linux-kernel - Re: [Cocci] Proposal for a new checkpatch check; matching _set_drvdata() & _get

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Message-ID: <4af50412-a22f-4ca1-adb0-d732438c6669@metafoo.de>
Date:   Fri, 20 Nov 2020 14:16:00 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Alexandru Ardelean <ardeleanalex@...il.com>,
        Julia Lawall <julia.lawall@...ia.fr>
Cc:     Joe Perches <joe@...ches.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Robo Bot <apw@...onical.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        LKML <linux-kernel@...r.kernel.org>,
        cocci <cocci@...teme.lip6.fr>
Subject: Re: [Cocci] Proposal for a new checkpatch check; matching
 _set_drvdata() & _get_drvdata()

On 11/20/20 12:54 PM, Alexandru Ardelean wrote:
> On Fri, Nov 20, 2020 at 12:47 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
>>
>>
>> On Thu, 19 Nov 2020, Joe Perches wrote:
>>
>>> On Thu, 2020-11-19 at 17:16 +0200, Andy Shevchenko wrote:
>>>> On Thu, Nov 19, 2020 at 4:09 PM Alexandru Ardelean
>>>> <ardeleanalex@...il.com> wrote:
>>>>> Hey,
>>>>>
>>>>> So, I stumbled on a new check that could be added to checkpatch.
>>>>> Since it's in Perl, I'm reluctant to try it.
>>>>>
>>>>> Seems many drivers got to a point where they now call (let's say)
>>>>> spi_set_drvdata(), but never access that information via
>>>>> spi_get_drvdata().
>>>>> Reasons for this seem to be:
>>>>> 1. They got converted to device-managed functions and there is no
>>>>> longer a remove hook to require the _get_drvdata() access
>>>>> 2. They look like they were copied from a driver that had a
>>>>> _set_drvdata() and when the code got finalized, the _set_drvdata() was
>>>>> omitted
>>>>>
>>>>> There are a few false positives that I can notice at a quick look,
>>>>> like the data being set via some xxx_set_drvdata() and retrieved via a
>>>>> dev_get_drvdata().
>>>> I can say quite a few. And this makes a difference.
>>>> So, basically all drivers that are using PM callbacks would rather use
>>>> dev_get_drvdata() rather than bus specific.
>>>>
>>>>> I think checkpatch reporting these as well would be acceptable simply
>>>>> from a reviewability perspective.
>>>>>
>>>>> I did a shell script to quickly check these. See below.
>>>>> It's pretty badly written but it is enough for me to gather a list.
>>>>> And I wrote it in 5 minutes :P
>>>>> I initially noticed this in some IIO drivers, and then I suspected
>>>>> that this may be more widespread.
>>>> It seems more suitable for coccinelle.
>>> To me as well.
>> To me as well, since it seems to involve nonlocal information.
>>
>> I'm not sure to understand the original shell script. Is there
>> something interesting about pci_set_drvdata?
> Ah, it's a stupid script I wrote in 5 minutes, so I did not bother to
> make things smart.
> In the text-matching I did in shell, there are some entries that come
> from comments and docs.
> It's only about 3-4 entries, so I just did a visual/manual ignore.
>
> In essence:
> The script searches for all strings that contain _set_drvdata.
> The separators are whitespace.
> It creates a list of all  xxxx_set_drvdata functions.
> For each xxxx_set_drvdata function:
>      It checks all files that have a xxxx_set_drvdata entry, but no
> xxxx_get_drvdata
>
> I piped this output into a file and started manually checking the drivers.
> There is one [I forget which function] that is xxxx_set_drvdata() but
> equivalent is xxxx_drvdata()
>
> As Andy said, some precautions must be taken in places where
> xxxx_set_drvdata() is called but dev_get_drvdata() is used.
> Cases like PM suspend/resume calls.
> And there may be some cases outside this context.
>
Doing something like this with coccinelle is fairly easy.

But I'd be very cautious about putting such a script into the kernel. It 
will result in too many false positive drive-by patches. Such a script 
will not detect cases such as:

  * Driver is split over multiple files. One file does 
..._set_drvdata(), another does ..._get_drvdata().

  * Framework uses drvdata to exchange data with the driver. E.g driver 
is expected to call set_drvdata() and then the framework uses 
get_drvdata() to retrieve the data. This is not a very good pattern, but 
there are some palces int he kernel where this is used. I believe for 
example V4L2 uses this.

- Lars