MSIBCM5718 Programmer’s Guide
Broadcom®
January 29, 2016 • 5718-PG108-R Page 236
MSI
PCI Specification 2.2 defines a new mechanism for a device to request services by its device driver. It is called
Message Signaled Interrupt (MSI). MSI will eventually deprecate the traditional interrupt mechanism. In MSI,
device DMAs a specified DWORD data to a specified host address if it needs to request services by its device
driver. The MSI state machine can be enabled/disabled by setting/resetting the Enable bit of MSI Mode register
(offset 0x6000). By default, this bit is set to 1 indicating that the MSI state machine is enabled. The main
advantages of MSI generation versus using a traditional interrupt are as follows:
• Eliminates the need for interrupt signal trace on the PCI device.
• Eliminates the need to perform a dummy read from the device by the device driver in its interrupt service
routine. The dummy read is done at the beginning of ISR to force all posted memory writes to be flushed to
the host memory.
Traditional Interrupt Scheme
A simplified block diagram showing traditional interrupt scheme is depicted in Figure 50.
Figure 50: Traditional Interrupt Scheme
To clarify second issue in traditional interrupt scheme, an example is given. The Ethernet controller receives one
or more packets from the networks. The Ethernet controller does the following:
BCM5718 Ethernet Controller
PCI Host
Bridge
Host Memory
CPU
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