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

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Date:   Tue, 22 Sep 2020 22:41:35 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
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 Pavel,

On 9/22/20 12:42 AM, Pavel Machek wrote:
> Hi!
> 
>>> Can I get details of your setup?
>>
>> I don't use this trigger, but I can imagine that someone does.
> 
> Well, if someone exists, we can increase the limit, or convince them
> to change their setup.

Linux is used in many commercial projects and each such change generates
a cost, so this is not a sheer matter of convincing someone.

>>> What CPU type that is, and how are you mapping CPU activity to LEDs?
>>
>> The type of CPU is irrelevant here. What is important is the fact
>> that with this trigger it is possible to visually monitor CPU core
>> online state. Probably it would be good to ask the author of that
>> trigger about his use case.
> 
> It is relevant -- cpu trigger never worked on x86. I had patch fixing
> it, but got pushback.

You mean literally x86 (32-bit)? Because I checked yesterday on my
x86_64 and it worked just fine, i.e. changing cpu online state
generated events on all userspace LEDs I registered for each cpuN
trigger.

>> I have spoken up, because I don't get the reason for your patch.
>> This driver was reworked year ago to remove PAGE_SIZE limit,
>> and I even applied it to my for-next tree, but that was at
>> the time of handling maintainership to yourself, and you
>> seem to not have picked that commit.
>>
>> Was that intentional? We had even Greg's ack [0].
> 
> I checked, and I believe the commit is in:

Indeed, I blindly sought the changeset in git log for ledtrig-cpu.c
file.

> #ifdef CONFIG_LEDS_TRIGGERS
> static BIN_ATTR(trigger, 0644, led_trigger_read, led_trigger_write,
> 0);
> static struct bin_attribute *led_trigger_bin_attrs[] = {
> 
> So.. no, it is not causing kernel crashes or something. But it is
> example of bad interface, and _that_ is causing problems. (And yes, if
> I realized it is simply possible to limit it, maybe the BIN_ATTR
> conversion would not be neccessary...)

The limitation you proposed breaks the trigger on many plafforms.

-- 
Best regards,
Jacek Anaszewski