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 15:09:59 +0000
From:   <Mario.Limonciello@...l.com>
To:     <greg@...ah.com>
CC:     <dvhart@...radead.org>, <andy.shevchenko@...il.com>,
        <linux-kernel@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>, <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

> -----Original Message-----
> From: Greg KH [mailto:greg@...ah.com]
> Sent: Tuesday, October 3, 2017 4:23 AM
> To: Limonciello, Mario <Mario_Limonciello@...l.com>
> Cc: dvhart@...radead.org; 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 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.
> 

Thanks for this feedback.  I'll look into this as an alternative.