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Communications Port Programming Interface (CPPI) 4.1 DMA
Data Buffer — A data buffer is a single data structure that contains payload information for
transmission to or reception from a port. A data buffer is a byte aligned contiguous block of memory
used to store packet payload data. A data buffer may hold any portion of a packet and may be linked
together (via descriptors) with other buffers to form packets. Data buffers may be allocated anywhere
within the 32-bit memory space. The Buffer Length field of the packet descriptor indicates the number
of valid data bytes in the buffer. There may be from 1 to 4M-1 valid data bytes in each buffer.
Host Buffer Descriptor (Buffer Descriptor) — A buffer descriptor is a single data structure that
contains information about one or more data buffers. This type of descriptor is required when more than
one descriptor is needed to define an entire packet, i.e., it either defines the middle of a packet or end
of a packet.
Start of Packet (SOP) — Packet along side Buffer Descriptors are used to hold information about
Packets. When multiple Descriptors are used to hold a single packet information or a single Descriptor
is used to hold a single packet information, the first descriptor is referred to as a Packet Descriptor
which is also Start-of-Packet Descriptor.
Host Packet Descriptor (Packet Descriptor) — A packet descriptor is another name for the first buffer
descriptor within a packet. Some fields within a data buffer descriptor are only valid when it is a packet
descriptor including the tags, packet length, packet type, and flags. This type of descriptor is always
used to define a packet since it provides packet level information that is useful to both the ports and the
Host in order to properly process the packet. It is the only descriptor used when single descriptor solely
defines a packet. When multiple descriptors are needed to define a packet, the packet descriptor is the
first descriptor used to define a packet.
Free Descriptor/Buffer Queue — A free descriptor/buffer queue is a hardware managed list of
available descriptors with pre-linked empty buffers that are to be used by the receive ports for host type
descriptors. Free Descriptor/Buffer Queues are implemented by the Queue Manager.
Teardown Descriptor — Teardown Descriptor is a special structure which is not used to describe
either a packet or a buffer but is instead used to describe the completion of a channel halt and
teardown event. Channel teardown is an important function because it ensures that when a connection
is no longer needed that the hardware can be reliably halted and any remaining packets which had not
yet been transmitted can be reclaimed by the Host without the possibility of losing buffer or descriptor
references (which results in a memory leak).
Packet Queue — A packet queue is hardware managed list of valid (i.e. populated) packet descriptors
that is used for forwarding a packet from one entity to another for any number of purposes.
NOTE: All descriptors (regardless of type) must be allocated at addresses that are naturally aligned to
the smallest power of 2 that is equal to or greater than the descriptor size.
20.4.2 Data Structures
Two data structures are mainly used to identify data buffers used by packet and buffer descriptors. A
third Descriptor, Teardown Descriptor, exists. The purpose of this Descriptor is a channel halt and
teardown event. Each of these Descriptor layouts as well as bit fields are shown below.
20.4.2.1 Host Packet Descriptor/ Packet Descriptor (SOP Descriptor)
Descriptors are designed to be used when USB like application requires support for true, unlimited
fragment count scatter/gather type operations. The Packet Descriptor is the first descriptor on multiple
descriptors setup or the only descriptor in a single descriptors setup. The Packet Descriptor contains
the following information:
•
Indicator which identifies the descriptor as a packet descriptor (always 10h
•
Source and destination tags (reserved)
•
Packet type
•
Packet length
•
Protocol specific region size
•
Protocol specific control/statusbits
•
Pointer to the first valid byte in the SOP data buffer
•
Length of the SOP data buffer
1796
Universal Serial Bus (USB)
Preliminary
© 2011, Texas Instruments Incorporated
www.ti.com
SPRUGX9 – 15 April 2011
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