Re: [PATCH v3 4/8] platform/x86: wmi: create character devices when requested by drivers

[Greg KH <greg@kroah.com>]

On Tue, Oct 03, 2017 at 09:48:05AM -0700, Andy Lutomirski wrote:
> On Tue, Oct 3, 2017 at 8:10 AM, Darren Hart <dvhart@infradead.org> wrote:
> > On Tue, Oct 03, 2017 at 11:23:23AM +0200, Greg KH wrote:
> >> On Wed, Sep 27, 2017 at 11:02:16PM -0500, Mario Limonciello wrote:
> >> > For WMI operations that are only Set or Query read or write sysfs
> >> > attributes created by WMI vendor drivers make sense.
> >> >
> >> > For other WMI operations that are run on Method, there needs to be a
> >> > way to guarantee to userspace that the results from the method call
> >> > belong to the data request to the method call.  Sysfs attributes don't
> >> > work well in this scenario because two userspace processes may be
> >> > competing at reading/writing an attribute and step on each other's
> >> > data.
> >> >
> >> > When a WMI vendor driver declares a set of functions in a
> >> > file_operations object the WMI bus driver will create a character
> >> > device that maps to those file operations.
> >> >
> >> > That character device will correspond to this path:
> >> > /dev/wmi/$driver
> >> >
> >> > This policy is selected as one driver may map and use multiple
> >> > GUIDs and it would be better to only expose a single character
> >> > device.
> >> >
> >> > The WMI vendor drivers will be responsible for managing access to
> >> > this character device and proper locking on it.
> >> >
> >> > When a WMI vendor driver is unloaded the WMI bus driver will clean
> >> > up the character device.
> >>
> >> Ok, thanks to Darren, I've gone and dug these up while my boxes were
> >> building stable kernels...
> >>
> >> Why are you not just using the misc device interface here?  Why do you
> >> need a whole new major and minor range?  Why not just register misc
> >> devices dynamically as-needed?  Should be much simpler and easier to
> >> maintain and reduce your code size a lot.
> >
> > Thank you Greg, this simplifies things quite a bit.
> >
> > Mario, the misc device interface will remove a lot of the boiler plate
> > setup and eliminate the need to allocate a new major number.
> >
> 
> In my mind, the problem with misc is that you may end up forever stuck
> with a misc device, and they're distinct (visibly to userspace) from
> all other character devices.

What do you mean by "distinct"?  No one cares about the major number for
a device node anymore.

> If you really want to be fancy, you could try to dust off a non-awful
> character device API, a la:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=u2f&id=d3ab93173d51cebf00dd2263fd0ce9f8cd6258f7

Yeah, fixing this up and doing something like this has been on my "TODO"
for over a decade now :(

thanks,

greg k-h