.Fa "u_int op"
 66.Fc
 67.Pp
 68.Fn MODULE_DEPEND "your_module" "libmbpool" 1 1 1
 69.Pp
 70.Cd "options LIBMBPOOL"
 71.Sh DESCRIPTION
 72Mbuf pools are intended to help drivers for interface cards that need huge
 73amounts of receive buffers, and additionally provides a mapping between these
 74buffers and 32-bit handles.
 75.Pp
 76An example of these cards are the Fore/Marconi ForeRunnerHE cards.
 77These
 78employ up to 8 receive groups, each with two buffer pools, each of which
 79can contain up to 8192.
 80This gives a total maximum number of more than
 81100000 buffers.
 82Even with a more moderate configuration the card eats several
 83thousand buffers.
 84Each of these buffers must be mapped for DMA.
 85While for
 86machines without an IOMMU and with lesser than 4GByte memory this is not
 87a problem, for other machines this may quickly eat up all available IOMMU
 88address space and/or bounce buffers.
 89On sparc64, the default I/O page size
 90is 16k, so mapping a simple mbuf wastes 31/32 of the address space.
 91.Pp
 92Another problem with most of these cards is that they support putting a 32-bit
 93handle into the buffer descriptor together with the physical address.
 94This handle is reflected back to the driver when the buffer is filled, and
 95assists the driver in finding the buffer in host memory.
 96For 32-bit machines,
 97the virtual address of the buffer is usually used as the handle.
 98This does not
 99work for 64-bit machines for obvious reasons, so a mapping is needed between
100these handles and the buffers.
101This mapping should be possible without
102searching lists and the like.
103.Pp
104An mbuf pool overcomes both problems by allocating DMA-able memory page wise
105with a per-pool configurable page size.
106Each page is divided into a number of
107equally-sized chunks, the last
108.Dv MBPOOL_TRAILER_SIZE
109of which are used by the pool code (4 bytes).
110The rest of each chunk is
111usable as a buffer.
112There is a per-pool limit on pages that will be allocated.
113.Pp
114Additionally, the code manages two flags for each buffer:
115.Dq on-card
116and
117.Dq used .
118A buffer may be in one of three states:
119.Bl -tag -width "on-card"
120.It free
121None of the flags is set.
122.It on-card
123Both flags are set.
124The buffer is assumed to be handed over to the card and
125waiting to be filled.
126.It used
127The buffer was returned by the card and is now travelling through the system.
128.El
129.Pp
130A pool is created with
131.Fn mbp_create .
132This call specifies a DMA tag
133.Fa dmat
134to be used to create and map the memory pages via
135.Xr bus_dmamem_alloc 9 .
136The
137.Fa chunk_size
138includes the pool overhead.
139It means that to get buffers for 5 ATM cells
140(240 bytes), a chunk size of 256 should be specified.
141This results in 12 unused
142bytes between the buffer, and the pool overhead of four byte.
143The total
144maximum number of buffers in a pool is
145.Fa max_pages
146*
147.Fa ( page_size
148/
149.Fa chunk_size ) .
150The maximum value for
151.Fa max_pages
152is 2^14-1 (16383) and the maximum of
153.Fa page_size
154/
155.Fa chunk_size
156is 2^9 (512).
157If the call is successful, a pointer to a newly allocated
158.Vt "struct mbpool"
159is set into the variable pointed to by
160.Fa mpb .
161.Pp
162A pool is destroyed with
163.Fn mbp_destroy .
164This frees all pages and the pool structure itself.
165If compiled with
166.Dv DIAGNOSTICS ,
167the code checks that all buffers are free.
168If not, a warning message is issued
169to the console.
170.Pp
171A buffer is allocated with
172.Fn mbp_alloc .
173This returns the virtual address of the buffer and stores the physical
174address into the variable pointed to by
175.Fa pa .
176The handle is stored into the variable pointed to by
177.Fa hp .
178The two most significant bits and the 7 least significant bits of the handle
179are unused by the pool code and may be used by the caller.
180These are
181automatically stripped when passing a handle to one of the other functions.
182If a buffer cannot be allocated (either because the maximum number of pages
183is reached, no memory is available or the memory cannot be mapped),
184.Dv NULL
185is returned.
186If a buffer could be allocated, it is in the
187.Dq on-card
188state.
189.Pp
190When the buffer is returned by the card, the driver calls
191.Fn mbp_get
192with the handle.
193This function returns the virtual address of the buffer
194and clears the
195.Dq on-card
196bit.
197The buffer is now in the
198.Dq used
199state.
200The function
201.Fn mbp_get_keep
202differs from
203.Fn mbp_get
204in that it does not clear the
205.Dq on-card
206bit.
207This can be used for buffers
208that are returned
209.Dq partially
210by the card.
211.Pp
212A buffer is freed by calling
213.Fn mbp_free
214with the virtual address of the buffer.
215This clears the
216.Dq used
217bit, and
218puts the buffer on the free list of the pool.
219Note that free buffers
220are NOT returned to the system.
221The function
222.Fn mbp_ext_free
223can be given to
224.Fn m_extadd
225as the free function.
226The user argument must be the pointer to
227the pool.
228.Pp
229Before using the contents of a buffer returned by the card, the driver
230must call
231.Fn mbp_sync
232with the appropriate parameters.
233This results in a call to
234.Xr bus_dmamap_sync 9
235for the buffer.
236.Pp
237All buffers in the pool that are currently in the
238.Dq on-card
239state can be freed
240with a call to
241.Fn mbp_card_free .
242This may be called by the driver when it stops the interface.
243Buffers in the
244.Dq used
245state are not freed by this call.
246.Pp
247For debugging it is possible to call
248.Fn mbp_count .
249This returns the number of buffers in the
250.Dq used
251and
252.Dq on-card
253states and
254the number of buffers on the free list.
255.Sh SEE ALSO
256.Xr mbuf 9
257.Sh AUTHORS
258.An Harti Brandt Aq harti@FreeBSD.org
259.Sh CAVEATS
260The function
261.Fn mbp_sync
262is currently a no-op because
263.Xr bus_dmamap_sync 9
264is missing the offset and length parameters.