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

[Guenter Roeck]

On Sun, Mar 17, 2013 at 06:19:33AM -0700, Guenter Roeck wrote:
> 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.

I found an alternate way of converting drivers with optional attributes - don't
use is_visible, but collect the attribute groups dynamically. With this,
conversion results look much better. For lm90:

 drivers/hwmon/lm90.c |   91 +++++++++++++++++++++-----------------------------
 1 file changed, 38 insertions(+), 53 deletions(-)

Code size is reduced by ~550 bytes. With that, this approach is much more
feasible.

I'll figure out how to attach the attributes to the hwmon device and send out
a new set of patches after I get it working.

Thanks,
Guenter
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