Re: [PATCH 4/4] iio: light: apds9960: convert to use maple tree register cache

[Jonathan Cameron]

On Mon, 27 Oct 2025 10:30:54 +0200
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> On Sat, Oct 25, 2025 at 11:36:45AM +0800, Gary Chu(楚光庆) wrote:
> > >On Fri, Oct 24, 2025 at 03:38:23PM +0800, Chu Guangqing wrote:  
> > >> The maple tree register cache is based on a much more modern data structure
> > >> than the rbtree cache and makes optimisation choices which are probably
> > >> more appropriate for modern systems than those made by the rbtree cache.  
> 
> ...
> 
> > >>   .reg_defaults = apds9960_reg_defaults,
> > >>   .num_reg_defaults = ARRAY_SIZE(apds9960_reg_defaults),  
> > >
> > >^^^^ Be careful with such cases, the cache implementations may behave
> > >differently. Have you tested this on the actual HW?
> > >  
> > We have conducted tests on some hardware, and performance improvements were observed,
> >  though tests have not been carried out on all hardware models.
> > Neither rbtree nor maple tree directly depends on hardware types (such as CPU or peripheral
> >  models). Instead, they rely on the address distribution characteristics (discrete/continuous)
> >  of hardware registers. The optimal cache type is determined by the hardware layout.
> > Red-black trees excel at individual operations on discrete addresses, while Maple Trees are
> >  proficient in range operations on contiguous addresses.  
> 
> It's not about the low-level cache implementation, it's about regmap
> abstraction implementation that might differ from cache to cache
> implementations. This all in regard how the cold cache is getting filled up.
> There is a separate discussion (unrelated to the topic of your series) where
> this was brought up.

I appreciate these things can be hard to track down with lots of threads in flight
but any chance of a reference for that? I'd be a little surprised if these uses
are complicated enough to hit corner cases but would like to know more.
I've taken a few similar changes in the past thinking there would be no
practical difference.

Jonathan



> That's why I asked how this was tested.
> 
> In any case, up to Jonathan, but I had to rise a potential misbehave, so in my
> opinion this kind of corner cases needs to be tested on real HW.
> 
> > >>   .max_register = APDS9960_REG_GFIFO_DIR(RIGHT),
> > >> - .cache_type = REGCACHE_RBTREE,
> > >> + .cache_type = REGCACHE_MAPLE,
> > >>  };  
>