Re: [PATCH] misc: always assign miscdevice to file->private_data in open()

[Martin Kepplinger]

Am 2014-10-09 17:50, schrieb Greg KH:
> On Thu, Oct 09, 2014 at 03:10:21PM +0200, Martin Kepplinger wrote:
>> Am 2014-10-08 15:43, schrieb Greg KH:
>>> On Wed, Oct 08, 2014 at 10:47:54AM +0200, Martin Kepplinger wrote:
>>>> As of now, a miscdevice driver has to provide an implementation of
>>>> the open() file operation if it wants to have misc_open() assign a
>>>> pointer to struct miscdevice to file->private_data for other file
>>>> operations to use (given the user calls open()).
>>>>
>>>> This leads to situations where a miscdevice driver that doesn't need
>>>> internal operations during open() has to implement open() that only
>>>> returns immediately, in order to use the data in private_data in other
>>>> fops.
>>>
>>> Yeah, that's messy, do we have any in-kernel misc drivers that do this?

yes, at least drivers/video/fbdev/pxa3xx-gcu.c , maybe more and I don't
know if others do the work theirselves.

An audit if changing to always-set-private_data breaks drivers should be
doable in a reasonable timeframe. I don't think there would be a
problem; it'd be good if others take a look aswell though.

>>>
>>>> This change provides consistent behaviour for miscdevice developers by
>>>> always providing the pointer in private_data. A driver's open() fop would,
>>>> of course, just overwrite it, when using private_data itself.
>>>>
>>>> Signed-off-by: Martin Kepplinger <martink@xxxxxxxxx>
>>>> ---
>>>> This is really only a question: Do I understand this correctly, and,
>>>> could this change then hurt any existing driver?
>>>
>>> I don't know, take a look at the existing ones and see please.
>>>
>>>> As a driver developer it took me a while to figure out what happens here,
>>>> and in my situation it would have been nice to just have this feature as
>>>> part of the miscdevice API. Possibly documented somewhere?
>>>
>>> Patches always accepted for documentation :)
>>
>> What would be a good place for this?
>> Documentation/driver-model/device.txt or
>> Documentation/filesystem/vfs.txt like so? I'm not sure.
> 
> There's no documentation for misc devices?  If not, just put it in
> kerneldoc format in the misc .c file.
> 
>> >From facd10cfa7539755e960dec8cc009934200e68ec Mon Sep 17 00:00:00 2001
>> From: Martin Kepplinger <martink@xxxxxxxxx>
>> Date: Thu, 9 Oct 2014 14:54:28 +0200
>> Subject: [PATCH] documentation: misc_open sets private_data for driver's
>>  open()
>>
>> Signed-off-by: Martin Kepplinger <martink@xxxxxxxxx>
>> ---
>>  Documentation/filesystems/vfs.txt |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/filesystems/vfs.txt
>> b/Documentation/filesystems/vfs.txt
>> index 61d65cc..06df9d9 100644
>> --- a/Documentation/filesystems/vfs.txt
>> +++ b/Documentation/filesystems/vfs.txt
>> @@ -869,7 +869,8 @@ otherwise noted.
>>  	done the way it is because it makes filesystems simpler to
>>  	implement. The open() method is a good place to initialize the
>>  	"private_data" member in the file structure if you want to point
>> -	to a device structure
>> +	to a device structure. In the case of "struct miscdevice", when
>> +	you implement open() this is done automatically.
> 
> No, no one will notice this in the vfs.txt file, and the vfs doesn't
> care about misc devices.
> 
>>>> misc_open() is called in any case, on open(). As long as miscdevice drivers
>>>> don't explicitly rely on private_data being NULL exactly IF they don't
>>>> implement an open() fop (which I wouldn't imagine), this would make things
>>>> even more convenient.
>>>
>>> I agree, but it would be great if you can audit the existing misc
>>> drivers to ensure we don't break anything with this change.  Can you do
>>> that please?
>>>
>>
>> I would grep -r "struct miscdevice" ./drivers/; and look at struct
>> file_operations of these results, see how their open() looks like, and
>> where they assign something to private_data.
>>
>> If you have an idea for a script that lists all relevant files for me,
>> please tell me.
> 
> You just came up with one there, that should be a good start.
> 
> good luck,
> 
> greg k-h
> 

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