saa7134: fix incorrect check to determine if no element is found in list

The bug is here: "if (dev == NULL)".

The list iterator value will *always* be set and non-NULL by
list_for_each_entry(), so it is incorrect to assume that the iterator
value will be NULL if the list is empty or no element is found in list.

Use a new value 'iter' as the list iterator, while use the old value
'req' as a dedicated pointer to point to the found element, which
1. can fix this bug, due to now 'req' is NULL only if it's not found.
2. do not need to change all the uses of 'req' after the loop.
3. can also limit the scope of the list iterator 'iter' *only inside*
   the traversal loop by simply declaring 'iter' inside the loop in the
   future, as usage of the iterator outside of the list_for_each_entry
   is considered harmful. https://lkml.org/lkml/2022/2/17/1032

Fixes: 4aabf6331f89c ("[PATCH] v4l: (951) Make saa7134-oss as a stand-alone module")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
---
 drivers/media/pci/saa7134/saa7134-alsa.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

> On 20. Mar 2022, at 03:57, Xiaomeng Tong <xiam0nd.tong@gmail.com> wrote:
> 
> The bug is here: "if (dev == NULL)".
> 
> The list iterator value will *always* be set and non-NULL by
> list_for_each_entry(), so it is incorrect to assume that the iterator
> value will be NULL if the list is empty or no element is found in list.
> 
> Use a new value 'iter' as the list iterator, while use the old value
> 'req' as a dedicated pointer to point to the found element, which
> 1. can fix this bug, due to now 'req' is NULL only if it's not found.
> 2. do not need to change all the uses of 'req' after the loop.
> 3. can also limit the scope of the list iterator 'iter' *only inside*
>   the traversal loop by simply declaring 'iter' inside the loop in the
>   future, as usage of the iterator outside of the list_for_each_entry
>   is considered harmful. https://lkml.org/lkml/2022/2/17/1032
> 
> Fixes: 4aabf6331f89c ("[PATCH] v4l: (951) Make saa7134-oss as a stand-alone module")
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
> ---
> drivers/media/pci/saa7134/saa7134-alsa.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/pci/saa7134/saa7134-alsa.c b/drivers/media/pci/saa7134/saa7134-alsa.c
> index fb24d2ed3621..4955f7e7c5bf 100644
> --- a/drivers/media/pci/saa7134/saa7134-alsa.c
> +++ b/drivers/media/pci/saa7134/saa7134-alsa.c
> @@ -1215,18 +1215,21 @@ static int alsa_device_exit(struct saa7134_dev *dev)
> static int saa7134_alsa_init(void)
> {
> 	struct saa7134_dev *dev = NULL;
> +	struct saa7134_dev *iter;
> 
> 	saa7134_dmasound_init = alsa_device_init;
> 	saa7134_dmasound_exit = alsa_device_exit;
> 
> 	pr_info("saa7134 ALSA driver for DMA sound loaded\n");
> 
> -	list_for_each_entry(dev, &saa7134_devlist, devlist) {
> -		if (dev->pci->device == PCI_DEVICE_ID_PHILIPS_SAA7130)
> +	list_for_each_entry(iter, &saa7134_devlist, devlist) {
> +		dev = iter;
> +
> +		if (iter->pci->device == PCI_DEVICE_ID_PHILIPS_SAA7130)
> 			pr_info("%s/alsa: %s doesn't support digital audio\n",
> -				dev->name, saa7134_boards[dev->board].name);
> +				iter->name, saa7134_boards[iter->board].name);
> 		else
> -			alsa_device_init(dev);
> +			alsa_device_init(iter);
> 	}
> 
> 	if (dev == NULL)

I could be wrong, but judging from the printed message "saa7134 ALSA: no saa7134 cards found"
and how the code is structured, I got the impression that the intention for this code was
to only execute if the list is empty. 

There is no break or anything so it's not looking for a specific element.

It might make more sense to simply replace this check with:
	if (list_empty(&saa7134_devlist))

> -- 
> 2.17.1
> 

	Jakob