System information

4 Graphic Card Implementation

4.3.3 CHARM Interface of the RPC

The CHARM sends or receives an RPC command with the aid of the RPC driver and

the RPC handler. The RPC driver is the RPC related interface between the host and the

CHARM. It handles the access to the RPC I/O ports from the host and provides a Linux

character device to the CHARM Linux system: /dev/charm/rpc/control. A program on

the CHARM can initiate an RPC command for the host by writing to the RPC device.

The following enumeration illustrates the order to transfer an RPC command to the host

computer:

1. Open a ﬁle handle to the RPC device.

2. Write the RPC message as one data packet to the RPC device.

3. Close the RPC device.

The RPC command is immediately added to the command queue. The RPC command

is delivered, if the host requests the CHARM for new RPC commands. There is no signal

like an interrupt to trigger the RPC request of the host. The host computer solely requests

for RPC commands while executing the expansion ROM of the CHARM. Section 4.3.4

illustrates the data ﬂow from the CHARM to the host and vice versa of an RPC message.

In contrast, the receiving and the execution of an RPC message sent by the host is

accomplished by the RPC Handler. The RPC Handler do a blocking read to the RPC

device. The RPC device driver wake ups the RPC handler, if a new RPC command from

the host arrives. The RPC Handler immediately executes the RPC command.

4.3.4 Data Flow of the RPC

Figure 4.18 depicts the data ﬂow of a host initiated RPC command. The host computer

sends the RPC message to the PCI I/O ports of the CHARM. The PCI Target Control

informs the BAR Switch driver about the I/O access. The BAR Switch driver reads out

the data and distributes them to the RPC driver. Section 4.2.3 illustrates the function of

the BAR switch driver precisely. The RPC driver wakes up the RPC Handler which reads

out and processes the RPC messages.

RPC

Driver

PCI

Target Control

BAR Switch

Driver

PCI

Core

CHARM

VGA BIOS

Host

CHARM

RPC

Handler

Figure 4.18: Data ﬂow of a host initiated RPC command. The dark boxes mark hardware

components and the white ones software units.