Re: [lm-sensors] [RFC PATCH 0/2] fs: sysfs: Add devres support

From: Guenter Roeck
Date: Sun Mar 17 2013 - 09:19:43 EST

On Sun, Mar 17, 2013 at 01:39:20PM +0100, Jean Delvare wrote:
> On Sat, 16 Mar 2013 14:25:40 -0700, Guenter Roeck wrote:
> > On Sat, Mar 16, 2013 at 12:50:02PM -0700, Greg Kroah-Hartman wrote:
> > > On Sat, Mar 16, 2013 at 11:12:53AM -0700, Guenter Roeck wrote:
> > > > My use case is primarily for hwmon drivers.
> > > >
> > > > hwmon has a separate API call to register a driver with the hwmon subsystem,
> > > > which creates a separate hwmon device and provides the user space notification.
> > > > As the created attribute files are often conditional on device variant and device
> > > > configuration, I don't see how this could be done through a default attribute
> > > > list (even though it might be worthwhile exploring if it can be used for some
> > > > of the simpler drivers).
> > >
> > > The default attribute list functionality offers you the ability to have
> > > callbacks to your driver to validate if you really want this sysfs file
> > > to be created or not. Just use that like other subsystems do, then you
> > > will never have to be making these create and remove calls at all.
> >
> > I thought about it, but right now I have no idea how to make it work.
> > Initialization sequence in hwmon drivers is
> > probe():
> > allocate and initialize local driver data structures
> > detect configuration and initialize hardware if necessary
> > create attribute files
> > register with hwmon subsystem
> > sometimes do additional work, such as enable interrupts
> >
> > If I use attribute_group of the device_driver structure to create the attribute
> > files, my understanding is that those would be created prior to calling
> > the probe function.
> >
> > This would be too early, since local data structures do not yet exist, and
> > the chip configuration is unknown and uninitialized.
> >
> > On the other side, attribute files must exist before hwmon_device_register()
> > is called, since otherwise userspace would get confused.
> I'd like to add something at this point.
> We have historically created the hwmon attributes in the hardware (i2c,
> platform...) device, and then created an empty hwmon class device on
> top of it so that libsensors etc. can locate all hardware monitoring
> chips on the system. This is probably wrong and this may explain the
> difference of views between Greg and Guenter.
> I suspect that ideally all hwmon-related attributes should belong to the
> hwmon-class device and not the physical device. Would doing so solve
> the problem of is_visible() needing chip-specific information that can
> only be gathered during probe()? Sure this is an interface change, but
> a few hwmon drivers already do it that way (the ones without an actual
> hardware device, e.g. ACPI thermal zones) and libsensors supports this
> since version 3.0.3, which was released in September 2008 - 4.5 years
> ago.
> This would require creating the attributes after calling
> hwmon_device_register() rather than before, but from the ongoing
> discussion I seem to understand that the driver core supports creating
> the attributes for us, possibly at the same time as the class device
> will be created. Would this solve the userspace timing issue?
This is what I had in mind as ultimate possibility when I created
the second API mentioned in my other e-mail.

struct device *devm_hwmon_device_register(struct device *dev,
const struct attribute_group **groups)

The attributes are still attached to dev (ie to the hardware device)
in my current code, but it should be possible to attach them to the
hwmon class device instead.

Problem with that approach is that it makes drivers larger, not smaller,
at least if is_visible is needed. So it kind of defeats the purpose.

We can go along that route anyway if people think it is the right or a better
approach, but I am not sure if it is worth it. I can send out the patches if
there is interest.

> Also note that libsensors is really old fashioned when it comes to
> device discovery. It doesn't support hot-plug nor hot-remove. So some
> work would be needed in this area anyway if we want libsensors-based
> applications to properly cope with device addition and removal.

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