Abstract:
A method for expanding the number of configuration registers available in a computer system. Unused configuration registers that correspond to non-existent functions are used for other purposes. Memory is configured to allow enumeration software to conclude that the non-existent functions do not in fact exist.

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 09/008,966, filed Jan. 20, 1998, entitled METHOD FOR EXTENDING THE AVAILABLE NUMBER OF CONFIGURATION REGISTERS, now U.S. Pat. No. 6,108,733. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to techniques for increasing the number of user-definable configuration registers available in a computer system. More particularly, the present invention relates to a procedure for making unused configuration memory space available for use as configuration registers. 
     BACKGROUND OF THE INVENTION 
     In computer systems employing the commonly-used Peripheral Component Interconnect (PCI) bus, the PCI architecture makes available configuration registers that can be used for implementing various logic and control functions. The configuration registers are typically programmed and accessed by the device that implements a particular function or feature. Often, the number of configuration registers available is sufficient for a given programming task. However, for implementations of some tasks, the number of configuration registers is too small, making PCI configuration registers a scarce resource. Increasing the number of PCI configuration registers is difficult, however, because the PCI standard requires that configuration registers be addressed in a specified manner, and conform to requirements of other software and hardware. Thus, there exists a need for an effective method for increasing the number of available configuration registers, while also maintaining compatibility with applicable bus standards or other standards. 
     SUMMARY OF THE INVENTION 
     The present invention relates to techniques for increasing the number of user-definable configuration registers, such as in a PCI-based computer system. In some embodiments, the present invention uses a non-existent device function to provide additional user definable configuration registers, while still maintaining compatibility with applicable standards or conventions. 
     In one embodiment of the invention, the invention relates to a computer comprising: (a) a processor; (b) a PCI bus attached to the processor through bridge logic; (c) configuration memory space that is accessible to the processor and that is allocated for devices attached to the PCI bus, wherein each PCI device has configuration space allocated for a plurality of functions; and (d) a PCI device attached to the PCI bus that has memory allocated in the configuration memory space for a first function for the PCI device and for a second function for the PCI device. The PCI device includes hardware for carrying out the first function, where the hardware for carrying out the first function accesses (i) configuration registers in the configuration memory space that corresponds to the first function and also (ii) configuration registers in the configuration memory space that corresponds to the second function. 
     In another embodiment of the present invention, the present invention relates to a method of configuring and using a device, where the device has configuration memory space allocated to it for a plurality of functions. At least one of the plurality of functions is implemented by the device, and at least one of the plurality of functions is not implemented by the device. The method comprises the acts of: (a) storing data in a header portion of the configuration space allocated to the function that is not implemented by the device, wherein the data indicates that the function is not implemented; (b) programming a register that is in the remaining portion of the configuration space allocated to the function that is not implemented by the device; and (c) accessing the programmed register. 
     Additional embodiments and features, and the nature of the present invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims, and to the several drawings herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a computer system including multiple PCI buses and a number of PCI devices on the PCI busses. 
     FIG. 2 is a memory map of the PCI memory space in a system such as that shown in FIG.  1 . 
     FIG. 3 is a memory map of a portion of the memory map of FIG. 2, wherein the memory space for a particular device is shown. 
     FIG. 4 is a memory map of a configuration space header and the associated configuration registers for a particular function in a given device. 
     FIG. 5 is a more detailed memory map of the configuration space header of FIG. 4, showing the configuration space header for a particular function in a given device. 
     FIG. 6 is a representation of the configuration address for a PCI bus, showing the hierarchical manner in which busses, devices, functions, and registers are addressed. 
     FIG. 7 is a flow chart illustrating a process for expanding the number of user-definable configuration registers. 
     FIG. 8 is a flow chart illustrating a process carried out by enumeration software. 
     FIG. 9 is a flow chart illustrating a process for accessing user-definable configuration registers in an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described herein in terms of a PCI-based computer system, but it should be understood that the techniques and procedures disclosed herein can be applicable to other systems and architectures. Not all embodiments of the present invention are limited to an implementation based on the Peripheral Component Interconnect architecture. 
     FIG. 1 is a block diagram of a hierarchical PCI-based computer system having a processor  10  attached to a host bus  12 . A host to PCI bridge  14  connects the host bus to a first PCI bus  20 , which has attached to it PCI devices  21 ,  22 . PCI to PCI bridge logic  24  and  26  connects the PCI bus  20  to two additional PCI busses  30  and  40 . A fourth PCI bus  50  is attached to the PCI bus  40  through bridge logic  44 . As shown in FIG. 1, a number of PCI busses can be implemented in a hierarchical or non-hierarchical manner within a computer system. 
     Each bus within the computer system is allocated an amount of memory within the PCI configuration space, a representation of which is shown in FIG.  2 . The memory map of FIG. 2 shows a typical PCI configuration space for a system such as that of FIG. 1, where each bus is allocated 64 kilobytes of PCI configuration space. Bus  0 , for example, is allocated the 64 kilobytes of configuration space indicated by the reference numeral  100 . Bus  1  is allocated the 64 kilobytes of configuration space indicated at  101 . In the embodiment of FIG. 2, within the configuration space allocated for each bus, a 2 kilobyte configuration space is allocated for each of 32 devices. For example, device  0  on bus  0  is allocated the 2 kilobyte configuration space  200  in FIG.  2 . 
     FIG. 3 is a representation of the device configuration space  200  from FIG.  2 . Within the 2 kilobyte device space  200 , configuration space is allocated for each of eight functions for the device that corresponds to the device space  200 . Some devices may have multiple functions, such as when a device has both video and audio capabilities. Often, however, devices have only a single function. But in the embodiment of FIG. 2, configuration space is allocated for eight functions for each device, even if one or more devices implement less than eight functions. Each of the eight possible functions for a given device is allocated a 256 byte configuration space. FIG. 3 shows the configuration space  400  for function  0  corresponding to the device space  200 . 
     In FIG. 4, the configuration space  400  for function  0  is illustrated. As shown this 256 byte configuration space is made up of a 64 byte header portion  401 , and a 192-byte portion  402  that includes that function&#39;s configuration registers. These configuration registers are used and/or programmed by device driver software. 
     The 64 byte header portion  401  of the 256 byte function configuration space is shown in FIG.  5 . The header portion of the function configuration space for each function is, in one embodiment, fixed as shown in FIG.  5 . As shown in FIG. 5, the header includes areas in which the device ID, the vendor ID, the header type, and other information may be stored. The definition allows, among other things, enumeration software (e.g., “Plug and Play” software) to identify, access, and program certain system resources. 
     FIG. 6 is a diagram of the configuration address that can be used to address the PCI configuration space. An eight-bit portion of the configuration address, bits  23  to  16 , are used to address the portion of configuration space that corresponds to a particular bus within the computer system. The five bits  15  to  11  address a particular device on the addressed bus, and the three bits  10  to  8  address a particular function on the addressed device. The eight bits  7  to  0  address the desired configuration register within the addressed function. 
     In operation, enumeration software, such as plug-and-play software, accesses portions of the configuration space in order to determine what types of devices are available in the computer system. The enumeration software may also determine how many functions each device supports. As described above, some devices may support only a single function, whereas other devices may support multiple functions. Typically, the number of functions supported by a particular device can be determined by accessing the configuration space header for one or more of that device&#39;s functions. 
     When enumeration software accesses the configuration space in a PCI-based system, it determines the number of functions each device supports by accessing the configuration space headers for at least one function for that device. Specifically, for each device on each bus, it may test the header type field  501  within function  0  for each device to determine if the device corresponding to the configuration header being accessed is a multi-function device. This is determined in a PCI-based system by testing the bit  502  of the configuration register  501  shown in FIG.  5 . This is bit  7  of the configuration register 0×10 within the configuration space header for function  0 . See Section 6.2.1 of the PCI Specification, Revision 2.1. 
     If the device is not a multi-function device, the enumeration logic does not scan the configuration space header for the remaining functions. Rather it assumes that the only function implemented by the device is function  0 , and it assumes that remaining functions  1  to  7  do not exist or are not implemented. 
     If, however, the device is a multi-function device, enumeration software scans the configuration space header for each of the eight functions that the device may have. Each configuration space header that corresponds to a non-existent function will not respond to the cycle in which that function is being accessed. Thus, the access will end in a master abort, which returns all logic 1s. When enumeration software receives all logic 1s, it concludes that the function does not exist. In this way, enumeration software determines the existence, or non-existence of a device function in a multi-function device by reading the configuration header for each function. If the configuration header returns all logic 1s, the function does not exist. If the configuration header returns anything else, the addressed function does exist. 
     The number of user-definable configuration registers can be effectively increased by using the configuration registers for a non-existent function. In some embodiments, the present invention will maintain compatibility with the PCI standard, and enumeration software will still operate in the manner described above. Because enumeration software will conclude that a function does not exist if the configuration space header for that non-existent function returns all logic Is, enumeration software will still operate correctly simply by setting every bit in the configuration space header to logic 1. The remaining portion of the configuration space for that function can then safely be used as additional configuration registers by a device-specific device driver. The device driver or other software that needs scarce configuration registers will be aware of the additional configuration registers, but these registers will effectively be hidden from enumeration software. 
     FIG. 7 is a flow chart of a process used during design in one embodiment of the present invention to expand the available number of user-definable configuration registers within a system. At  701  the device configuration space is checked for a given device to determine whether there are any non-existent functions for the device. If there are no non-existent functions, then the embodiment of the invention illustrated in FIG. 7 yields no additional configuration registers from this particular device configuration space. If, however, there are non-existent functions for the device, then one or more is chosen at  705 . For each chosen function, the configuration space header bits are programmed to be set to logic 1 when they are accessed by enumeration software. The remaining portion of the configuration space for the non-existent function can then be used for additional configuration registers at  709 . 
     The device implemented by the process illustrated in FIG. 7 will still maintain compatibility with enumeration software, as can be demonstrated by the flow chart of FIG.  8 . FIG. 8 is a flow chart of the process carried out by enumeration software when it accesses configuration space for a given device. At  801  the enumeration software accesses the header type field for the first function for a given device. This type field indicates whether the device is a multi-function device or not. If at  803  it is determined that the device is not a multi-function device, then the enumeration software accesses function  0  at  805 , and does not bother accessing any of the other possible functions. 
     If the device is determined at  803  to be a multi-function device, then the configuration space header for each of the remaining functions for that device is scanned at  807 . If, for each of those functions, the configuration space header returns all logic 1s at  809 , then the enumeration software concludes that the addressed function does not exist. For those functions that do not return all logic 1s, the function does exist, and can be accessed by the enumeration software at  809 . 
     The use of non-existent device functions for additional configuration registers will not affect or disrupt the process illustrated in FIG. 8, which is carried out by enumeration software. Thus, enumeration software will continue to work properly in a device implemented in accordance with the present invention. In some embodiments of the present invention, even configuration space headers are used as additional configuration registers when the device has only a single function. With a single-function device, the enumeration software will only scan the configuration space header for function  0 . This makes it unnecessary for configuration space headers for functions  1  to  7  to return logic 1s when accessed. Thus, these configuration space headers can also be used as configuration registers in a single-function device. 
     FIG. 9 is a flow chart of the process carried out by a device driver to access configuration registers, including those made available through use of configuration space corresponding to non-existent device functions. The device driver knows of the existence of the configuration registers that are hidden from the enumeration software, and configuration registers made available through non-existent functions are accessed in a manner similar to that in which conventional configuration registers are accessed. FIG. 9 illustrates a process for accessing these additional configuration registers. The configuration space address is decoded to determine the addressed function at  901 , and also decoded to determine the addressed register at  903  within the configuration space for the addressed function. Once this decoding is complete, the decoded register can be accessed at  905 . This access may even be to a function that does not correspond to the function that is currently being used or is currently implicated. 
     In another embodiment, and for an additional level of security, it may be useful to include a stealth bit in the function  0  user definable configuration space for each device. When the stealth bit is set, even the upper 192 bits of the non-existent function would return logic Is when they are read, and they would also be write-protected. When such a stealth bit mode is enabled, the configuration registers in the non-existent function would not be used. 
     The present invention can be used to expand the number of configuration registers available in a system that implements a bus-parking diagnostic technique, such as that disclosed in co-pending and commonly-assigned application Ser. No. 08/984,115, filed Dec. 3, 1997, now U.S. Pat. No. 6,092,219 entitled “Method for Use of Bus Parking States to Communicate Diagnostic Information,” which is hereby fully incorporated by reference. 
     Although the present invention has been described in terms of certain embodiments, various changes and modifications, even if not shown or specifically described herein, are deemed to lie within the spirit and scope of the invention and the appended claims.