Re: [PATCH] drm/radeon: Inline r100_mm_rreg

[Lauri Kasanen <cand@gmx.com>]

On Thu, 10 Apr 2014 21:30:03 +0200
Christian König <deathsimple@vodafone.de> wrote:

> >>> Quick thought from someone entirely unfamiliar with the hardware:
> >>> perhaps you can get the performance benefit without the size increase
> >>> by moving the else portion into a non-inline function? I'm guessing
> >>> that most accesses happen in the "if" branch.
> >> The function call overhead is about equal to branching overhead, so
> >> splitting it would only help about half that. It's called from many
> >> places, and a lot of calls per sec.
> 
> Actually direct register access shouldn't be necessary so often. Apart 
> from page flips, write/read pointer updates and irq processing there 
> shouldn't be so many of them. Could you clarify a bit more what issue 
> you are seeing here?

Too much cpu usage for such a simple function. 2% makes it #2 in top-10
radeon.ko functions, right after evergreen_cs_parse. For reference, #3
(radeon_cs_packet_parse) is only 0.5%, one fourth of this function's
usage.

As proved by the perf increase, it's called often enough that getting
rid of the function call overhead (and compiling the if out
compile-time) helps measurably.

- Lauri