Transmit Channelized Segmented AXI4-Stream - 2.3 English

Channelization represents a further superset of control to a segmented AXI4-Stream bus. It allows TDM subdivision of the bus bandwidth between unrelated channels. Channels in this case are identified on individual AXI4-Stream transactions by the tid field; because DCMAC Subsystem supports up to 40 channels, the id fields are all six bits.

The tid field in this case takes on a significance that goes beyond the data. It is a request that some form of processing be performed (or at least evaluated) for the indicated channel. As a result, the tid field becomes active AXI information, even when no data moves on the bus (that is to say, when all segments have ena==0).

The DCMAC Subsystem channelized segmented AXI4-Stream interface is defined to include an advance indication of channel requested (using the id_req_vld and id_req output ports). It employs destination-sequenced operation wherein the DCMAC Subsystem indicates the ID that must be returned by the data source on the tid field after a fixed response interval. The specific interval can vary by application but must be static after the first tvalid/tid response is returned. The signals added to the segmented AXI4-Stream interface for channelization are shown in the following table.

The channel status fields provide a means for the DCMAC Subsystem to indicate its internal state and to request flow control on a per-channel basis.

The channelized AXI4-Stream bus has its overall rate controlled by the flow of id_req_vld, which inherently regulates the bus so that internal consumption is exactly matched. When id_req_vld is deasserted, the corresponding tvalid is deasserted. This ensures that the source rate is limited to the DCMAC Subsystem rate. Individual channels within the channelized AXI4-Stream scheme, however, require individualized control mechanisms to ensure they do not overrun the internal resources allocated to them.

The channel status signals then provide the means for the DCMAC Subsystem to request moderation of the ingress rate of each channel. This is accomplished by indicating the need for skip (no valid data) cycles. When a skip is requested, the DCMC is indicating that it has a sufficient accumulation of internal data to fully satisfy its per-channel processing for a cycle-worth of source data. These requests are pulse signals (not level, so a cycle of response is expected for each cycle of assertion) and the required reaction time of the source is determined by calendar interleaving and the internal resources allocated to the channel.

These signals are overlaid on top of the base AXI4-Stream signal set. The id_req_vld and id_req signals are temporally related to tvalid and tid, and precede them by a fixed number of cycles. The following list further clarifies the purpose and use of these signals.

tx_axis_id_req_vld

Indicates that tvalid must be asserted.

tx_axis_id_req[5:0]

Pre-indication of tid (channel ID) value that must accompany tvalid assertion.

tx_axis_ch_status_vld

Channel status valid; asserted to indicate valid information on the following ch_status_id and ch_status_skip_req signals.

tx_axis_ch_status_id

Channel ID.

tx_axis_ch_status_skip_req

Skip request. Asserted for a single cycle any time the DCMAC Subsystem accumulates a full bus-cycle worth of data in its internal context (meaning that the DCMAC Subsystem can completely meet its data needs for a single cycle without requiring any new data from the AXI data source). Used for per-channel rate management (for example, to offset bandwidth expansion).

tx_axis_tuser_skip_response

Skip response. Indicates an idle cycle (must be accompanied by all segments tuser_ena == 0) that is explicitly in response to a skip_req. These cycles allow the DCMAC Subsystem to replace inactive segments in the source stream with accumulated bytes from its local context.

Assertion of a skip request is a situation in which the DCMAC Subsystem needs to operate from its per-channel storage before it overflows. For that to happen, the transmit AXI4-Stream driver has to indicate that it is that channel’s turn in the overall sequence, and correspondingly provide no data while explicitly asserting the skip response indicator. This means that tvalid must be asserted alongside the corresponding tid and tuser_skip_response (while tuser_ena0 in all port segments should be deasserted). When provided with a skip response cycle, the DCMAC Subsystem correspondingly replaces the provided idle segments with its own accumulated data.