M1: A timer driver

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, 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.

Group Work & Version Control

By now you should have got yourself in a group and you should have a group account setup for you. 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. 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.

Goals

The aim of this milestone is to design and implement device driver to accurately provide timeouts 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 libs/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 notification object, and initialise the device.

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 timer_interrupt(void)

Function that will be called whenever a message is received on the interrupt_ep given to start_timer

int stop_timer(void)

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

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 sleep syscall that is 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 timers provided on the OdroidC2's S905 system on chip (SOC). An overview of the parts in the OdroidC2 can be found here.

We have provided constants and definitions for the timer device in projects/aos/libclock/include/device.h. Use these constants, rather than those from the manual, as these have been tested to work (based on the source code for the Odroid-C2 Linux branch, which is more accurate than the manual). However, still refer to chapter 26 in the SOC manual to understand how the timer functions. We have tested timer F through I, and recommend you use as many of these timers as you desire for this milestone.

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.

• 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[F|G|H|I] 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.

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 timers F, G, H or I.
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 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 perform a series of steps to register to receive interrupts and manage interrupts for the timer device.

• You need to create an interrupt handling cap using our cspace library irq_handler = cspace_irq_control_get_cap(cur_cspace, seL4_CapIRQControl, irq, 1). The last argument is important as it tells the kernel that this is an edge-triggered interrupt.
• We have already created and bound a notification endpoint within the framework we have given you. Its cptr is ntfn, as initialised in main_continued. You should create a new badged copy of this cap and pass it to start_timer, who should then register your new interrupt handler with the kernel and direct it to forward the interrupt to the provided notification object. An example of badging is done for network interrupts, see the call to badge_irq_ep in main.c

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. A function to achieve this has been provided for you in apps/sos/src/mapping.c. You can also look at the basic uart driver in apss/sos/src/devices/uart.c.

Issues

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

• At what address is the frame with the timer 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?
• 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 timestamp_* 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 timestamp_ms 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.

Better Solutions

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

• A tickless timer where interrupts are only generated when threads need waking (or due to the finite size of the timer).