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SubjectRe: [PATCH v5 5/6] iio: core: add simple centralized mechanism for ioctl() handlers
On Sun, 26 Apr 2020 11:10:37 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Sun, 26 Apr 2020 10:38:16 +0300
> Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
>
> > The aim of this is to reduce the organization violation of ioctl() calls in
> > IIO core. Currently, since the chardev is split across files, event ioctl()
> > calls need to be called in buffer ioctl() calls.
> >
> > The 'industrialio-core.c' file will provide a 'iio_device_ioctl()' which
> > will iterate over a list of ioctls registered with the IIO device. These
> > can be event ioctl() or buffer ioctl() calls, or something else.
> > This is needed, since there is currently one chardev per IIO device and
> > that is used for both event handling and reading from the buffer.
> >
> > Each ioctl() will have to return a IIO_IOCTL_UNHANDLED code (which is
> > positive 1), if the ioctl() did not handle the call in any. This eliminates
> > any potential ambiguities; if we were to have used error codes it would
> > have been uncertain whether they were actual errors, or whether
> > the registered ioctl() doesn't service the command.
> >
> > If any ioctl() returns 0, it was considered that it was serviced
> > successfully and the loop will exit.
> >
> > One assumption for all registered ioctl() handlers is that they are
> > statically allocated, so the iio_device_unregister() which just remove all
> > of them from the device's ioctl() handler list.
> >
> > Also, something that is a bit hard to do [at this point] and may not be
> > worth the effort of doing, is to check whether registered ioctl()
> > calls/commands overlap. This should be unlikely to happen, and should get
> > caught at review time. Though, new ioctl() calls would likely not be added
> > too often.
> >
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
>
> A question on locking inline. Otherwise this looks fairly clean and simple
> to me.
>
> Jonathan
>
>
> > ---
> > drivers/iio/iio_core.h | 14 ++++++++++++++
> > drivers/iio/industrialio-core.c | 33 +++++++++++++++++++++++++++++++++
> > include/linux/iio/iio.h | 2 ++
> > 3 files changed, 49 insertions(+)
> >
> > diff --git a/drivers/iio/iio_core.h b/drivers/iio/iio_core.h
> > index a527a66be9e5..34c3e19229d8 100644
> > --- a/drivers/iio/iio_core.h
> > +++ b/drivers/iio/iio_core.h
> > @@ -17,6 +17,20 @@ struct iio_dev;
> >
> > extern struct device_type iio_device_type;
> >
> > +#define IIO_IOCTL_UNHANDLED 1
> > +struct iio_ioctl_handler {
> > + struct list_head entry;
> > + long (*ioctl)(struct iio_dev *indio_dev, struct file *filp,
> > + unsigned int cmd, unsigned long arg);
> > +};
> > +
> > +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> > + unsigned int cmd, unsigned long arg);
> > +
> > +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> > + struct iio_ioctl_handler *h);
> > +void iio_device_ioctl_handler_unregister(struct iio_ioctl_handler *h);
> > +
> > int __iio_add_chan_devattr(const char *postfix,
> > struct iio_chan_spec const *chan,
> > ssize_t (*func)(struct device *dev,
> > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> > index aec585cc8453..79e8fa8ff70b 100644
> > --- a/drivers/iio/industrialio-core.c
> > +++ b/drivers/iio/industrialio-core.c
> > @@ -1531,6 +1531,7 @@ struct iio_dev *iio_device_alloc(int sizeof_priv)
> > }
> > dev_set_name(&dev->dev, "iio:device%d", dev->id);
> > INIT_LIST_HEAD(&dev->buffer_list);
> > + INIT_LIST_HEAD(&dev->ioctl_handlers);
> >
> > return dev;
> > }
> > @@ -1584,6 +1585,33 @@ struct iio_dev *devm_iio_device_alloc(struct device *dev, int sizeof_priv)
> > }
> > EXPORT_SYMBOL_GPL(devm_iio_device_alloc);
> >
> > +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> > + struct iio_ioctl_handler *h)
> > +{
> > + /* this assumes that all ioctl() handlers are statically allocated */
> > + list_add_tail(&h->entry, &indio_dev->ioctl_handlers);
> > +}
> > +
> > +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> > + unsigned int cmd, unsigned long arg)
> > +{
> > + struct iio_ioctl_handler *h;
> > + int ret;
> > +
> > + if (!indio_dev->info)
> > + return -ENODEV;
> > +
> > + list_for_each_entry(h, &indio_dev->ioctl_handlers, entry) {
> > + ret = h->ioctl(indio_dev, filp, cmd, arg);
> > + if (ret == 0)
> > + return 0;
> > + if (ret != IIO_IOCTL_UNHANDLED)
> > + return ret;
> > + }
> > +
> > + return -EINVAL;
> > +}
> > +
> > static int iio_check_unique_scan_index(struct iio_dev *indio_dev)
> > {
> > int i, j;
> > @@ -1695,6 +1723,8 @@ EXPORT_SYMBOL(__iio_device_register);
> > **/
> > void iio_device_unregister(struct iio_dev *indio_dev)
> > {
> > + struct iio_ioctl_handler *h, *t;
> > +
> > if (indio_dev->chrdev)
> > cdev_device_del(indio_dev->chrdev, &indio_dev->dev);
> > else
> > @@ -1708,6 +1738,9 @@ void iio_device_unregister(struct iio_dev *indio_dev)
> >
> > iio_disable_all_buffers(indio_dev);
> >
> > + list_for_each_entry_safe(h, t, &indio_dev->ioctl_handlers, entry)
> > + list_del(&h->entry);
> > +
>
> Is there any chance anything is walking that list whilst we are deleting it?
> I think this needs to happen under a lock as does the walk.

We may want to use the rcu methods to make the walk and delete safe whilst
avoiding heavy weight locking on the read path

linux/rculist.h

I've not thought about this in much depth though so take that advice as
a vague suggestion and nothing more!


>
> > indio_dev->info = NULL;
> >
> > iio_device_wakeup_eventset(indio_dev);
> > diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> > index 52992be44e9e..b6ca8d85629e 100644
> > --- a/include/linux/iio/iio.h
> > +++ b/include/linux/iio/iio.h
> > @@ -488,6 +488,7 @@ struct iio_buffer_setup_ops {
> > * @currentmode: [DRIVER] current operating mode
> > * @dev: [DRIVER] device structure, should be assigned a parent
> > * and owner
> > + * @ioctl_handlers: [INTERN] list of registered ioctl handlers
> > * @event_interface: [INTERN] event chrdevs associated with interrupt lines
> > * @buffer: [DRIVER] any buffer present
> > * @buffer_list: [INTERN] list of all buffers currently attached
> > @@ -529,6 +530,7 @@ struct iio_dev {
> > int modes;
> > int currentmode;
> > struct device dev;
> > + struct list_head ioctl_handlers;
> >
> > struct iio_event_interface *event_interface;
> >
>

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