Re: [PATCH v1 1/5] media: mc: entity: Add pad iterator for media_pipeline

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

On 15/12/2022 14:48, Laurent Pinchart wrote:
> Hi Tomi,
> 
> On Thu, Dec 15, 2022 at 02:33:43PM +0200, Tomi Valkeinen wrote:
>> On 12/12/2022 16:16, Laurent Pinchart wrote:
>>> Add a media_pipeline_for_each_pad() macro to iterate over pads in a
>>> pipeline. This should be used by driver as a replacement of the
>>> media_graph_walk API, as iterating over the media_pipeline uses the
>>> cached list of pads and is thus more efficient.
>>>
>>> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>>> ---
>>>    drivers/media/mc/mc-entity.c | 18 ++++++++++++++++++
>>>    include/media/media-entity.h | 29 +++++++++++++++++++++++++++++
>>>    2 files changed, 47 insertions(+)
>>>
>>> diff --git a/drivers/media/mc/mc-entity.c b/drivers/media/mc/mc-entity.c
>>> index b8bcbc734eaf..70df2050951c 100644
>>> --- a/drivers/media/mc/mc-entity.c
>>> +++ b/drivers/media/mc/mc-entity.c
>>> @@ -932,6 +932,24 @@ __must_check int media_pipeline_alloc_start(struct media_pad *pad)
>>>    }
>>>    EXPORT_SYMBOL_GPL(media_pipeline_alloc_start);
>>>    
>>> +struct media_pad *
>>> +__media_pipeline_pad_iter_next(struct media_pipeline *pipe,
>>> +			       struct media_pipeline_pad_iter *iter,
>>> +			       struct media_pad *pad)
>>> +{
>>> +	if (!pad)
>>> +		iter->cursor = pipe->pads.next;
>>> +
>>> +	if (iter->cursor == &pipe->pads)
>>> +		return NULL;
>>> +
>>> +	pad = list_entry(iter->cursor, struct media_pipeline_pad, list)->pad;
>>> +	iter->cursor = iter->cursor->next;
>>> +
>>> +	return pad;
>>> +}
>>> +EXPORT_SYMBOL_GPL(__media_pipeline_pad_iter_next);
>>> +
>>>    /* -----------------------------------------------------------------------------
>>>     * Links management
>>>     */
>>> diff --git a/include/media/media-entity.h b/include/media/media-entity.h
>>> index 85ed08ddee9d..e881e483b550 100644
>>> --- a/include/media/media-entity.h
>>> +++ b/include/media/media-entity.h
>>> @@ -130,6 +130,15 @@ struct media_pipeline_pad {
>>>    	struct media_pad *pad;
>>>    };
>>>    
>>> +/**
>>> + * struct media_pipeline_pad_iter - Iterator for media_pipeline_for_each_pad
>>> + *
>>> + * @cursor: The current element
>>> + */
>>> +struct media_pipeline_pad_iter {
>>> +	struct list_head *cursor;
>>> +};
>>> +
>>
>> Is there any reason to have this iter struct in a public header, vs.
>> having it in mc-entity.c?
> 
> It has to be instantiated on the stack by the user of the
> media_pipeline_for_each_pad() macro. A typical usage is
> 
> 	struct media_pipeline_pad_iter iter;
> 	struct media_pad *pad
> 
> 	media_pipeline_for_each_pad(pipe, &iter, pad) {
> 		...
> 	};
> 
> Note how iter is not a pointer.

Ah, right.

>>>    /**
>>>     * struct media_link - A link object part of a media graph.
>>>     *
>>> @@ -1163,6 +1172,26 @@ void media_pipeline_stop(struct media_pad *pad);
>>>     */
>>>    void __media_pipeline_stop(struct media_pad *pad);
>>>    
>>> +struct media_pad *
>>> +__media_pipeline_pad_iter_next(struct media_pipeline *pipe,
>>> +			       struct media_pipeline_pad_iter *iter,
>>> +			       struct media_pad *pad);
>>> +
>>> +/**
>>> + * media_pipeline_for_each_pad - Iterate on all pads in a media pipeline
>>> + * @pipe: The pipeline
>>> + * @iter: The iterator (struct media_pipeline_pad_iter)
>>> + * @pad: The iterator pad
>>
>> If I understand this correctly, both iter and pad are just variables the
>> macro will use. In other words, they are not used to pass any values.
>>
>> Would it be better to declare those variables in the macro itself? Well,
>> that has its downsides. But perhaps at least clarify in the doc that
>> they are only variables used by the loop, and do not need to be initialized.
> 
> Now that the kernel uses C99, I suppose we could make the pad variable
> locally declared within the loop:
> 
> #define media_pipeline_for_each_pad(pipe, pad)				\
> 	for (struct media_pipeline_pad *pad = __media_pipeline_pad_iter_next((pipe), iter, NULL);	\
> 	     pad != NULL;						\
> 	     pad = __media_pipeline_pad_iter_next((pipe), iter, pad))
> 
> Hiding the iter variable would be more difficult, as I don't think you
> can declare multiple variables of different types.
> 
> I'm a bit concerned about backporting though, so I'd rather not do this
> in this patch, but on top.

I was thinking about using a do {} while(0) around the for loop to 
declare the variables, but.. of course that can't be used here. One 
shouldn't do reviews when one has a cold =).

  Tomi