Timing API Specification

Introduction
This specification is for a timing API for the Linux kernel which is available early during the bootup process.

Rationale
The purpose of this specification is to provide a platform-neutral API for timing instrumentation of the Linux kernel. Currently, there are a few different timing APIs in the kernel used for such instrumentation. For example, on the i386 architecture the Time Stamp Counter (TSC) register of the processor is used for getting fast timestamps for low overhead and high resolution time measurements. (This has historically been made available with the get_cycles function call.) Many timing measurement systems for the Linux kernel have been based on either get_cycles or directly on a call to read the TSC register, such as rdtscll.

There are a few problems with this setup. First, these functions are not supported on all architectures and platforms of interest. Thus, measurement systems that utilize them are not portable to these other platforms. Also, other functions which ARE available on all platforms are either high overhead, or are not usable from the very beginning of kernel bootup, or both.

This specification, then, is intended to address the need for a common API available on all embedded platforms, for a low-overhead, high resolution clock source accessible very early (even before setup_arch) in the boot sequence of the kernel.

Specifications

 * 1) The Linux kernel SHALL support a configuration to provide the following described timing API.
 * 2) The configuration option to enable this feature MUST be called CONFIG_FAST_TIMESTAMPS
 * 3) When CONFIG_FAST_TIMESTAMPS is enabled, the kernel SHALL provide the following 2 routines:
 * 4) void store_timestamp(timestamp_t *t)
 * 5) void timestamp_to_timeval(timestamp_t *t, struct timeval *tv)
 * 6) The store_timestamp API MUST be available and useful no later than upon the completion of the setup_arch routine in the kernel.
 * 7) The store_timestamp API SHOULD be available and useful as soon as possible after start_kernel
 * 8) The size and resolution of timestamp_t *t SHOULD be such that timestamps can record at least 30 seconds of timing information.
 * 9) The size of timestamp_t SHOULD be no greater than 64 bits.
 * 10) store_timestamp MUST produce values for timestamp that are monotonically increasing with each call on the same processor (with the exception of overruns or explicit clock value changes).
 * 11) The resolution of timestamp, after conversion to timeval, MUST NOT be less than 1 jiffy.
 * 12) The resolution of timestamp, after conversion to timeval, SHOULD be at least 100 usec
 * 13) A resolution of at least 1 usec for values of timestamp is preferred.

Notes (informational and non-normative)

 * The clock needs to be accessible at least during the boot up period of the kernel.
 * A free-running clock could be set up by firmware (before kernel start) and polled afterwards by the kernel in order to measure total system bootup time. Note that this implies that the timestamp value does not need to start at 0 with the beginning of execution of the kernel or the setup of the timer device.
 * It is preferred that the timestamp value returned should not fluctuate with changes in CPU frequency, but this is not mandatory.
 * Alternative, it should be possible to avoid changes in the CPU frequency during the timing period.
 * The values returned need not be linearly related. That is, it is acceptable for the values to be non-linear, as long as the conversion to timeval (sec, nsec) is correct. Thus, as one example of value management, it is possible to store the hardware clock value in the low 32 bits, and the number of rollovers in the high 32 bits. This works even if the clock source itself is less than 32 bits wide (eg 12 bits, or 16 bits).
 * It is not necessary for there to be an accurate correlation of timevals produced by timestamp_to_timeval to wall clock times. That is, there is no requirement to implement a strict gettimeofday call.
 * the purpose of separating the "store" function from the "to_timeval" function, is to make the overhead of actually acquiring the timestamp as little as possible. Some callers may immediately convert the timestamp to a timeval, but other time-sensitive code may defer the conversion to timeval units until some later time (or indefinitely).
 * The size of timestamp_t should be as small as possible. It is expected that timestamp values will be stored in trace logs which may accumulate data points quickly.  It is expected that on many embedded architectures, timestamp_t will be defined as an unsigned long (32 bits), and that it will NOT be defined to be any greater in size than an unsigned long long (64 bits).
 * Embedded processors have CPU clock frequencies ranging from a few MHZ to a few GHZ. A 1 GHZ clock will overrun 32 bits in about 4 seconds. This means that for fast-running hardware timestamp_t should probably be an unsigned long long.

Remaining Issues

 * It would be good to try this out somewhere