linux-kernel - RE: [RFC 2/4] media: videobuf2: Replace bufs array by a list

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Date:   Mon, 13 Mar 2023 23:16:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Laurent Pinchart' <laurent.pinchart@...asonboard.com>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>
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Subject: RE: [RFC 2/4] media: videobuf2: Replace bufs array by a list

From: Laurent Pinchart
> Sent: 13 March 2023 18:12
> 
> Hi Benjamin,
> 
> Thank you for the patch.
> 
> On Mon, Mar 13, 2023 at 02:59:14PM +0100, Benjamin Gaignard wrote:
> > Replacing bufs array by a list allows to remove the 32 buffers
> > limit per queue.

Is the limit actually a problem?
Arrays of pointers have locking and caching advantages over
linked lists.

...
> > @@ -1239,8 +1242,12 @@ static inline void vb2_clear_last_buffer_dequeued(struct vb2_queue *q)
> >  static inline struct vb2_buffer *vb2_get_buffer(struct vb2_queue *q,
> >  						unsigned int index)
> >  {
> > -	if (index < q->num_buffers)
> > -		return q->bufs[index];
> > +	struct vb2_buffer *vb;
> > +
> > +	list_for_each_entry(vb, &q->allocated_bufs, allocated_entry)
> > +		if (vb->index == index)
> > +			return vb;
> > +
> >  	return NULL;

You really don't want to be doing that....

There are schemes for unbounded arrays.
Scanning a linked list isn't a very good one.

	David

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