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

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Date:   Tue, 3 Oct 2017 08:10:39 -0700
From:   Darren Hart <dvhart@...radead.org>
To:     Greg KH <greg@...ah.com>
Cc:     Mario Limonciello <mario.limonciello@...l.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>, quasisec@...gle.com,
        pali.rohar@...il.com
Subject: Re: [PATCH v3 4/8] platform/x86: wmi: create character devices when
 requested by drivers

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.

-- 
Darren Hart
VMware Open Source Technology Center