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3.2 Host Buffer Descriptor

The host buffer descriptor is identical in size and organization to a host packet descriptor but does not include valid information in the packet level fields and does not include a populated region for protocol-specific information. Host buffer descriptors are designed to be linked onto a host packet descriptor or another host buffer descriptor to provide support for unlimited scatter / gather type operations. Host buffer descriptors provide information about a single corresponding data buffer. Every host buffer descriptor stores the following information:

Pointer to the first valid byte in the data buffer
Length of the data buffer
Pointer to the next buffer descriptor in the packet

Host buffer descriptors always contain 32 bytes of required information. Because it is a requirement that it is possible to convert a host descriptor between a buffer descriptor and a packet descriptor (by filling in the appropriate fields), in practice, host buffer descriptors will be allocated using the same sizes as host packet descriptors. The descriptor layout is shown in Table 3-15.

Table 3-15 Host Buffer Descriptor Layout

Reserved
(10 bytes)

Buffer reclamation info
(2 bytes)

Buffer info (8 bytes)

Linking info (4 bytes)

Original buffer info
(8 bytes)

Reserved
(10 bytes)

Buffer reclamation info
(2 bytes)

Buffer info (8 bytes)

Linking info (4 bytes)

Original buffer info
(8 bytes)

A host packet descriptor and zero or more host buffer descriptors may be linked together using the next descriptor pointer fields to form packets. The last descriptor in a packet has a 0 next descriptor pointer. Each host buffer descriptor also points to a single data buffer.

The contents of the host buffer descriptor words are detailed in Table 3-16 through Table 3-23. The host buffer descriptor is designed to be interchangeable with host packet descriptors, with common fields residing in the same locations.

Table 3-16 Host Buffer Descriptor Reserved Word 0 (BD Word 0)

Bits	Name	Description	RX Overwrite
31-0	Reserved	Reserved for host packet fields	No

Table 3-17 Host Buffer Descriptor Reserved Word 1 (BD Word 1)

Bits	Name	Description	RX Overwrite
31-0	Reserved	Reserved for host packet fields	No

Table 3-18 Host Buffer Descriptor Buffer Reclamation Info (BD Word 2)

Bits	Name	Description	RX Overwrite
31-15	Reserved	Reserved for host packet fields	No
14	Return Push Policy	This field indicates how a transmit DMA should return the descriptor pointers to the free queues. This field is encoded as follows: 0 = Descriptor must be returned to tail of queue 1 = Descriptor must be returned to head of queue This bit is used only when the Return Policy bit is set to 1.	No
13-12	Packet Return Queue Mgr #	This field indicates which of the four potential queue managers in the system the descriptor is to be returned to after transmission is complete. This field is not altered by the DMA during transmission or reception and should be initialized by the host.	No
11-0	Packet Return Queue #	This field indicates the queue number within the selected queue manager that the descriptor is to be returned to after transmission is complete.	No

Table 3-19 Host Buffer Descriptor Buffer N Info Word 0 (BD Word 3)

Bits	Name	Description	RX Overwrite
31-22	Reserved	Reserved for host packet fields	Yes
21-0	Buffer N Length	The buffer length field indicates how many valid data bytes are in the buffer. Unused or protocol-specific bytes at the beginning of the buffer are not counted in the buffer length field. This value will be overwritten during reception.	Yes

Table 3-20 Host Buffer Descriptor Buffer N Info Word 1 (BD Word 4)

Bits	Name	Description	RX Overwrite
31-0	Buffer N Pointer	The buffer pointer is the byte-aligned memory address of the buffer associated with the buffer descriptor. This value will not be overwritten during reception.	Yes

Table 3-21 Host Buffer Descriptor Linking Word (BD Word 5)

Bits	Name	Description	RX Overwrite
31-0	Next Descriptor Pointer	The 32-bit word aligned memory address of the next buffer descriptor in the packet. This is the mechanism used to reference the next buffer descriptor from the current buffer descriptor. If the value of this pointer is 0, then the current buffer is the last buffer in the packet. This value will be overwritten during reception.	Yes

Table 3-22 Host Buffer Descriptor Original Buffer Info Word 0 (BD Word 6)

Bits	Name	Description	RX Overwrite
31-28	Original Buffer 0 Pool Index	This field is used to identify which pool the attached buffer was originally allocated from. This is distinct from the descriptor pool/queue index because a single buffer may be referenced by more that one descriptor. This is a software-only field that is not touched by the hardware.	No
27-22	Original Buffer 0 Reference Count	This field is used to indicate how many references have been made to the attached buffer by different descriptors. Multiple buffer references are commonly used to implement broadcast and multicast packet forwarding when zero packet data copies are desired. This is a software-only field that is not touched by the hardware.	No
21-0	Original Buffer 0 Length	The buffer length field indicates the original size of the buffer in bytes. Data bytes are in the buffer. This value will not be overwritten during reception. This value is read by the RX DMA to determine the actual buffer size as allocated by the host at initialization. Because the buffer length in word 3 is overwritten by the RX port during reception, this field is necessary to permanently store the buffer size information.	No

Table 3-23 Host Buffer Descriptor Original Buffer Info Word 1 (BD Word 7)

Bits	Name	Description	RX Overwrite
31-0	Original Buffer 0 Pointer	The buffer pointer is the byte-aligned memory address of the buffer associated with the buffer descriptor. This value will not be overwritten during reception. This value is read by the RX DMA to determine the actual buffer location as allocated by the host at initialization. Because the buffer pointer in word 4 is overwritten by the RX port during reception, this field is necessary to permanently store the buffer pointer information.	No