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 09:48:05 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Darren Hart <dvhart@...radead.org>
Cc:     Greg KH <greg@...ah.com>,
        Mario Limonciello <mario.limonciello@...l.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, quasisec@...gle.com,
        Pali Rohár <pali.rohar@...il.com>
Subject: Re: [PATCH v3 4/8] platform/x86: wmi: create character devices when
 requested by drivers

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