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

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Mon, 23 Oct 2023 13:51:04 +0200
Nuno Sá <noname.nuno@gmail.com> wrote:

> On Mon, 2023-10-23 at 10:53 +0100, Jonathan Cameron wrote:
> > On Mon, 23 Oct 2023 10:55:56 +0200
> > Nuno Sá <noname.nuno@gmail.com> wrote:
> >   
> > > 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. 
> > >   
> > 
> > I fully agree it's ugly and a little unintuitive but I don't see a way an
> > "lvalue"
> > can work work cleanly (due to magic types under the hood) and I suspect we
> > will
> > have to get used to this pattern.
> >   
> 
> Yeah, given the games being played with the constructor and the _lock definition
> so we return the variable we want to "release" I agree it would be hard to have
> anything clean and likely even harder to read (more than it is already :)).
> 
> However, I think users of the cleanup.h stuff could build on top of it... For
> instance, in our case we could have something like:
> 
> #define IIO_CLAIM_DIRECT(dev) 
> 	int __ret = 0;
> 	CLASS(iio_claim_direct, claimed_dev)(dev);
> 	if ((IS_ERR(claimed_dev))
> 		__ret = PTR_ERR(claimed_dev);
> 	__ret

Maybe, but we'll have to deal with people perpetually trying to brackets around
the complex macro... 
> 
> Then we could use it in the same way as before... Or at the very least I would
> simply make it a bit more readable for IIO (rather than the plain CLASS() call):
> 
> #define IIO_CLAIM_DIRECT(claimed_dev, dev)
> 	CLASS(iio_claim_direct, claimed_dev)(dev)
> 
> Just some thoughts...

Maybe. I'm not sure it's worth it though. This class stuff is
odd and I don't really want to hid it from people too much.

Sometimes better just to make people deal with the ugly on basis they hopefully
go figure out what it is doing.

Jonathan

> 
> - Nuno Sá
> 
>