Home Teaching Research Publications

Advanced Operating Systems (LV 7680)

M1 - A timer driver

  • Due: Week 2
  • Marks: 5 (-1 per week late, discontinue course if more than one week late)
  • Write a simple device driver for the timers available on the platform.

Your first milestone is to write a basic timer driver. However, you should use this as an opportunity to get to used working with your partner(s), and probably work out exactly how you can work together so you don't end up duplicating work, or worse still not completing essential parts of the project.

  1. Group Work and Version Control
  2. Goals
  3. Motivation
  4. The Driver Interface
  5. The timer device
  6. Supplied Code
  7. seL4/ARM Interrupts
  8. Device Mappings
  9. Issues
  10. Assessment
    1. Background
    2. Demonstration
    3. Show Stoppers
    4. Better Solutions

Group Work and Version Control

We expect that you are using Git to maintain your source code, and that a repository be setup in your group account with the correct permissions and sticky bits set so that you can both access it. Comprehensive information on git can be found in this free ebook. Also see our git overview page for suggestions.

You should consider using a merge tool such as the meld (or equivalent) as the default merge program to avoid painful merges.

You have been provided with a git repository in gitlab for your group. This repository will be used for submitting all further milestones. You can clone the repository with the following command.

    git clone https://gitlab.cs.hs-rm.de/advanced-operating-systems/2021_aos/group{group-number}/aos-2021.git aos2021

You will need to provide your username and password when cloning. If you set up your ssh-keys on our gitlab server you can use the following command instead:

    git git@gitlab.cs.hs-rm.de:advanced-operating-systems/2021_aos/group{group-number}/aos-2021.git aos2021

Goals

The aim of this milestone is to design and implement device driver to accurately provide time stamps and trigger activities. You should add a file for the timer implementation and modify the main system call loop to handle timer interrupts. See the framework page for how to add a file to the SOS build system.

  • Learn and understand the fundamentals of writing a device driver.
  • Play with real hardware.
  • Learn about interrupt handling in seL4.
  • Learn about memory mapped device access and control.
  • Time system behaviour on seL4.

Motivation

Applications will eventually need to be able to make sleep system calls.

The Driver Interface

Your driver needs to export the interface specified in projects/aos/libclock/include/clock/clock.h. There are the following functions:

    int start_timer(seL4_CPtr interrupt_ep)

    Initialises the driver. Should set up any interrupts needed by the timer to be delivered to the given endpoint

    timestamp_t get_time(void)

    Read the current time in milliseconds from the internal counter timer (timer E). 

    uint32_t register_timer(uint64_t delay, void (*callback)(uint32_t id, void *data), void *data)

    Registers a callback function be called after the specified interval (in microseconds, though actual wakeup resolution will depend on the timer resolution). Several registrations may be pending at any time. The return value is zero on failure, otherwise a unique identifier for this timeout. This identifier can be used to remove a timeout. After a time out has occured, or the timeout has been removed, the identifier may be reused.

    int remove_timer(uint32_t id)

    Remove a previously registered timer callback, using the unique identifier returned by register_timer.

    int stop_timer(void)

    Stops operation of the driver. This will remove any outstanding time requests.

    int timer_irq(void *data, seL4_Word irq, seL4_IRQHandler irq_handler)

    Function to be called by the IRQ dispatch whenever an IRQ is triggered by the timer hardware. The IRQ dispatch will pass in the data and IRQ number that the callback was registered with and an IRQ handler capability that must be used to acknowledge the IRQ.

The above interface is just an internal function call interface. You do not need to export this interface to the users. User programs will indirectly access the clock driver through the time and sleep syscalls that are implemented in a later milestone.

Note: After registering an interrupt, you must call seL4_IRQHandler_Ack to ensure the kernel in a sane initial state.

The timer device

Your main job is to learn how to program the platform's timer devices. For the OdroidC2's S905 SoC, information on the relevant parts can be found here.

For the previously used i.MX6 platforms (SabreLite and Wandboard), reference information can be found here. Similarly, for the BeagleBone Black, a hardware reference manual is found here. The i.MX6 as well as the am335x SoCs have many peripherals for various purposes, such as GPU, UART, video, watchdog and so on. For our purposes, we are interested in using the timers EPIT1, EPIT2 and GPT, to set them up as our sources of timer interrupts. The EPIT and GPT timer modules have very similar interfaces.

We have provided a small wrapper library for interacting with the timer device. Each of the first four timeout timers (A, B, C, and D) have identifiers in projects/aos/libclock/include/clock/device.h and a function that can be used to find the IRQ numbers associated with each timer. You must use one or more of these timers in your clock implementation.

There is also an internal interface to be used by libclock which is found in projects/aos/libclock/src/device.h which has been implemented in projects/aos/libclock/src/device.c. The functions here can be used to configure the timestamp (counter) timer (timer E) and each of the timeout timers. Use the constants provided here rather than those in the manual as these have been determined to work correctly. However, still refer to chapter 26 in the SOC manual to understand how the timer functions.

Implementing a device driver really just a matter of learning about its registers, what values to read and write to those registers, and when to do it.

The minimal subset of a timer module's functionality that you must understand and use is listed below. Refer to the SoC manuals for more complete descriptions.

  • Timer MUX: A 32-bit register Used to configure the timers. At minimum to use a timer you must enable it, and set the input clock.
  • Timer[A|B|C|D] Register: The upper 16 bits of this register are the current value of the timer. The lower-16 bits are the start value for this timer, which is loaded into the upper 16 bits on write. If the timer is enabled, the upper 16 bits will then count down to 0 at the rate of the selected input clock. (Timers E through I have not been provided as the higher 16 bits do not seem to operate as described by the manual.)

The following is a deliberately short breakdown related to the i.MX6. You are encouraged to explore your specific SoC and other registers for a deeper understanding. Also, for the BeagleBone, you may find some interesting hints and sample code in the libclock library.

  • Timer Control Register (EPITxCR, GPTCR) : The actual timer/counters. You need to use at least one of them (take your pick). You will need to understand one of these to properly configure and enable the corresponding timers. As part of the configuration process, you will need to select an input clock to drive these timers. We recommend the Peripheral Clock (ipg_clk) which you may assume to be configured to 66MHz (see Chapter 18 of the i.MX6 reference manual).
  • Timer Counter Register (EPITx_CNR, GPT_CNT) : Use this register as the lower 32-bits of your timestamp value.
  • Timer Load Register (EPITx_LR): Contains the value that is to be loaded on EPIT overflow.
  • Timer Compare Register (EPITx_CMPR, GPT_OCRn): Contains the comparison value which will trigger a timer interrupt.
  • Timer Status (EPITx_SR, GPT_SR) : When an interrupt occurs, this register provides more information about the source of the interrupt. When you have finished processing the interrupt, you you must clear the appropriate flag in this register and then call seL4_IRQHandler_Ack to clear the interrupt event in the kernel. Note that writing a '1' to any bit in this register will clear the corresponding flag while writing a '0' will have no effect.

Note: This section is deliberately kept short (e.g., we do not dictate which timer to use or in what mode to use it in). The idea is for you to develop your own design and implementation. There are only two conditions that must be satisfied:

  1. You must use interrupt(s) generated from at least one of the timers.
  2. You must implement the driver interface described above.

Supplied Code

For this project you have been supplied with skeleton code to help you along the way. This code is intended as an implementation guide, not as a 'black-box' library.

It is important that you fully understand all provided code that you use. For the purposes of assessment, we treat any supplied code that you call as your code and as such you may be asked to describe how it works.

Now might be a good time to get familiar with the the resources, especially the framework documentation.

seL4/ARM Interrupts

The seL4/ARM kernel exports specific interrupts to a user level interrupt handler via asynchronous notification.

You will need to register the timer_irq function to handle the IRQs for any of the timers you decide to use. The sos_register_irq_handler function in projects/aos/sos/src/irq.h can be used to register the handler and the meson_timeout_irq function from projects/aos/libclock/include/device.h can be used to determine the IRQ for each of the timers.

The provided code already calls sos_handle_irq_notification whenever SOS receives a notification on its IRQ endpoint. This is then responsible for calling the timer_irq callback.

Before attempting this, you should read Chapter 6 of the sel4 documentation to gain an understanding of TCBs, and Section 8.1 to understand how interrupts are delivered.

Device Mappings

In seL4/ARM, device registers are memory mapped. That is, hardware registers can be accessed via normal load/store operations to special addresses. To access device registers, you must first map the device into the driver's virtual address space with the appropriate attributes. The provided code performs this mapping for the timer driver.

Issues

You may need to resolve some or all of these issues:

  • At what address do the timer device registers need to be mapped and accessed through?
  • What value must be programmed to the timer to get a frequency of x milliseconds?
  • How are the interrupts acknowledged?
  • Periodic timer ticks or variable length time-outs (a so called tickless kernel)?
  • Single or multi-threaded driver?
  • Which data structures should I use?
  • What race conditions might exist? (imagine the timer hardware as parallel thread of execution). More specifically, how do you derive a 64-bit timer from a 32-bit one.
  • What is an acceptable granularity/resolution for timeouts/timestamps? Hint: 100ms is too long, 1ms ticks are too frequent for timer ticks, though your timestamp should be more accurate than 1ms.
  • In principle, good device drivers attempt to minimise the length of interrupt handling code.
  • How will you test your implementation? You may want to test a lower-level timer interface that you can unit test without requiring callbacks to be registered.

Assessment

Background

For the remainder of the semester progress through the milestones is contingent on passing the demonstration requirements below, and not having any show stoppers.

In general, we don't mark down for milestones that don't meet minimal requirements. Instead, we'll point out what is required, which groups can then fix, and then submit the following week for a small late penalty. Thus less than perfect project marks usually come about via late penalties, not lower marks for poor solutions.

It's in your best interest to fix problems, rather than letting them snowball into something more problematic as the semester progresses.

Better solutions outlines (only) some potential differentiators of solution quality that are expected to have more favourable marks at the end of the semester, however they are not required.

In general, do not jeopardise the progression of your project by chasing better solutions at the expense of correctness of your project.

Demonstration

You should be able to show some test code that uses all the functions specified in the driver interface. You may use the functions from projects/aos/libclock/include/clock/timestamp.h to demonstrate the accuracy of your timeouts.

Specifically set up and demonstrate:

  • A regular 100ms timer tick. Register a timeout to fire a callback every 100ms. Then, print the value returned by get_time every time this callback is received. Note: Your timestamps must be at least accurate to the nearest 10ms.
  • Register another timeout at a different interval in addition to the 100ms running concurrently (i.e. demo more than one timeout registered at a time).
  • Before entering the main system call loop, set up a few calls to register_timer. Make sure the delay used is long enough such that the system call loop is entered before these wake up. These callbacks should just print out the current timestamp as each delay expires. You will later use register_timer to implement a sleep system call for your user-level processes.

Show Stoppers

Note this is not a complete list. The following designs are considered unsatisfactory:

  • Only supporting a single timeout registered at a time.
  • Delivering callbacks in the wrong order
  • O(n) searches in the interrupt path of the timer interrupt handler.
  • Interrupt frequencies greater than 100Hz, if your timer ticks regularly.
  • Race conditions when managing the hardware timer.

Better Solutions

The following approaches are considered better than minimum, and favourably contribute to the final project mark.

  • Timestamp accuracy to sub-millisecond without increasing the tick rate of the timer.
  • A tickless timer where interrupts are only generated when threads need waking (or due to the finite size of the timer).