diff options
Diffstat (limited to 'Documentation/ia64/IRQ-redir.txt')
| -rw-r--r-- | Documentation/ia64/IRQ-redir.txt | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/Documentation/ia64/IRQ-redir.txt b/Documentation/ia64/IRQ-redir.txt new file mode 100644 index 000000000000..f7bd72261283 --- /dev/null +++ b/Documentation/ia64/IRQ-redir.txt @@ -0,0 +1,69 @@ +IRQ affinity on IA64 platforms +------------------------------ + 07.01.2002, Erich Focht <efocht@ess.nec.de> + + +By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be +controlled. The behavior on IA64 platforms is slightly different from +that described in Documentation/IRQ-affinity.txt for i386 systems. + +Because of the usage of SAPIC mode and physical destination mode the +IRQ target is one particular CPU and cannot be a mask of several +CPUs. Only the first non-zero bit is taken into account. + + +Usage examples: + +The target CPU has to be specified as a hexadecimal CPU mask. The +first non-zero bit is the selected CPU. This format has been kept for +compatibility reasons with i386. + +Set the delivery mode of interrupt 41 to fixed and route the +interrupts to CPU #3 (logical CPU number) (2^3=0x08): + echo "8" >/proc/irq/41/smp_affinity + +Set the default route for IRQ number 41 to CPU 6 in lowest priority +delivery mode (redirectable): + echo "r 40" >/proc/irq/41/smp_affinity + +The output of the command + cat /proc/irq/IRQ#/smp_affinity +gives the target CPU mask for the specified interrupt vector. If the CPU +mask is preceded by the character "r", the interrupt is redirectable +(i.e. lowest priority mode routing is used), otherwise its route is +fixed. + + + +Initialization and default behavior: + +If the platform features IRQ redirection (info provided by SAL) all +IO-SAPIC interrupts are initialized with CPU#0 as their default target +and the routing is the so called "lowest priority mode" (actually +fixed SAPIC mode with hint). The XTP chipset registers are used as hints +for the IRQ routing. Currently in Linux XTP registers can have three +values: + - minimal for an idle task, + - normal if any other task runs, + - maximal if the CPU is going to be switched off. +The IRQ is routed to the CPU with lowest XTP register value, the +search begins at the default CPU. Therefore most of the interrupts +will be handled by CPU #0. + +If the platform doesn't feature interrupt redirection IOSAPIC fixed +routing is used. The target CPUs are distributed in a round robin +manner. IRQs will be routed only to the selected target CPUs. Check +with + cat /proc/interrupts + + + +Comments: + +On large (multi-node) systems it is recommended to route the IRQs to +the node to which the corresponding device is connected. +For systems like the NEC AzusA we get IRQ node-affinity for free. This +is because usually the chipsets on each node redirect the interrupts +only to their own CPUs (as they cannot see the XTP registers on the +other nodes). + |