The requester side of the integrated block maintains the state of all pending Non-Posted transactions (memory reads, I/O reads and writes, configuration reads and writes, Atomic Operations) initiated by the user application, so that the completions returned by the targets can be matched against the corresponding requests. The state of each outstanding transaction is held in a Split Completion Table in the requester side of the interface, which has a capacity of 64 Non-Posted transactions. The returning Completions are matched with the pending requests using a 6 -bit tag. There are two options for management of these tags.
• Internal Tag Management : This mode of operation is selected by setting the Enable Client Tag option in the Vivado IDE, which is the default setting for the core. In this mode, logic within the integrated block is responsible for allocating the tag for each Non-Posted request initiated from the requester side. The integrated block maintains a list of free tags and assigns one of them to each request when the user application initiates a Non-Posted transaction, and communicates the assigned tag value to the user application through the output pcie_rq_tag[5:0] . The value on this bus is valid when the integrated block asserts pcie_rq_tag_vld . The user logic must copy this tag so that any Completions delivered by the integrated block in response to the request can be matched to the request.
In this mode, logic within the integrated block checks for the Split Completion Table full condition, and back pressures a Non-Posted request from the user application (using s_axis_rq_tready ) if the total number of Non-Posted requests currently outstanding has reached its limit ( 64 ).
• External Tag Management: In this mode, the user logic is responsible for allocating the tag for each Non-Posted request initiated from the requester side. The user logic must choose the tag value without conflicting with the tags of all other Non-Posted transactions outstanding at that time, and must communicate this chosen tag value to the integrated block through the request descriptor. The integrated block still maintains the outstanding requests in its Split Completion Table and matches the incoming Completions to the requests, but does not perform any checks for the uniqueness of the tags, or for the Split Completion Table full condition.
When internal tag management is in use, the integrated block asserts pcie_rq_tag_vld for one cycle for each Non-Posted request, after it has placed its allocated tag on pcie_rq_tag[ 5 :0] . There can be a delay of several cycles between the transfer of the request on the RQ interface and the assertion of pcie_rq_tag_vld by the integrated block to provide the allocated tag for the request. The user application can, meanwhile, continue to send new requests. The tags for requests are communicated on the pcie_rq_tag bus in FIFO order, so it is easy to associate the tag value with the request it transferred. A tag is reused when the end-of-frame (EOF) of the last completion of a split completion is accepted by the user application.