[RFC PATCH 0/3] Add accurate boot timing to a Linux system

Hi Russell,

On Sat, Sep 24, 2011 at 1:32 AM, Russell King - ARM Linux
[off-list ref] wrote:

On Fri, Sep 23, 2011 at 04:03:15PM -0700, Simon Glass wrote:

quoted

An accurate timer is required to make the numbers meaningful. Many
modern platforms have a microsecond timer. This patch set uses a
function called timer_get_us() to read the timer.

Not another 'get a time value' function. ?Why do we need soo many?
We have - at least:

ktime_get (and various flavours of it)
do_gettimeofday
getnstimeofday
sched_clock

Do we really need yet another one which will have to be multiplexed
amongst platforms, requiring scaling and so forth from whatever the
platform provides?

No we don't need another! It is just a convenience for this RFC since
the actual timer mechanism is not clear and not important for this
RFC. Insert your favourite timer mechanism instead.

Remember that ARM timers are virtually all MMIO mapped, which means
they don't work during early kernel bringup when the MMU mappings for
the hardware have not been setup. ?(That's the reason stuff like
sched_clock for printk doesn't work early.) ?That can't be solved by
creating yet another per-platform method to get microseconds.

While it would be useful to measure early things like kernel
decompression, it's hard to make that work cross-platform and there
are other problems like you raise, and where to store timing info.