Memory Parity Workarounds

Figure 18-1 : The SysAD Bus in Relation to GIO Some GIO cards do not drive all 32 GIO data lines during CPU PIO reads. These reads from the GIO card are either 8-bit or 16-bit transfers, so the lines are left floating. The problem is that to generate parity bits for the SysAD bus, the Memory Controller (MC) must calculate parity for all 32 bits. Since the calculation must occur before the CPU read completes, it is possible that one (or more) of the floating bits may change while parity is being calculated. Thus, when the CPU read completes, it may be received as a parity error on the SysAD bus.

Note: Diagnosis is complicated by the fact that this problem may not show up on every transaction. It occurs only when one of the data lines that is left floating happens to change state between the start of the MC parity calculation and the completion of the CPU read. A device and its driver can appear to function correctly for some time before the problem occurs. When writing a driver for a GIO card that does not drive all 32 data lines, you must either disable SysAD parity checking completely, or disable it during the time your driver is performing PIO transfers. Three kernel functions are supplied for these purposes; none of them take arguments.

• is_sysad_parity_enabled() returns a nonzero value if SysAD parity checking is enabled.

• disable_sysad_parity() turns off parity checking on the SysAD bus.

• enable_sysad_parity() returns SysAD parity checking to normal.

It is much better to disable parity checking only during the time the device is being used. The advantage here is that the software recovery procedures for memory parity errors are almost always in effect.

To selectively disable parity checking, put wrappers around your driver's PIO transactions to disable SysAD parity checking before a transfer, and to re-enable it after the PIO completes. Example 18-5 shows a skeleton of such a wrapper.

Example 18-5 : Disabling SysAD Parity Checking During PIO

void
do_PIO_without_parity()
{
   int was_enabled = is_sysad_parity_enabled();
   if (was_enabled) disable_sysad_parity();
/* do driver PIO transfers */
if (was_enabled) enable_sysad_parity();
}