Re: [PATCH] leds: remove PAGE_SIZE limit of /sys/class/leds/<led>/trigger

From: Akinobu Mita <akinobu.mita@gmail.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>,
	linux-leds@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Pavel Machek <pavel@ucw.cz>, Dan Murphy <dmurphy@ti.com>
Subject: Re: [PATCH] leds: remove PAGE_SIZE limit of /sys/class/leds/<led>/trigger
Date: Tue, 3 Sep 2019 22:55:40 +0900	[thread overview]
Message-ID: <CAC5umyjTJSMdKMtZbF8Uxky6nOrAHesTHmZRV5VA1uPwX2rtxA@mail.gmail.com> (raw)
In-Reply-To: <20190902190843.GB25019@kroah.com>

2019年9月3日(火) 4:08 Greg KH <gregkh@linuxfoundation.org>:
>
> On Mon, Sep 02, 2019 at 08:47:02PM +0200, Jacek Anaszewski wrote:
> > On 9/2/19 8:12 PM, Greg KH wrote:
> > > On Sun, Sep 01, 2019 at 06:53:34PM +0200, Jacek Anaszewski wrote:
> > >> Hi Akinobu,
> > >>
> > >> Thank you for the patch.
> > >>
> > >> I have one nit below but in general it looks good to me.
> > >> I've tested it with 2000 mtd triggers (~14kB file size)
> > >> and it worked flawlessly.
> > >>
> > >> Still, I would like to have ack from Greg for it.
> > >>
> > >> Adding Greg on Cc.
> > >>
> > >> On 8/29/19 4:49 PM, Akinobu Mita wrote:
> > >>> Reading /sys/class/leds/<led>/trigger returns all available LED triggers.
> > >>> However, the size of this file is limited to PAGE_SIZE because of the
> > >>> limitation for sysfs attribute.
> > >>>
> > >>> Enabling LED CPU trigger on systems with thousands of CPUs easily hits
> > >>> PAGE_SIZE limit, and makes it impossible to see all available LED triggers
> > >>> and which trigger is currently activated.
> > >>>
> > >>> This converts /sys/class/leds/<led>/trigger to bin attribute and removes
> > >>> the PAGE_SIZE limitation.
> > >
> > > But this is NOT a binary file.  A sysfs binary file is used for when the
> > > kernel passes data to or from hardware without any parsing of the data
> > > by the kernel.
> > >
> > > You are not doing that here, you are abusing the "one value per file"
> > > rule of sysfs so much that you are forced to work around the limitation
> > > it put in place on purpose to keep you from doing stuff like this.
> > >
> > > Please fix this "correctly" by creating a new api that works properly
> > > and just live with the fact that this file will never work correctly and
> > > move everyone to use the new api instead.
> > >
> > > Don't keep on abusing the interface by workarounds like this, it is not
> > > ok.
> >
> > In the message [0] you pledged to give us exception for that, provided
> > it will be properly documented in the code. I suppose you now object
> > because the patch does not meet that condition.
>
> Well, I honestly don't remember writing that email, but it was 5 months
> and many thousands of emails ago :)
>
> Also, you all didn't document the heck out of this.  So no, I really do
> not want to see this patch accepted as-is.
>
> > Provided that will be fixed, can we count on your ack for the
> > implementation of the solution you proposed? :-)
>
> Let's see the patch that actually implements what I suggested first :)

I'd propose introducing a new procfs file (/proc/led-triggers) and new
/sys/class/leds/<led>/current-trigger api.

Reading /proc/led-triggers file shows all available triggers.
This violates "one value per file", but it's a procfs file.

The /sys/class/leds/<led>/current-trigger is almost identical to
/sys/class/leds/<led>/trigger.  The only difference is that
'current-trigger' only shows the current trigger name.
This file follows the "one value per file" rule of sysfs.