Real-time performance using FIQ interrupt handling in SPEAr MPUs
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4. completion of the code use the following assembly instruction subs pe lr 4 This command restores the instruction pointer to the original location it was before FIQ arrived FIQ ISR in C Writing the FIQ handler function in ARM assembly language can be cumbersome so it is highly recommended to use C language instead This is especially true for large functions when the handler function is larger than the maximum size available for FIQ vector 500 bytes In such cases branching to the C function can be possible This involves saving of the register context r0 r12 general mode registers and initialization of the function frame pointer Doc ID 16927 Rev 1 ky AN3129 IRQ latency measurement 4 4 1 IRQ latency measurement The latency measurement module is a Linux kernel module LKM available as part of the STLinux SPEAr BSP release kernel code This module can be statically linked with the kernel at the time of the build This module is useful for measuring the IRQ or FIQ latency in a running system The statistics collected are e Maximum latency in microseconds Minimum latencies in microseconds Total number of samples captured Frequency distribution of IRQ handling latency in range 0 200 microseconds Figure 3 Linux Kernel IRQ latency measurement module System Type Arrow keys navigate the menu lt Enter gt selects submenus gt Highlighted letters are hotkeys Pressing lt Y gt i
5. crash Doc ID 16927 Rev 1 3 12 FIQ handler environment AN3129 3 3 1 3 2 3 3 3 4 4 12 FIQ handler environment The FIQ registration the ISR function and the runtime environment of a FIQ handler are different from a standard IRQ handler A working example of an FIQ handler can be found in latency measurement module in Section 4 IRQ latency measurement FIQ registration For FIQ registration set_fiq_handler API is used The two arguments of this API are e Function pointer e Size of handler function in bytes This handler function is copied to the FIQ vector reserved area The FIQ vector area is last in the memory area reserved for interrupt vectors About five hundred 500 bytes are available in this area for copying the ISR In the SPEAr Linux Support Package LSP the following API must be used to select the configuration of the IRQ source e vic_unmask_irq int number enables interrupt source on VIC as IRQ e vic_select_fiq int number selects interrupt source on VIC as FIQ instead of IRQ FIQ mode registers setup When the FIQ handler runs it utilizes FIQ mode specific registers r8 r14 and general mode registers r0 r7 This set of FIQ mode registers is exclusive to FIQ mode and not available to other handlers To pass any information to the FIQ handler the FIQ mode registers have to be set using the call set_fiq_regs Return from FIQ mode To return from FIQ mode on
6. ncludes lt N gt excludes lt M gt modularizes features Press lt Esc gt lt Esc gt to exit lt gt for Help lt gt for Search Legend built in excluded lt M gt module lt gt LAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA ARH system type SPE amp ar gt ttt Boot options ttt Power management Architecture ST Spear600 gt ST Spear GPIO FSMC support for SPE amp Ar Soc SPEar Interrupt Latency Test SPEAR600 Machine SPEAR600 target board ST SPEAr600 Evaluation board gt SPEAR Miscellaneous Register Configuration x x x x x x x x x x m LOEKIE lt Help gt AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA To see above modes in the kernel source directory 1 Run make menuconfig 2 Then navigate to System Type gt SPEAr Interrupt Latency Test The source code of the latency measurement module is available in the STLinux SPEAr BSP Linux kernel source code The file location is arch arm plat spear latency_test c You can use this module as a starting point for writing a new FIQ handler Using the latency measurement module The latency test module exchanges configurable parameters and measured results through Linux sysfs filesystem Doc ID 16927 Rev 1 5 12 IRQ latency measurement AN3129 6 12 This module is linked statically to the kernel image and has user mode interface at sys kernel irq_latency To u
7. nel Interrupt IRQ handler Interrupt latency Interrupt handler Interrupt disabling in Kernel duration OR k gt other interrupt handler An FIQ is a highest priority interrupt and preempts both of these sources of latency It can preempt IRQ top half handlers as well as Linux kernel code sections where IRQs are disabled Further to this the FIQ handler can not be preempted by other IRQs This ensures that the platform has predictable worst case timings for FIQ handler latency making FIQ suitable for real time systems The FIQ handler can not permit tasks that require other IRQs to be active including the timer This also makes it suitable for real time systems but imposes the following limitations e Kernel mode APIs and device driver framework can not be used inside the FIQ handler These APIs available for Linux driver development may internally rely upon timers schedulers spinlocks etc for functioning Linux code also needs to guard against multiple access to objects by spinlocks or other relevant mechanisms FIQ ISRs can come in the middle of anything and have to get out quickly again too So unless it is a simple macro you can not use any Linux APIs in the FIQ ISR e Page faults have to be avoided inside the handler the memory used in the handler should be already allocated and initialized In case that a page fault occurs inside the FIQ handler it can not be recovered by the kernel and it would lead to a kernel
8. nterrupt request FIQ interrupts 2 Vectored IRQ interrupts 3 Non vectored IRQ interrupts FIQs are intended for fast low latency interrupt handling In FIQ mode the ARM core uses seven 32 bit banked registers which enable the VIC to process interrupts as quickly as possible Multiple interrupts can be handled as FIQs but in this case the IRQ vector needs to be loaded in the program counter PC In order to reduce interrupt latency it is advised to use a single FIQ source because the ISR can be directly executed without determining the source of the interrupt As the Linux kernel never disables FIQs they have ultimate priority over anything else in the system When using an FIQ the FIQ latency will be the time it takes the CPU to enter your FIQ function Moreover an FIQ can preempt Linux kernel code and IRQ handlers and it can also be used for faster response to interrupts The use of an FIQ in place of an IRQ does not require any hardware wiring modifications on the SPEAr board The VIC has a 32 bit register VICINTSELECT which can toggle an IRQ source to raise an FIQ instead of the default mechanism of IRQ Doc ID 16927 Rev 1 ky AN3129 Linux kernel IRQ handling latency 2 Linux kernel IRQ handling latency In a Linux kernel based system interrupt latency may occur in the following cases Interrupt latency for top half e Various sections where IRQs are disabled Figure 2 IRQ handling latency in Linux Ker
9. or IRQ shoots up to 177 us The maximum latency for FIQ is close to 8 us e The overall throughput of system remains the same in both cases Doc ID 16927 Rev 1 ky AN3129 Conclusion 5 Conclusion The fast interrupt mechanism ensures a bounded performance with the Linux kernel without any decrease in the overall system throughput Its worst case is less than ten microseconds lt 10usec However this deterministic behavior comes at the cost of some limitations in ISR handler development which can not utilize Linux kernel API and device driver framework The FIQ interrupt handling mechanism used as described in this application note is well suited for critical real time applications with SPEAr ky Doc ID 16927 Rev 1 9 12 References AN3129 6 References Table 2 References Title Revision Path Location PrimeCell Vectored Interrupt Controller PL190 rip2 ARM website ARM Architecture Reference Manual 2005 ARM website SPEAr300 User Manual 1 2 STLinux SPEAr BSP SPEAr600 User Manual 1 3 STLinux SPEAr BSP 10 12 Doc ID 16927 Rev 1 AN3129 Revision history 7 Revision history Table 3 Document revision history Date Revision Changes 05 Jan 2010 1 Initial release Doc ID 16927 Rev 1 11 12 AN3129 Please Read Carefully Information in this document is provided solely in connection with ST products STMicroelectronics NV and its subsidiaries
10. pture use one of the following commands depending on the mode echo 0 gt irg mode or echo 0 gt fiq_mode Once the mode stopped the timer is stopped and the IRQ handling is deactivated Also the statistics are cleared 4 2 Module internals The module uses a general purpose hardware timer to raise an IRQ FIQ and capture the ticks elapsed The timer automatically reloads itself after raising an interrupt In this way when the IRQ handler reads the timer value this value is the exact measurement of the system latency for handling this IRQ Depending upon the choice of IRQ handling mechanism IRQ or FIQ the corresponding selection is made in VIC handling FIQ registration and de registration is part of the standard Linux kernel Timer GPT interval 1 millisecond Timer tick interval 1 48 microsecond Interrupt handler latency frequency counter is captured from 0 to 9600 ticks This corresponds to a range of 0 to 200 microseconds This frequency range is sufficient for determining the interrupt latency performance 4 3 Performance measurements The details of setup used for making the measurements are e Kernel Linux kernel version 2 6 27 Board SPEAr Plus 600 application board CPU ARM core running 333 MHz RAM 128 MB DDR2 running 333 MHz Flash 8 MB NOR Doc ID 16927 Rev 1 7 12 IRQ latency measurement AN3129 4 3 1 4 4 8 12 System load using netperf While the timer is recording values for ISR latenc
11. se the module 1 Run the following command ls sys kernel irq_latency The command output shows four entries _irq_mode shows if IRQ latency measurement is 1 on or O off no capture figq mode shows if FIQ latency measurement is 1 on or O off no capture stats shows the running statistics of measurements as summary stats_ details shows detailed log of IRQ latency counters During kernel booting the statically linked module is loaded automatically By default the module lies dormant and does not capture latency statistics 2 To start the capture you must activate one of the two IRQ handling modes using the command echo 1 gt irq_mode or echo 1 gt fiq_mode irq_mode in this timer interrupts are handled with an IRQ mechanism fiq_mode in this timer interrupts are handled as FIQ interrupts FIQ mode is designed to handle real time scenarios To get the statistics captured by the module use the command cat stats repeatedly while performing other tasks A typical output is Latency max usec 48 min usec 6 totalsamples 7798 To get detailed statistics use the command cat stats_details repeatedly while performing other tasks A typical output is Latency Counter usec 0 0 1 0 2 0 3 0 4 0 5 0 6 12001 7 70 Doc ID 16927 Rev 1 ky AN3129 IRQ latency measurement 8 35 9 29 10 13 11 0 198 O 199 O 200 0 3 To stop statistics ca
12. y the system is loaded using high traffic on Ethernet For this the netperf utility is used Netperf is an open source free software used for benchmarking of Ethernet performance between two systems There are two components to the netperf Ethernet benchmark server and client When the client netperf is invoked it connects to the server netserver and starts to send data through sockets The total bytes transferred in 10 seconds are measured and used to calculate the network throughput The SPEAr board is connected directly to a Linux PC through Ethernet cross cable Netperf client is run on the SPEAr board and netserver server is run on the Linux PC The netperf utility sends Ethernet packets at the maximum possible rate to the netserver which sends them back Results This experiment was done both for IRQ mode and FIQ mode The values from the timer module and the netperf throughput are recorded in the following table In all three tests the overall system throughput was measured as peak network performance Table 1 Timer ISR latency measurements minimum maximum Interrupt Latency ps IRQ FIQ Under Network load Test1 Test2 Test3 Test1 Test2 Test3 Maximum us 175 177 173 7 7 8 Minimum us 1 4 1 0 0 0 Throughput Mbps 1 94 94 94 94 94 94 1 Throughput was calculated using the netperf client data transfer rate According to the table we can infer that The maximum latency f
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