Re: [RFC PATCH 1/8] iio: locking: introduce __cleanup() based direct mode claiming infrastructure

["Nuno Sá" <noname.nuno@gmail.com>]

On Sun, 2023-10-22 at 16:10 -0500, David Lechner wrote:
> On Sun, Oct 22, 2023 at 10:47 AM Jonathan Cameron <jic23@kernel.org> wrote:
> > 
> > From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > 
> > Allows use of:
> > 
> >         CLASS(iio_claim_direct, claimed_dev)(indio_dev);
> >         if (IS_ERR(claimed_dev))
> >                 return PTR_ERR(claimed_dev);
> > 
> >         st = iio_priv(claimed_dev);
> > 
> > to automatically call iio_device_release_direct_mode() based on scope.
> > Typically seen in combination with local device specific locks which
> > are already have automated cleanup options via guard(mutex)(&st->lock)
> > and scoped_guard().  Using both together allows most error handling to
> > be automated.
> > 
> > Note that whilst this pattern results in a struct iio_dev *claimed_dev
> > that can be used, it is not necessary to do so as long as that pointer
> > has been checked for errors as in the example.
> > 
> > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > ---
> >  drivers/iio/industrialio-core.c |  4 ++++
> >  include/linux/iio/iio.h         | 25 +++++++++++++++++++++++++
> >  2 files changed, 29 insertions(+)
> > 
> > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-
> > core.c
> > index c77745b594bd..93bfad105eb5 100644
> > --- a/drivers/iio/industrialio-core.c
> > +++ b/drivers/iio/industrialio-core.c
> > @@ -2065,6 +2065,10 @@ EXPORT_SYMBOL_GPL(iio_device_claim_direct_mode);
> >   */
> >  void iio_device_release_direct_mode(struct iio_dev *indio_dev)
> >  {
> > +       /* Auto cleanup can result in this being called with an ERR_PTR */
> > +       if (IS_ERR(indio_dev))
> > +               return;
> > +
> >         mutex_unlock(&to_iio_dev_opaque(indio_dev)->mlock);
> >  }
> >  EXPORT_SYMBOL_GPL(iio_device_release_direct_mode);
> > diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> > index d0ce3b71106a..11c42170fda1 100644
> > --- a/include/linux/iio/iio.h
> > +++ b/include/linux/iio/iio.h
> > @@ -9,6 +9,7 @@
> > 
> >  #include <linux/device.h>
> >  #include <linux/cdev.h>
> > +#include <linux/cleanup.h>
> >  #include <linux/slab.h>
> >  #include <linux/iio/types.h>
> >  /* IIO TODO LIST */
> > @@ -644,6 +645,30 @@ int __devm_iio_device_register(struct device *dev,
> > struct iio_dev *indio_dev,
> >  int iio_push_event(struct iio_dev *indio_dev, u64 ev_code, s64 timestamp);
> >  int iio_device_claim_direct_mode(struct iio_dev *indio_dev);
> >  void iio_device_release_direct_mode(struct iio_dev *indio_dev);
> > +/*
> > + * Auto cleanup version of iio_device_claim_direct_mode,
> > + *
> > + *     CLASS(iio_claim_direct, claimed_dev)(indio_dev);
> > + *     if (IS_ERR(claimed_dev))
> > + *             return PTR_ERR(claimed_dev);
> > + *
> > + *     st = iio_priv(claimed_dev);
> > + *     ....
> > + */
> > +DEFINE_CLASS(iio_claim_direct, struct iio_dev *,
> > +            iio_device_release_direct_mode(_T),
> > +            ({
> > +                       struct iio_dev *dev;
> > +                       int d = iio_device_claim_direct_mode(_T);
> > +
> > +                       if (d < 0)
> > +                               dev = ERR_PTR(d);
> > +                       else
> > +                               dev = _T;
> > +                       dev;
> > +            }),
> > +            struct iio_dev *_T);
> > +
> >  int iio_device_claim_buffer_mode(struct iio_dev *indio_dev);
> >  void iio_device_release_buffer_mode(struct iio_dev *indio_dev);
> > 
> > --
> > 2.42.0
> > 
> 
> What is the benefit of exposing `claimed_dev` rather than just the int
> return value? It seems like it just makes more noise in the error
> check.
> 

I don't really have a very strong opinion on this but what I really don't like
much is the pattern:

CLASS(type, ret), where the return value is an argument of the macro... It would
be nice if we could just make it like:

ret = guard(type)(...); //or any other variation of the guard() macro
if (ret) 
	return ret;

the above could also be an error pointer or even have one variation of each. but
yeah, that likely means changing the cleanup.h file and that might be out of
scope for Jonathan's patch series. 

- Nuno Sá