On Tue, Oct 3, 2017 at 8:10 AM, Darren Hart <dvhart@xxxxxxxxxxxxx> 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. 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
