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

[Greg KH <greg@kroah.com>]

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.
> 
> Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
> ---
>  drivers/platform/x86/wmi.c | 98 +++++++++++++++++++++++++++++++++++++++++++---
>  include/linux/wmi.h        |  1 +
>  2 files changed, 94 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/platform/x86/wmi.c b/drivers/platform/x86/wmi.c
> index 4d73a87c2ddf..17399df87948 100644
> --- a/drivers/platform/x86/wmi.c
> +++ b/drivers/platform/x86/wmi.c
> @@ -34,7 +34,9 @@
>  #define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
>  
>  #include <linux/acpi.h>
> +#include <linux/cdev.h>
>  #include <linux/device.h>
> +#include <linux/idr.h>
>  #include <linux/init.h>
>  #include <linux/kernel.h>
>  #include <linux/list.h>
> @@ -50,6 +52,9 @@ MODULE_AUTHOR("Carlos Corbacho");
>  MODULE_DESCRIPTION("ACPI-WMI Mapping Driver");
>  MODULE_LICENSE("GPL");
>  
> +#define WMI_MAX_DEVS  MINORMASK
> +static DEFINE_IDR(wmi_idr);

You never free the idr's memory when you unload the module :(