[PATCH v2 1/4] ARM: dts: omap5: Update GPIO with address space and interrupts

Hi Mitch,

On 10/23/2012 11:55 AM, Mitch Bradley wrote:

On 10/23/2012 4:49 AM, Jon Hunter wrote:

quoted

Therefore, I believe it will improve search time and hence, boot time if
we have interrupt-parent defined in each node.

I strongly suspect (based on many years of performance tuning, with
special focus on boot time) that the time difference will be completely
insignificant.  The total extra time for walking up the interrupt tree
for every interrupt in a large system is comparable to the time it takes
to send a few characters out a UART.  So you can get more improvement
from eliminating a single printk() than from globally adding per-node
interrupt-parent.

Furthermore, the cost of processing all of the interrupt-parent
properties is probably similar to the cost of the avoided tree walks.

CPU cycles are very fast compared to I/O register accesses, say a factor
of 100.  Now consider that many modern devices contain embedded
microcontrollers (SD cards, network interface modules, USB hubs and
devices, ...), and those devices usually require various delays measured
in milliseconds, to ensure that the microcontroller is ready for the
next initialization step.  Those delays are extremely long compared to
CPU cycles.  Obviously, some of that can be overlapped by careful
multithreading, but that isn't free either.

The bottom line is that I'm pretty sure that adding per-node
interrupt-parent would not be worthwhile from the standpoint of speeding
up boot time.

Absolutely, I don't expect this to miraculously improve the boot time or
suggest that this is a major contributor to boot time, but what is the
best approach in general in terms of efficiency (memory and time). In
other words, is there a best practice? And from your feedback, I
understand that adding a global interrupt-parent is a good practice.

For a bit of fun, I took an omap4430 board and benchmarked the time
taken by the of_irq_find_parent() when interrupt-parent was defined for
each node using interrupts and without.

There were a total of 47 device nodes using interrupts. Adding the
interrupt-parent to all 47 nodes increased the dtb from 13211 bytes to
13963 bytes.

On boot-up I saw 117 calls to of_irq_find_parent() for this platform
(there appears to be multiple calls for a given device). Without
interrupt-parent defined for each node total time spent in
of_irq_find_parent() was 1.028 ms where as with interrupt-parent defined
for each node the total time was 0.4032 ms. This was done using a
38.4MHz timer and the overhead of reading the timer 117 times was about
36 us.

I understand that this does not provide the full picture, but I wanted
to get a better handle on the times here. So yes the overall overhead
here is not significant for us to worry about.

Cheers
Jon