linux-kernel - Re: ledtrig-cpu: Limit to 4 CPUs

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Message-ID: <20201008101051.GB16084@duo.ucw.cz>
Date:   Thu, 8 Oct 2020 12:10:51 +0200
From:   Pavel Machek <pavel@....cz>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     linux-leds@...r.kernel.org, dmurphy@...com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ledtrig-cpu: Limit to 4 CPUs

Hi!

> > I believe probability someone uses that with more than 4 CPUs is <
> > 5%.
> 
> So you even didn't bother to check:
> 
> $ git grep "default-trigger = \"cpu[4-9]"
> arch/arm/boot/dts/vexpress-v2m-rs1.dtsi: linux,default-trigger = "cpu4";
> arch/arm/boot/dts/vexpress-v2m-rs1.dtsi: linux,default-trigger = "cpu5";
> arch/arm/boot/dts/vexpress-v2m.dtsi: linux,default-trigger = "cpu4";
> arch/arm/boot/dts/vexpress-v2m.dtsi: linux,default-trigger = "cpu5";
> 
> cpus are enumerated starting from 0, so there are more reasons for which
> your patch is broken:
> 
> 1. There are mainline users.
> 2. You claim that you limit trigger use to 4 cpus, while the number is
>    actually 5, due to your condition:
> 	+		if (cpu > 4)
> 	+			continue;

Ok, fixed.

> 3. For platforms exceeding the limit the number of triggers registered
>    would not match the number all available cpus, for no obvious reason.
>    Better solution would be to prevent use of the trigger entirely
>    in such cases, which would need only to alter first instruction in
>    ledtrig_cpu_init(), which currently is:
> 
> 	BUILD_BUG_ON(CONFIG_NR_CPUS > 9999);

Hmm. If I do that I'll get complains from various build bots...

But I might do dependency in Kconfig...

> The correct approach would be to create new trigger with better
> interface and then advise people switching to it.

Patch would be accepted.

> > Probability that someone uses it with more than 100 CPUs is << 1%
> > I'd say. Systems just don't have that many LEDs. I'll take the risk.
> > 
> > If I broke someone's real, existing setup, I'll raise the limit.
> 
> Is this professional approach - throw a potential bug at users and
> check if it will hit them? :-) And for no reason - you're not fixing
> anything.

I'm sorry I failed to meet your expectations.

I raised limit to 8.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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