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

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Date:   Fri, 25 Sep 2020 10:51:38 +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

On 9/22/20 10:41 PM, Jacek Anaszewski wrote:
> 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.

Actually it precludes its use.

I still see the patch in your linux-next, so I reserve myself the
right to comment on your pull request.

-- 
Best regards,
Jacek Anaszewski