Re: [PATCH v3 03/15] iio: inkern: only return error codes in iio_channel_get_*() APIs

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 15 Jul 2022 14:28:51 +0200
Nuno Sá <nuno.sa@analog.com> wrote:

> APIs like of_iio_channel_get_by_name() and of_iio_channel_get_all() were
> returning a mix of NULL and pointers with NULL being the way to
> "notify" that we should do a "system" lookup for channels. This make
> it very confusing and prone to errors as commit 9f63cc0921ec
> ("iio: inkern: fix return value in devm_of_iio_channel_get_by_name()")
> proves. On top of this, patterns like 'if (channel != NULL) return
> channel' were being used where channel could actually be an error code
> which makes the code hard to read.
> 
> This change also makes some functional changes on how errors were being
> handled. In the original behavior, even if we get an error like '-ENOMEM',
> we still continue with the search. We should only continue to lookup for
> the channel when it makes sense to do so. Hence, the main error handling
> in 'of_iio_channel_get_by_name()' is changed to the following logic:
> 
>  * If a channel 'name' is provided and we do find it via
> 'io-channel-names', we should be able to get it. If we get any error,
> we should not proceed with the lookup. Moreover, we should return an error
> so that callers won't proceed with a system lookup.
>  * If a channel 'name' is provided and we cannot find it ('index < 0'),
> 'of_parse_phandle_with_args()' is expected to fail with '-EINVAL'. Hence,
> we should only continue if we get that error.
>  * If a channel 'name' is not provided we should only carry on with the
> search if 'of_parse_phandle_with_args()' returns '-ENOENT'.
> 
> Also note that a system channel lookup is only done if the returned
> error code (from 'of_iio_channel_get_by_name()' or
> 'of_iio_channel_get_all()' is -ENODEV.
> 
> Signed-off-by: Nuno Sá <nuno.sa@analog.com>
> Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>

Hi Nuno,

Sorry for delay on getting to this.  Sometimes the most useful work
on the core gets queued up at the back of my review queue because it's harder
than the average driver :)

Anyhow, only totally trivial stuff in here from me.
If you don't respin because of other comments, I can clean these up
whilst applying.

Thanks,

Jonathan

> ---
>  drivers/iio/inkern.c | 54 +++++++++++++++++++++++++++++++-------------
>  1 file changed, 38 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> index 87fd2a0d44f2..c6f1cfe09bd3 100644
> --- a/drivers/iio/inkern.c
> +++ b/drivers/iio/inkern.c
> @@ -214,7 +214,7 @@ static struct iio_channel *of_iio_channel_get(struct device_node *np, int index)
>  struct iio_channel *of_iio_channel_get_by_name(struct device_node *np,
>  					       const char *name)
>  {
> -	struct iio_channel *chan = NULL;
> +	struct iio_channel *chan;
>  
>  	/* Walk up the tree of devices looking for a matching iio channel */
>  	while (np) {
> @@ -231,13 +231,33 @@ struct iio_channel *of_iio_channel_get_by_name(struct device_node *np,
>  							 name);
>  		chan = of_iio_channel_get(np, index);
>  		if (!IS_ERR(chan) || PTR_ERR(chan) == -EPROBE_DEFER)
> -			break;
> -		else if (name && index >= 0) {
> -			pr_err("ERROR: could not get IIO channel %pOF:%s(%i)\n",
> -				np, name ? name : "", index);
> -			return NULL;
> +			return chan;
> +		if (name) {
> +			if (index >= 0) {
> +				pr_err("ERROR: could not get IIO channel %pOF:%s(%i)\n",
> +				       np, name, index);
> +				/*
> +				 * In this case, we found 'name' in 'io-channel-names'
> +				 * but somehow we still fail so that we should not proceed
> +				 * with any other lookup. Hence, explicitly return -EINVAL
> +				 * (maybe not the better error code) so that the caller
> +				 * won't do a system lookup.
> +				 */
> +				return ERR_PTR(-EINVAL);
> +			}

Trivial, but wrong comment syntax for IIO + inconsistent with surrounding comments.

> +			/* If index < 0, then of_parse_phandle_with_args() fails
> +			 * with -EINVAL which is expected. We should not proceed
> +			 * if we get any other error.
> +			 */
> +			if (PTR_ERR(chan) != -EINVAL)
> +				return chan;
> +		} else if (PTR_ERR(chan) != -ENOENT) {
> +			/*
> +			 * if !name, then we should only proceed the lookup if
> +			 * of_parse_phandle_with_args() returns -ENOENT.
> +			 */
> +			return chan;
>  		}
> -

Try to avoid noise like this + I like this whitespace :)

>  		/*
>  		 * No matching IIO channel found on this node.
>  		 * If the parent node has a "io-channel-ranges" property,
> @@ -245,10 +265,10 @@ struct iio_channel *of_iio_channel_get_by_name(struct device_node *np,
>  		 */
>  		np = np->parent;
>  		if (np && !of_get_property(np, "io-channel-ranges", NULL))
> -			return NULL;
> +			return ERR_PTR(-ENODEV);
>  	}
>  
> -	return chan;
> +	return ERR_PTR(-ENODEV);
>  }
>  EXPORT_SYMBOL_GPL(of_iio_channel_get_by_name);
>  
> @@ -267,8 +287,8 @@ static struct iio_channel *of_iio_channel_get_all(struct device *dev)
>  			break;
>  	} while (++nummaps);
>  
> -	if (nummaps == 0)	/* no error, return NULL to search map table */
> -		return NULL;
> +	if (nummaps == 0)
> +		return ERR_PTR(-ENODEV);
>  
>  	/* NULL terminated array to save passing size */
>  	chans = kcalloc(nummaps + 1, sizeof(*chans), GFP_KERNEL);
> @@ -295,7 +315,7 @@ static struct iio_channel *of_iio_channel_get_all(struct device *dev)
>  
>  static inline struct iio_channel *of_iio_channel_get_all(struct device *dev)
>  {
> -	return NULL;
> +	return ERR_PTR(-ENODEV);
>  }
>  
>  #endif /* CONFIG_OF */
> @@ -362,7 +382,7 @@ struct iio_channel *iio_channel_get(struct device *dev,
>  	if (dev) {
>  		channel = of_iio_channel_get_by_name(dev->of_node,
>  						     channel_name);
> -		if (channel != NULL)
> +		if (!IS_ERR(channel) || PTR_ERR(channel) != -ENODEV)
>  			return channel;
>  	}
>  
> @@ -412,8 +432,6 @@ struct iio_channel *devm_of_iio_channel_get_by_name(struct device *dev,
>  	channel = of_iio_channel_get_by_name(np, channel_name);
>  	if (IS_ERR(channel))
>  		return channel;
> -	if (!channel)
> -		return ERR_PTR(-ENODEV);
>  
>  	ret = devm_add_action_or_reset(dev, devm_iio_channel_free, channel);
>  	if (ret)
> @@ -436,7 +454,11 @@ struct iio_channel *iio_channel_get_all(struct device *dev)
>  		return ERR_PTR(-EINVAL);
>  
>  	chans = of_iio_channel_get_all(dev);
> -	if (chans)
> +	/*
> +	 * We only want to carry on if the error is -ENODEV.  Anything else
> +	 * should be reported up the stack.
> +	 */
> +	if (!IS_ERR(chans) || PTR_ERR(chans) != -ENODEV)
>  		return chans;
>  
>  	name = dev_name(dev);