The PCI Express Lock Protocol
The only source of lock supported by PCI Express is the system processor, and, as a consequence, the source of all locked operations in PCI Express is the Root Complex (acting as the processor's surrogate). A locked operation is performed between a Root Complex downstream port and the PCI Express downstream port to which the targeted legacy device is attached. In most systems, the legacy device is typically a PCI Express-to-PCI or PCI Express-to-PCI-X bridge. Only one locked sequence at a time is supported for a given hierarchical path.
PCI Express limits locked transactions to Traffic Class 0 and Virtual Channel 0. All transactions with TC values other than zero that are mapped to a VC other than zero are permitted ...
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