Infinite Flow Control Advertisement

PCI Express defines an infinite Flow Control credit value. A device that advertises infinite Flow Control credits need not send Flow Control Update packets following initialization and the transmitter will never be blocked from sending transactions. During flow control initialization, a device advertises "infinite" credits by delivering a zero in the credit field of the FC_INIT1 DLLP.

Who Advertises Infinite Flow Control Credits?

It's interesting to note that the minimum Flow Control credits that must be advertised includes infinite credits for completion transactions in certain situations. See Table 7-1 on page 303. These requirements involve devices that originate requests for which completions are expected to be returned (i.e., Endpoints and root ports that do not support peer-to-peer transfers). It does not include devices that merely forward completions (switches and root ports that support peer-to-peer transfers). This implies a requirement that any device initiating a request must commit buffer space for the expected completion header and data (if applicable). This guarantees that no throttling would ever occur when completions cross the final link to the original requester. This type of rule is required of PCI-X devices that initiate split transactions. Multiple searches of the specification failed to reveal this requirement explicitly stated for PCI Express devices; however, it is implied by the requirement to advertise infinite Flow Control credits.

Note also that infinite flow control credits can only be advertised during initializtion. This must be true, because the CA counter in the receiver could rollover to 00h and send an Update FC packet with the credit field set to 00h. If the Link is in the DL_Init state, this means infinite credits, but if the Link is in the DL_Active state, this does not mean infinite credits.

Special Use for Infinite Credit Advertisements.

The specification points out a special consideration for devices that do not need to implement all the FC buffer types for all VCs. For example, the only Non-Posted writes are I/O Writes and Configuration Writes both of which are permitted only on VC0. Thus, Non-Posted data buffers are not needed for VC1 - VC7. Because no Flow Control tracking is needed, a device can simply advertise infinite Flow Control credits during initialization, thereby eliminating the need to send needless FC_Update packets.

Header and Data Advertisements May Conflict

An infinite Flow Control advertisement might be sent for either the Data or header buffers (with same FC type) but not both. In this case, Update DLLPs are required for one buffer but not the other. This simply means that the device requiring credits will send an Update DLLP with the corresponding field containing the CREDITS_ALLOCATED credit information, and the other field must be set to zero (consistent with its advertisement).