Re: [Cocci] Proposal for a new checkpatch check; matching _set_drvdata() & _get_drvdata()

[Alexandru Ardelean]

On Fri, Nov 20, 2020 at 3:16 PM Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
>
> On 11/20/20 12:54 PM, Alexandru Ardelean wrote:
> > On Fri, Nov 20, 2020 at 12:47 PM Julia Lawall <julia.lawall@xxxxxxxx> 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@xxxxxxxxx> 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:

Yeah, it would probably be a good idea to start with a few simple
checks, then scale it.
If we go for the existing _set_drvdata() / _get_drvdata() pair checks,
there is a risk of breaking things.

>
>   * 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
>