Date: Sun, 13 Dec 2015 17:02:44 -0500
From: Damien Riegel <damien.riegel@savoirfairelinux.com>
To: Wim Van Sebroeck <wim@iguana.be>
Cc: Guenter Roeck <linux@roeck-us.net>, Pratyush Anand <panand@redhat.com>,
        linux-watchdog@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC/RFT PATCH] watchdog: Move watchdog device creation to
 watchdog_dev.c
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	Wim Van Sebroeck <wim@iguana.be>,
	Guenter Roeck <linux@roeck-us.net>,
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On Mon, Dec 07, 2015 at 09:41:03PM +0100, Wim Van Sebroeck wrote:
> Hi All,
> 
> > On 12/07/2015 08:15 AM, Damien Riegel wrote:
> > >On Sun, Dec 06, 2015 at 11:51:41AM -0800, Guenter Roeck wrote:
> > >>The watchdog character device s currently created in
> > >>watchdog_dev.c, and the watchdog device in watchdog_core.c. This
> > >>results in cross-dependencies, as the device creation needs to
> > >>know the watchdog character device number.
> > >>
> > >>On top of that, the watchdog character device is created before
> > >>the watchdog device is created. This can result in race conditions
> > >>if the watchdog device node is accessed before the watchdog device
> > >>has been created.
> > >>
> > >>To solve the problem, move watchdog device creation into
> > >>watchdog_dev.c, and create the watchdog device prior to creating
> > >>its device node. Also move device class creation into
> > >>watchdog_dev.c, since this is now the only place where the
> > >>watchdog class is needed.
> > >>
> > >>Inspired by an earlier patch set from Damien Riegel.
> > >>
> > >>Cc: Damien Riegel <damien.riegel@savoirfairelinux.com>
> > >>Signed-off-by: Guenter Roeck <linux@roeck-us.net> --- Hi Damien,
> > >>
> > >>I think this approach would be a bit better. The watchdog device
> > >>isn't really used in the watchdog core code, so it is better
> > >>created in watchdog_dev.c. That also fits well with other pending
> > >>changes, such as sysfs attribute support, and my attempts to move
> > >>the ref/unref functions completely into the watchdog core. As a
> > >>side effect, it also cleans up the error path in
> > >>__watchdog_register_device().
> > >>
> > >>What do you think ?
> > >
> > >Hi Guenter,
> > >
> > >Like the idea, but I don't really get the separation. For instance,
> > >you move watchdog_class in watchdog_dev.c but you keep watchdog_ida
> > >in watchdog_core.c whereas it is only used for device
> > >creation/deletion.
> > >
> > The class is watchdog driver internal, and it is device related, so
> > I think it made sense to move it to watchdog_dev.c. On top of that,
> > it will be needed there if/when we introduce sysfs attributes.
> > 
> > The watchdog id can be determined by obtaining an id using ida, or
> > it can be provided through the watchdog alias. The operation to get
> > it is not device related, and it is not straightforward to obtain
> > it, so I thought it makes sense to keep the code in watchdog_core.c.
> > 
> > Of course a lot of it is personal preference.
> > 
> 
> Let me go back to how I saw the design when I created the generic
> watchdog framework: When using watchdog device drivers we need to be
> able to support the /dev/watchdog system. I also foresaw that we
> should have a sysfs interface and I saw the future for watchdog
> devices that you should be able to choose between the 2 different
> systems. You should be able to use only the /dev/watchdog interfacing,
> but you should also be able to use both a sysfs interface and a
> /dev/watchdog interface and it should even be possible to have only a
> sysfs interface in certain embedded devices. So that's why I split the
> watchdog framework over 3 files: core code, the /dev/watchdog
> interfacing and the sysfs code. Since I want to have compiled code
> small enough when choosing either /Dev/watchdog or sysfs or both this
> sounded the most logical thing to do (Unless you have a single file
> full of #ifdef-ery that becomes unreadable).
> 
> So I do not agree to have sysfs code in watchdog_dev.c . It belongs in
> watchdog_sysfs.c imho. If someone has a better idea, I'll be glad to
> listen to it and see what the benefits are. But I want a clean system
> for excluding both /dev/ (current watchdog_dev.c) and/or sysfs
> (watchdog_sysfs.c) in the future. Off-course the current behaviour is
> to have the /dev/ interface and have the option to add sysfs
> attributes.

I agree that keeping sysfs code separate makes sense, as someone might
want to not use it.

The question is: can we make the /dev/watchdog entries optional ? That
would break the compatibility, right? Imho, it would be saner to keep
only one way to interact with watchdogs (ie. keep /dev/watchdog as is
and don't make it optional, and sysfs read-only and eventually
optional). I think that question should be answered before we can decide
how we want to split the code between watchdog_dev.c and watchdog_core.c

Thanks,
Damien

> 
> Kind regards, Wim.
> 
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