Re: [PATCH v2 4/8] v4l: vsp1: Use reference counting for fragments

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Kieran,

Thank you for the patch.

On Monday 14 Aug 2017 16:13:27 Kieran Bingham wrote:
> Extend the display list body with a reference count, allowing bodies to
> be kept as long as a reference is maintained. This provides the ability
> to keep a cached copy of bodies which will not change, so that they can
> be re-applied to multiple display lists.
> 
> Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
> 
> ---
> This could be squashed into the fragment update code, but it's not a
> straightforward squash as the refcounts will affect both:
>   v4l: vsp1: Provide a fragment pool
> and
>   v4l: vsp1: Convert display lists to use new fragment pool
> therefore, I have kept this separate to prevent breaking bisectability
> of the vsp-tests.

Sounds good to me.

> ---
>  drivers/media/platform/vsp1/vsp1_clu.c |  7 ++++++-
>  drivers/media/platform/vsp1/vsp1_dl.c  | 15 ++++++++++++++-
>  drivers/media/platform/vsp1/vsp1_lut.c |  7 ++++++-
>  3 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/platform/vsp1/vsp1_clu.c
> b/drivers/media/platform/vsp1/vsp1_clu.c index 52c523625e2f..175717018e11
> 100644
> --- a/drivers/media/platform/vsp1/vsp1_clu.c
> +++ b/drivers/media/platform/vsp1/vsp1_clu.c
> @@ -257,8 +257,13 @@ static void clu_configure(struct vsp1_entity *entity,
>  		clu->clu = NULL;
>  		spin_unlock_irqrestore(&clu->lock, flags);
> 
> -		if (dlb)
> +		if (dlb) {
>  			vsp1_dl_list_add_fragment(dl, dlb);
> +
> +			/* release our local reference */
> +			vsp1_dl_fragment_put(dlb);
> +		}
> +
>  		break;
>  	}
>  }
> diff --git a/drivers/media/platform/vsp1/vsp1_dl.c
> b/drivers/media/platform/vsp1/vsp1_dl.c index 6ffdc3549283..37feda248946
> 100644
> --- a/drivers/media/platform/vsp1/vsp1_dl.c
> +++ b/drivers/media/platform/vsp1/vsp1_dl.c
> @@ -14,6 +14,7 @@
>  #include <linux/device.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/gfp.h>
> +#include <linux/refcount.h>
>  #include <linux/slab.h>
>  #include <linux/workqueue.h>
> 
> @@ -58,6 +59,8 @@ struct vsp1_dl_body {
>  	struct list_head list;
>  	struct list_head free;
> 
> +	refcount_t refcnt;
> +
>  	struct vsp1_dl_fragment_pool *pool;
>  	struct vsp1_device *vsp1;
> 
> @@ -230,6 +233,7 @@ struct vsp1_dl_body *vsp1_dl_fragment_get(struct
> vsp1_dl_fragment_pool *pool)
>  	if (!list_empty(&pool->free)) {
>  		dlb = list_first_entry(&pool->free, struct vsp1_dl_body,
> free);
>  		list_del(&dlb->free);
> +		refcount_set(&dlb->refcnt, 1);
>  	}
> 
>  	spin_unlock_irqrestore(&pool->lock, flags);
> @@ -244,6 +248,9 @@ void vsp1_dl_fragment_put(struct vsp1_dl_body *dlb)
>  	if (!dlb)
>  		return;
> 
> +	if (!refcount_dec_and_test(&dlb->refcnt))
> +		return;
> +
>  	dlb->num_entries = 0;
> 
>  	spin_lock_irqsave(&dlb->pool->lock, flags);
> @@ -428,7 +435,11 @@ void vsp1_dl_list_write(struct vsp1_dl_list *dl, u32
> reg, u32 data)
>   * list, in the order in which fragments are added.
>   *
>   * Adding a fragment to a display list passes ownership of the fragment to
> the
> - * list. The caller must not touch the fragment after this call.
> + * list. The caller must not modify the fragment after this call, but can
> retain
> + * a reference to it for future use if necessary, to add to subsequent
> lists.

I think there's a bit of contradiction here, if the ownership passes to the 
list then the caller shouldn't touch it anymore. How about stating it as 
follows ?

 * The caller retains its reference to the fragment when adding it to a
 * display list, but is not allowed to add new entries to the fragment.
 * The reference must be explicitly released by a call to
 * vsp1_dl_fragment_put() when the fragment isn't needed anymore.

> the
> - * list. The caller must not touch the fragment after this call.
> + * list. The caller must not modify the fragment after this call, but can
> retain
> + * a reference to it for future use if necessary, to add to subsequent
> lists.
> + *
> + * The reference count of the body is incremented by this attachment, and
> thus
> + * the caller should release it's reference if does not want to cache the
> body.
>   *
>   * Fragments are only usable for display lists in header mode. Attempt to
>   * add a fragment to a header-less display list will return an error.
> @@ -440,6 +451,8 @@ int vsp1_dl_list_add_fragment(struct vsp1_dl_list *dl,
>  	if (dl->dlm->mode != VSP1_DL_MODE_HEADER)
>  		return -EINVAL;
> 
> +	refcount_inc(&dlb->refcnt);
> +
>  	list_add_tail(&dlb->list, &dl->fragments);
>  	return 0;
>  }
> diff --git a/drivers/media/platform/vsp1/vsp1_lut.c
> b/drivers/media/platform/vsp1/vsp1_lut.c index 57482e057e54..388bd89ade0b
> 100644
> --- a/drivers/media/platform/vsp1/vsp1_lut.c
> +++ b/drivers/media/platform/vsp1/vsp1_lut.c
> @@ -213,8 +213,13 @@ static void lut_configure(struct vsp1_entity *entity,
>  		lut->lut = NULL;
>  		spin_unlock_irqrestore(&lut->lock, flags);
> 
> -		if (dlb)
> +		if (dlb) {
>  			vsp1_dl_list_add_fragment(dl, dlb);
> +
> +			/* release our local reference */
> +			vsp1_dl_fragment_put(dlb);
> +		}
> +
>  		break;
>  	}
>  }

-- 
Regards,

Laurent Pinchart