I have big confusion on this. Can any one explain me the difference between this??
When do we use Enable and when do we use restore. Both mean the same or are they different???
I know enable is used when disable is used . And restore is used when we save the interrupts.
But, when and where do we use them???
Thanks a lot.
Where excatly did you come across these? I would like to know the context to give more details. AFAIK its like this
Restore interrupt: means restore interrupt flag to the state prior ie old state
void _restore_interrupts(unsigned int);
Enable interrupt: It enables the interrupt flag and enables Interrupt service routines to run ie STI instruction under x86
unsigned int _enable_interrupts();
Normally for a critical section cli instruction is used to disable interrupts so that no other ISR is run. When the critical section is over the STI instruction is used to enable interrupts
Related
I'm working with TIM2 on the STM32L068K which is a Cortex M0 processor. When I write to the timer enable bit, all the interrupt flags immediately get set. This in itself is a known issue and apparently endemic to the processor design based on the online commentary I've read.
I can clear out the interrupt flags by writing to the status register, but the problem is that the NVIC pending IRQ bit for this source (#15) is also set. This means that the second I execute cpsie i I get vectored to the ISR for source #15 (confirmed by seeing that this is the reported source in IPSR). I've tried multiple techniques for writing to NVIC_ICPR, but the bit remains set. As one example of many things I've tried, check out this site : https://www.sciencedirect.com/topics/engineering/pending-interrupt. I've also tried the CMSIS calls to no good effect. Do writes to this register only work in handler mode, not thread mode? And if so, how then can you stop a spurious interrupt from happening? Is it possible to manually enable handler mode without triggering an exception?
Note that this website does say "If the interrupt source generates an interrupt request continuously (level output), then the pending status could remain high even if you try to clear it at the NVIC." I wouldn't expect the TIM2 IRQ to fall into this category as it should only be triggering when the count reaches zero, which is not happening here, and the interrupt flags for it have already been cleared anyway.
I'm using the blue pill, and trying to figure out interrupts. I have an interrupt handler:
void __attribute__ ((interrupt ("TIM4_IRQHandler"))) myhandler()
{
puts("hi");
TIM4->EGR |= TIM_EGR_UG; // send an update even to reset timer and apply settings
TIM4->SR &= ~0x01; // clear UIF
TIM4->DIER |= 0x01; // UIE
}
I set up the timer:
RCC_APB1ENR |= RCC_APB1ENR_TIM4EN;
TIM4->PSC=7999;
TIM4->ARR=1000;
TIM4->EGR |= TIM_EGR_UG; // send an update even to reset timer and apply settings
TIM4->EGR |= (TIM_EGR_TG | TIM_EGR_UG);
TIM4->DIER |= 0x01; // UIE enable interrupt
TIM4->CR1 |= TIM_CR1_CEN;
My timer doesn't seem to activate. I don't think I've actually enabled it though. Have I??
I see in lots of example code commands like:
NVIC_EnableIRQ(USART1_IRQn);
What is actually going in NVIC_EnableIRQ()?
I've googled around, but I can't find actual bare-metal code that's doing something similar to mine.
I seem to be missing a crucial step.
Update 2020-09-23 Thanks to the respondents to this question. The trick is to set the bit for the interrupt number in an NVIC_ISER register. As I pointed out below, this doesn't seem to be mentioned in the STM32F101xx reference manual, so I probably would never have been able to figure this out on my own; not that I have any real skill in reading datasheets.
Anyway, oh joy, I managed to get interrupts working! You can see the code here: https://github.com/blippy/rpi/tree/master/stm32/bare/04-timer-interrupt
Even if you go bare metal, you might still want to use the CMSIS header files that provide declarations and inline version of very basic ARM Cortex elements such NVIC_EnableIRQ.
You can find NVIC_EnableIRQ at https://github.com/ARM-software/CMSIS_5/blob/develop/CMSIS/Core/Include/core_cm3.h#L1508
It's defined as:
#define NVIC_EnableIRQ __NVIC_EnableIRQ
__STATIC_INLINE void __NVIC_EnableIRQ(IRQn_Type IRQn)
{
if ((int32_t)(IRQn) >= 0)
{
__COMPILER_BARRIER();
NVIC->ISER[(((uint32_t)IRQn) >> 5UL)] = (uint32_t)(1UL << (((uint32_t)IRQn) & 0x1FUL));
__COMPILER_BARRIER();
}
}
If you want to, you can ignore __COMPILER_BARRIER(). Previous versions didn't use it.
The definition is applicable to Cortex M-3 chips. It's different for other Cortex versions.
With the libraries is still considered bare metal. Without operating system, but anyway, good that you have a desire to learn at this level. Someone has to write the libraries for others.
I was going to do a full example here, (it really takes very little code to do this), but will take from my code for this board that uses timer1.
You obviously need the ARM documentation (technical reference manual for the cortex-m3 and the architectural reference manual for armv7-m) and the data sheet and reference manual for this st part (no need for programmers manual from either company).
You have provided next to no information related to making the part work. You should never dive right into a interrupt, they are advanced topics and you should poll your way as far as possible before finally enabling the interrupt into the core.
I prefer to get a uart working then use that to watch the timer registers when the roll over, count, etc. Then see/confirm the status register fired, learn/confirm how to clear it (sometimes it is just a clear on read).
Then enable it into the NVIC and by polling see the NVIC sees it, and that you can clear it.
You didn't show your vector table this is key to getting your interrupt handler working. Much less the core booting.
08000000 <_start>:
8000000: 20005000
8000004: 080000b9
8000008: 080000bf
800000c: 080000bf
...
80000a0: 080000bf
80000a4: 080000d1
80000a8: 080000bf
...
080000b8 <reset>:
80000b8: f000 f818 bl 80000ec <notmain>
80000bc: e7ff b.n 80000be <hang>
...
080000be <hang>:
80000be: e7fe b.n 80000be <hang>
...
080000d0 <tim1_handler>:
The first word loads the stack pointer, the rest are vectors, the address to the handler orred with one (I'll let you look that up).
In this case the st reference manual shows that interrupt 25 is TIM1_UP at address 0x000000A4. Which mirrors to 0x080000A4, and that is where the handler is in my binary, if yours is not then two things, one you can use VTOR to find an aligned space, sometimes sram or some other flash space that you build for this and point there, but your vector table handler must have the proper pointer or your interrupt handler won't run.
volatile unsigned int counter;
void tim1_handler ( void )
{
counter++;
PUT32(TIM1_SR,0);
}
volatile isn't necessarily the right way to share a variable between interrupt handler and foreground task, it happens to work for me with this compiler/code, you can do the research and even better, examine the compiler output (disassemble the binary) to confirm this isn't a problem.
ra=GET32(RCC_APB2ENR);
ra|=1<<11; //TIM1
PUT32(RCC_APB2ENR,ra);
...
counter=0;
PUT32(TIM1_CR1,0x00001);
PUT32(TIM1_DIER,0x00001);
PUT32(NVIC_ISER0,0x02000000);
for(rc=0;rc<10;)
{
if(counter>=1221)
{
counter=0;
toggle_led();
rc++;
}
}
PUT32(TIM1_CR1,0x00000);
PUT32(TIM1_DIER,0x00000);
A minimal init and runtime for tim1.
Notice that the NVIC_ISER0 is bit 25 that is set to enable interrupt 25 through.
Well before trying this code, I polled the timer status register to see how it works, compare with docs, clear the interrupt per the docs. Then with that knowledge confirmed with the NVIC_ICPR0,1,2 registers that it was interrupt 25. As well as there being no other gates between the peripheral and the NVIC as some chips from some vendors may have.
Then released it through to the core with NVIC_ISER0.
If you don't take these baby steps and perhaps you have already, it only makes the task much worse and take longer (yes, sometimes you get lucky).
TIM4 looks to be interrupt 30, offset/address 0x000000B8, in the vector table. NVIC_ISER0 (0xE000E100) covers the first 32 interrupts so 30 would be in that register. If you disassemble the code you are generating with the library then we can see what is going on, and or look it up in the library source code (as someone already did for you).
And then of course your timer 4 code needs to properly init the timer and cause the interrupt to fire, which I didn't check.
There are examples, you need to just keep looking.
The minimum is
vector in the table
set the bit in the interrupt set enable register
enable the interrupt to leave the peripheral
fire the interrupt
Not necessarily in that order.
I'm learning computer organization and structure (I'm using Linux OS with x86-64 architecture). we've studied that when an interrupt occurs in user mode, the OS is notified and it switches between the user stack and the kernel stack by loading the kernels rsp from the TSS, afterwards it saves the necessary registers (such as rip) and in case of software interrupt it also saves the error-code. in the end, just before jumping to the adequate handler routine it zeroes the TF and in case of hardware interrupt it zeroes the IF also. I wanted to ask about few things:
the error code is save in the rip, so why loading both?
if I consider a case where few interrupts happen together which causes the IF and TF to turn on, if I zero the TF and IF, but I treat only one interrupt at a time, aren't I leave all the other interrupts untreated? in general, how does the OS treat few interrupts that occur at the same time when using the method of IDT with specific vector for each interrupt?
does this happen because each program has it's own virtual memory and thus the interruption handling processes of all the programs are unrelated? where can i read more about it?
how does an operating system keep other necessary progresses running while handling the interrupt?
thank you very much for your time and attention!
the error code is save in the rip, so why loading both?
You're misunderstanding some things about the error code. Specifically:
it's not generated by software interrupts (e.g. instructions like int 0x80)
it is generated by some exceptions (page fault, general protection fault, double fault, etc).
the error code (if used) is not saved in the RIP, it's pushed on the stack so that the exception handler can use it to get more information about the cause of the exception
2a. if I consider a case where few interrupts happen together which causes the IF and TF to turn on, if I zero the TF and IF, but I treat only one interrupt at a time, aren't I leave all the other interrupts untreated?
When the IF flag is clear, mask-able IRQs (which doesn't include other types of interrupts - software interrupts, exceptions) are postponed (not disabled) until the IF flag is set again. They're "temporarily untreated" until they're treated later.
The TF flag only matters for debugging (e.g. single-step debugging, where you want the CPU to generate a trap after every instruction executed). It's only cleared in case the process (in user-space) was being debugged, so that you don't accidentally continue debugging the kernel itself; but most processes aren't being debugged like this so most of the time the TF flag is already clear (and clearing it when it's already clear doesn't really do anything).
2b. in general, how does the OS treat few interrupts that occur at the same time when using the method of IDT with specific vector for each interrupt? does this happen because each program has it's own virtual memory and thus the interruption handling processes of all the programs are unrelated? where can i read more about it?
There's complex rules that determine when an interrupt can interrupt (including when it can interrupt another interrupt). These rules mostly only apply to IRQs (not software interrupts that the kernel won't ever use itself, and not exceptions which are taken as soon as they occur). Understanding the rules means understanding the IF flag and the interrupt controller (e.g. how interrupt vectors and the "task priority register" in the local APIC influence the "processor priority register" in the local APIC, which determines which groups of IRQs will be postponed when the IF flag is set). Information about this can be obtained from Intel's manuals, but how Linux uses it can only be obtained from Linux source code and/or Linux specific documentation.
On top of that there's "whatever mechanisms and practices the OS felt like adding on top" (e.g. deferred procedure calls, tasklets, softIRQs, additional stack management) that add more complications (which can also only be obtained from Linux source code and/or Linux specific documentation).
Note: I'm not a Linux kernel developer so can't/won't provide links to places to look for Linux specific documentation.
how does an operating system keep other necessary progresses running while handling the interrupt?
A single CPU can't run 2 different pieces of code (e.g. an interrupt handler and user-space code) at the same time. Instead it runs them one at a time (e.g. runs user-space code, then switches to an IRQ handler for very short amount of time, then returns to the user-space code). Because the IRQ handler only runs for a very short amount of time it creates the illusion that everything is happening at the same time (even though it's not).
Of course when you have multiple CPUs, different CPUs can/do run different pieces of code at the same time.
system call -- It is an instruction that generates an interrupt that causes OS to gain
control of processor.
so if a running process issue a system call (e.g. create/terminate/read/write etc), a interrupt is generated which cause the KERNEL TO TAKE CONTROL of the processor which then executes the required interrupt handler routine. correct?
then can anyone tell me how the processor known that this instruction is supposed to block the process, go to privileged mode, and bring kernel code.
I mean as a programmer i would just type stream1=system.io.readfile(ABC) or something, which translates to open and read file ABC.
Now what is monitoring the execution of this process, is there a magical power in the cpu to detect this?
As from what i have read a PROCESSOR can only execute only process at a time, so WHERE IS THE MONITOR PROGRAM RUNNING?
How can the KERNEL monitor if a system call is made or not when IT IS NOT IN RUNNING STATE!!
or does the computer have a SYSTEM CALL INSTRUCTION TABLE which it compares with before executing any instruction?
please help
thanku
The kernel doesn't monitor the process to detect a system call. Instead, the process generates an interrupt which transfers control to the kernel, because that's what software-generated interrupts do according to the instruction set reference manual.
For example, on Unix the process stuffs the syscall number in eax and runs an an int 0x80 instruction, which generates interrupt 0x80. The CPU reacts to this by looking in the Interrupt Descriptor Table to find the kernel's handler for that interrupt. This handler is the entry point for system calls.
So, to call _exit(0) (the raw system call, not the glibc exit() function which flushes buffers) in 32-bit x86 Linux:
movl $1, %eax # The system-call number. __NR_exit is 1 for 32-bit
xor %ebx,%ebx # put the arg (exit status) in ebx
int $0x80
Let's analyse each questions you have posed.
Yes, your understanding is correct.
See, if any process/thread wants to get inside kernel there are only two mechanisms, one is by executing TRAP machine instruction and other is through interrupts. Usually interrupts are generated by the hardware, so any other process/threads wants to get into kernel it goes through TRAP. So as usual when TRAP is executed by the process it issues interrupt (mostly software interrupt) to your kernel. Along with trap you will also mentions the system call number, this acts as input to your interrupt handler inside kernel. Based on system call number your kernel finds the system call function inside system call table and it starts to execute that function. Kernel will set the mode bit inside cs register as soon as it starts to handle interrupts to intimate the processor as current instruction is a privileged instruction. By this your processor will comes to know whether the current instruction is privileged or not. Once your system call function finished it's execution your kernel will execute IRET instruction. Which will clear mode bit inside CS register to inform whatever instruction from now inwards are from user mode.
There is no magical power inside processor, switching between user and kernel context makes us to think that processor is a magical thing. It is just a piece of hardware which has the capability to execute tons of instructions at a very high rate.
4..5..6. Answers for all these questions are answered in above cases.
I hope I've answered your questions up to some extent.
The interrupt controller signals the CPU that an interrupt has occurred, passes the interrupt number (since interrupts are assigned priorities to handle simultaneous interrupts) thus the interrupt number to determine wich handler to start. The CPu jumps to the interrupt handler and when the interrupt is done, the program state reloaded and resumes.
[Reference: Silberchatz, Operating System Concepts 8th Edition]
What you're looking for is mode bit. Basically there is a register called cs register. Normally its value is set to 3 (user mode). For privileged instructions, kernel sets its value to 0. Looking at this value, processor knows which kind of instruction it is. If you're interested digging more please refer this excellent article.
Other Ref.
Where is mode bit
Modern hardware supports multiple user sessions. If your hw supports multi user mode, i provides a mechanism called interrupt. An interrupt basically stops the execution of the current code to execute other code (e.g kernel code).
Which code is executed is decided by parameters, that get passed to the interrupt, by the code that issues the interrupt. The hw will increase the run level, load the kernel code into the memory and forces the cpu to execute this code. When the kernel code returns, it again directly informs the hw and the run level gets decreased.
The HW will then restore the cpu state before the interrupt and set the cpu the the next line in the code that started the interrupt. Done.
Since the code is actively calling the hw, which again actively calls the kernel, no monitoring needs to be done by the kernel itself.
Side note:
Try to keep your question short. Make clear what you want. The first answer was correct for the question you posted, you just didnt phrase it well. Make clear that you are new to the topic and need a detailed explanation of basic concepts instead of explaining what you understood so far and don't use caps lock.
Please accept the answer cnicutar provided. thank you.
I am new to linux driver and writing a char driver for hardware.
What is the exact way to prevent interrupt(software/hardware) jamming in while the driver functions (eg. ioctl) is executing?
Thanks,
Pui
Did you mean disabling all interrupts in the system ? That is not actually not a good idea.
If you have a section of code and you want to make sure that an unexpected interrupt doesn't come in the way, have a look at spin_lock_irqsave(). This will disable interrupts locally. When you are done, you can then use spin_lock_irqrestore().
If you are concerned about only updating a variable, you might consider making it atomic(atomic_t).
Lastly, if you are only thinking of disabling interrupts from your char hardware when the driver is executing a certain kind of function, that will be hardware dependent. For example, for the LSI 1068E, you have to write 0xFFFFFFFF to the IntMask register. You can also ask the kernel to disable an interrupt line by using disable_irq() on that interrupt line but that is not probably what you want.