The following example shows how a sequence of Read transactions can result in deadlock. A similar situation also applies to a sequence of Write transactions when a slave device can reorder its Write response. This example shows a case where there are two master devices (M0 and M1) and two slave devices (S0 and S1) connected using the AXI Crossbar core.
1.Master device M0 reads from Slave device S0.
2.Master device M0 then reads from Slave device S1 (using the same ID thread).
3.Master device M1 then reads from Slave device S1.
4.Master device M1 then reads from Slave device S0 (using the same ID thread).
5.Slave device S0 responds to Master device M1 first. It re-orders the Read response, which is allowable because the received transaction IDs are different. However, the AXI Crossbar core cannot pass the response to Master device M1 because Master device M1 must first receive its response from Slave device S1.
6.Slave device S1 responds to Master device M0 (it does not re-order). But the AXI Crossbar core cannot pass the response to Master device M0 because Master device M0 must first receive its response from Slave device S0.
This results in deadlock.