Protecting against configuration changes

Do not write a driver that relies on particular configuration parameters such as the interrupt vector. Avoiding such ``hardcoded'' assumptions helps prevent collisions with other drivers, and insulates the driver from system configuration changes.

DDI 7 and earlier implementation

• Most drivers should not, and do not need to, be aware of their own major device number. However, the device number contains the major number if you want to display it. Use getmajor(D3) to extract the major number from the device number.

  The only drivers that need to know this information fall into one of two categories: drivers performing some form of physical I/O that use the major device number to determine the type of I/O, and block device drivers that need to know if the device they control is the root or the swap device.

ODDI implementation

• Drivers that need to know their own major number can extract it from the device number with the major(D3oddi) function.

  The only drivers that need to know this information fall into one of two categories: drivers performing some form of physical I/O that use the major device number to determine the type of I/O, and block device drivers that need to know if the device they control is the root or the swap device.

• Drivers doing physical I/O differentiate it either by using the block/character parameter of the combined open(D2oddi) routine, or by marking the transfer in the b_dev member of the transfer's buffer. Drivers needing to know if they are the root device can find out using code such as the following:

     #include "sys/conf.h"
     #include "sys/cmn_err.h"
     
  
     extern struct bdevsw bdevsw[];
     
  
     if ( bdevsw[major(rootdev)].d_open == xxopen ) {
       cmn_err(CE_CONT,"the xx driver is the root device\n");
     	.
     	.
     	.
     }