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

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Message-ID: <aP8tvj_IPbv65m0T@smile.fi.intel.com>
Date: Mon, 27 Oct 2025 10:30:54 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Gary Chu(楚光庆) <chuguangqing@...pur.com>
Cc: lars <lars@...afoo.de>,
	"Michael.Hennerich" <Michael.Hennerich@...log.com>,
	jic23 <jic23@...nel.org>, dlechner <dlechner@...libre.com>,
	"nuno.sa" <nuno.sa@...log.com>, andy <andy@...nel.org>,
	"subhajit.ghosh" <subhajit.ghosh@...aklogic.com>,
	"javier.carrasco.cruz" <javier.carrasco.cruz@...il.com>,
	linux-iio <linux-iio@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH 4/4] iio: light: apds9960: convert to use maple tree
 register cache

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. 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,
> >>  };

-- 
With Best Regards,
Andy Shevchenko