linux-kernel - Re: [PATCH v2 3/4] iio: light: veml3235: extend regmap to add cache

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Message-ID: <20250114132306.00005358@huawei.com>
Date: Tue, 14 Jan 2025 13:23:06 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
CC: Javier Carrasco <javier.carrasco.cruz@...il.com>, Matti Vaittinen
	<mazziesaccount@...il.com>, Jonathan Cameron <jic23@...nel.org>, "Lars-Peter
 Clausen" <lars@...afoo.de>, <linux-iio@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] iio: light: veml3235: extend regmap to add cache

On Sun, 12 Jan 2025 20:50:41 +0200
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Sun, Jan 12, 2025 at 6:21 PM Javier Carrasco
> <javier.carrasco.cruz@...il.com> wrote:
> > On Sun Jan 12, 2025 at 5:11 PM CET, Andy Shevchenko wrote:  
> > > On Sun, Jan 12, 2025 at 6:07 PM Javier Carrasco
> > > <javier.carrasco.cruz@...il.com> wrote:  
> > > > On Sun Jan 12, 2025 at 4:18 PM CET, Andy Shevchenko wrote:  
> > > > > Tue, Dec 24, 2024 at 11:59:02AM +0100, Javier Carrasco kirjoitti:  
> > > > > > The configuration and ID registers are not volatile and are not affected
> > > > > > by read operations (i.e. not precious), making them suitable to be
> > > > > > cached in order to reduce the number of accesses to the device.  
> 
> ...
> 
> > > > > > +   .cache_type = REGCACHE_RBTREE,  
> > > > >
> > > > > Any specific reason why this is NOT a maple tree?  
> > > >
> > > > I followed the most common approach in IIO (52 RBTREE vs 2 MAPLE),  
> > >
> > > But it's historical and can't be taken as an argument.
> > >  
> > > > assuming that the "low-end systems" comment for the different REGCACHE_*
> > > > applies well to the typical systems that will make use of this driver,
> > > > and many others under IIO. I considered that *possible* performance
> > > > advantage for low-end systems above other considerations, like the
> > > > general rule about using maple tree.  
> > >
> > > https://elixir.bootlin.com/linux/v6.13-rc3/source/include/linux/regmap.h#L58
> > >
> > > "Any new caches
> > >  * should usually use the maple tree cache unless they specifically
> > >  * require that there are never any allocations at runtime and can't
> > >  * provide defaults in which case they should use the flat cache."
> > >
> > > Can you reconsider?  
> >
> > That was exactly the comment I referenced, actually the part about
> > low-end systems that appears right after what you highlighted.
> >
> > I have nothing against switching to MAPLE, if that is preferred even if
> > the main user of this driver will be a low-end system. I think that IIO
> > is a typical subsystem that addresses needs for very low-end systems
> > that are sometimes slightly more powerful than a microcontroller, but on
> > the other hand I am by no means an expert, and if MAPLE is the way to go
> > here as well, I will send a follow-up patch for it.  
> 
> Ah, I see now. Okay, I leave it then to Jonathan as I am okay with any
> choice as long as it's understood and justified.

For a fairly small regmap, I doubt there is a strong reason to care about the choice.
So stick to what you have.  For future drivers we can reassess as makes sense.

Jonathan

> 
>