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Subject: Re: [PATCH v6 20/24] iio: buffer: add ioctl() to support opening
 extra buffers for IIO device
To:     Alexandru Ardelean <alexandru.ardelean@analog.com>,
        linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org
Cc:     Michael.Hennerich@analog.com, jic23@kernel.org, nuno.sa@analog.com,
        dragos.bogdan@analog.com
References: <20210215104043.91251-1-alexandru.ardelean@analog.com>
 <20210215104043.91251-21-alexandru.ardelean@analog.com>
From:   Lars-Peter Clausen <lars@metafoo.de>
Message-ID: <d0d2aece-e333-bd87-ad0a-60cf3e387ae4@metafoo.de>
Date:   Sun, 28 Feb 2021 08:57:00 +0100
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On 2/15/21 11:40 AM, Alexandru Ardelean wrote:
> [...]
>   /**
>    * iio_buffer_wakeup_poll - Wakes up the buffer waitqueue
>    * @indio_dev: The IIO device
> @@ -1343,6 +1371,96 @@ static void iio_buffer_unregister_legacy_sysfs_groups(struct iio_dev *indio_dev)
>   	kfree(iio_dev_opaque->legacy_scan_el_group.attrs);
>   }
>   
> [...]
> +static long iio_device_buffer_getfd(struct iio_dev *indio_dev, unsigned long arg)
> +{
> +	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
> +	int __user *ival = (int __user *)arg;
> +	struct iio_dev_buffer_pair *ib;
> +	struct iio_buffer *buffer;
> +	int fd, idx, ret;
> +
> +	if (copy_from_user(&idx, ival, sizeof(idx)))
> +		return -EFAULT;

If we only want to pass an int, we can pass that directly, no need to 
pass it as a pointer.

int fd = arg;

> +
> +	if (idx >= iio_dev_opaque->attached_buffers_cnt)
> +		return -ENODEV;
> +
> +	iio_device_get(indio_dev);
> +
> +	buffer = iio_dev_opaque->attached_buffers[idx];
> +
> +	if (test_and_set_bit(IIO_BUSY_BIT_POS, &buffer->flags)) {
> +		ret = -EBUSY;
> +		goto error_iio_dev_put;
> +	}
> +
> +	ib = kzalloc(sizeof(*ib), GFP_KERNEL);
> +	if (!ib) {
> +		ret = -ENOMEM;
> +		goto error_clear_busy_bit;
> +	}
> +
> +	ib->indio_dev = indio_dev;
> +	ib->buffer = buffer;
> +
> +	fd = anon_inode_getfd("iio:buffer", &iio_buffer_chrdev_fileops,
> +			      ib, O_RDWR | O_CLOEXEC);

I wonder if we need to allow to pass flags, like e.g. O_NONBLOCK.

Something like 
https://elixir.bootlin.com/linux/latest/source/fs/signalfd.c#L288

> +	if (fd < 0) {
> +		ret = fd;
> +		goto error_free_ib;
> +	}
> +
> +	if (copy_to_user(ival, &fd, sizeof(fd))) {
> +		put_unused_fd(fd);
> +		ret = -EFAULT;
> +		goto error_free_ib;
> +	}

Here we copy back the fd, but also return it. Just return is probably 
enough.

> +
> +	return fd;
> +
> +error_free_ib:
> +	kfree(ib);
> +error_clear_busy_bit:
> +	clear_bit(IIO_BUSY_BIT_POS, &buffer->flags);
> +error_iio_dev_put:
> +	iio_device_put(indio_dev);
> +	return ret;
> +}
> [...]
> diff --git a/include/linux/iio/iio-opaque.h b/include/linux/iio/iio-opaque.h
> index b6ebc04af3e7..32addd5e790e 100644
> --- a/include/linux/iio/iio-opaque.h
> +++ b/include/linux/iio/iio-opaque.h
> @@ -9,6 +9,7 @@
>    * @event_interface:		event chrdevs associated with interrupt lines
>    * @attached_buffers:		array of buffers statically attached by the driver
>    * @attached_buffers_cnt:	number of buffers in the array of statically attached buffers
> + * @buffer_ioctl_handler:	ioctl() handler for this IIO device's buffer interface
>    * @buffer_list:		list of all buffers currently attached
>    * @channel_attr_list:		keep track of automatically created channel
>    *				attributes
> @@ -28,6 +29,7 @@ struct iio_dev_opaque {
>   	struct iio_event_interface	*event_interface;
>   	struct iio_buffer		**attached_buffers;
>   	unsigned int			attached_buffers_cnt;
> +	struct iio_ioctl_handler	*buffer_ioctl_handler;

Can we just embedded this struct so we do not have to 
allocate/deallocate it?

>   	struct list_head		buffer_list;
>   	struct list_head		channel_attr_list;
>   	struct attribute_group		chan_attr_group;