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

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Message-ID: <20190903144432.GA12889@kroah.com>
Date:   Tue, 3 Sep 2019 16:44:32 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Akinobu Mita <akinobu.mita@...il.com>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        linux-leds@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Pavel Machek <pavel@....cz>, Dan Murphy <dmurphy@...com>
Subject: Re: [PATCH] leds: remove PAGE_SIZE limit of
 /sys/class/leds/<led>/trigger

On Tue, Sep 03, 2019 at 11:21:59PM +0900, Akinobu Mita wrote:
> 2019年9月3日(火) 23:07 Greg KH <gregkh@...uxfoundation.org>:
> >
> > On Tue, Sep 03, 2019 at 10:55:40PM +0900, Akinobu Mita wrote:
> > > 2019年9月3日(火) 4:08 Greg KH <gregkh@...uxfoundation.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.
> >
> > No, procfs files are ONLY for process-related things.  Don't keep the
> > insanity of this file format by just moving it out of sysfs and into
> > procfs :)
> 
> I see.
> 
> How about creating one file or directory for each led-trigger in
> /sys/kernel/led-triggers directory?
> 
> e.g.
> 
> $ ls /sys/kernel/led-triggers
> audio-micmute                              ide-disk        phy0assoc
> audio-mute                                 kbd-altgrlock   phy0radio
> ...
> hidpp_battery_3-full                       panic

Fine with me if you think that will work.