[PATCH 1/1] ARM: OMAP2+: hwmod: Print an error message if no match exists for a hwmod class

Hello Vaibhav

On Fri, 10 Aug 2012, Bedia, Vaibhav wrote:

quoted

Ok. I'll try to do some profiling using a timer to see how much time we
spend over here. On AM335x there are 2 such lookups, one for the dma
and one for the mcbsp. I could get this done today but will get to it
tomorrow. Based on how this goes I'll try out the omap_hwmod_class list
like you suggested.

I used a dmtimer clocked at 24MHz (41ns resolution) to get the time
spent in the lookup. There are two failed lookups that happen in AM335x
and following are the times taken

Run1
Lookup for dma - 435 cycles -> 17.835 usecs
Lookup for mcbsp - 83 cycles -> 3.4 usecs

Run2
Lookup for dma - 503 cycles -> 20.623 usecs
Lookup for mcbsp - 90 cycles -> 3.69 usescs

Run3
Lookup for dma - 375 cycles -> 15.375 usecs
Lookup for mcbsp - 91 cycles -> 3.731 usecs

The 2nd lookup happens immediately after the 1st one so probably
the difference is coming due to the data sitting in the cache?

The average times don't look too high to me but I guess you already
knew that ;)

If you are ok with a pr_debug() instead of a pr_err() I'll resubmit
the patch with this change.

Well I'd much rather see a patch that maintains a list of hwmods 
pertaining to each class.  That would solve the performance issue at 
fairly minimal cost (especially if those lists were freed after kernel 
init). 

It sounds like the basic difference in opinion is about whether an 
omap_hwmod_for_each_by_class() that doesn't find anything is an error 
condition or not.  My sense is that it is not.


- Paul