Re: WMI and Kernel:User interface

Andy Lutomirski
2017-05-10T22:51:10Z

On Wed, May 10, 2017 at 3:02 PM, <Mario.Limonciello@dell.com> wrote:
> So here's a "more" realistic scenario:
>
> OEM has support through a WMI function to control keyboard backlight timeouts
> and intensity. That same WMI function also can support turning on/off an individual
> USB port. Backlight timeouts are done by setting the first argument to "1" and USB
> port control is done by setting first argument to "2".
>
> Some userspace app is developed that can control both of these functions through
> the chardev. Later an enterprising young kernel developer realizes that backlight
> control should be done through a platform driver instead.
>
> They write a platform driver to do it, and add a filter to block "1" arguments from
> userspace. Now if the userspace app tries to call the chardev with the "1" argument
> some error code is returned indicating this request is not supported.
>
> The result is the userspace app broke, but it broke because the kernel is supporting
> the method in a much smarter and more scalable way.

There's another possibility: the filter could intercept the "1"
argument and change the brightness.

FWIW, I think that almost anything the kernel did with a real WMI API
would be better than the ugly things that drivers like dcdbas allow.

--Andy