Re: [RFC RESEND 05/11] v4l2-core: Don't sync cache for a buffer if so requested

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

Hi Sakari,

Thank you for the patch.

On Friday 11 Sep 2015 14:50:28 Sakari Ailus wrote:
> From: Samu Onkalo <samu.onkalo@intel.com>
> 
> The user may request to the driver (vb2) to skip the cache maintenance
> operations in case the buffer does not need cache synchronisation, e.g. in
> cases where the buffer is passed between hardware blocks without it being
> touched by the CPU.
> 
> Also document that the prepare and finish vb2_mem_ops might not get called
> every time the buffer ownership changes between the kernel and the user
> space.
> 
> Signed-off-by: Samu Onkalo <samu.onkalo@intel.com>
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> ---
>  drivers/media/v4l2-core/videobuf2-core.c | 52 ++++++++++++++++++++++-------
>  include/media/videobuf2-core.h           | 12 +++++---
>  2 files changed, 49 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c
> b/drivers/media/v4l2-core/videobuf2-core.c index c5c0707a..b664024 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -187,6 +187,28 @@ static void __vb2_queue_cancel(struct vb2_queue *q);
>  static void __enqueue_in_driver(struct vb2_buffer *vb);
> 
>  /**
> + * __mem_prepare_planes() - call finish mem op for all planes of the buffer
> + */
> +static void __mem_prepare_planes(struct vb2_buffer *vb)
> +{
> +	unsigned int plane;
> +
> +	for (plane = 0; plane < vb->num_planes; ++plane)
> +		call_void_memop(vb, prepare, vb->planes[plane].mem_priv);
> +}
> +
> +/**
> + * __mem_finish_planes() - call finish mem op for all planes of the buffer
> + */
> +static void __mem_finish_planes(struct vb2_buffer *vb)
> +{
> +	unsigned int plane;
> +
> +	for (plane = 0; plane < vb->num_planes; ++plane)
> +		call_void_memop(vb, finish, vb->planes[plane].mem_priv);
> +}
> +
> +/**
>   * __vb2_buf_mem_alloc() - allocate video memory for the given buffer
>   */
>  static int __vb2_buf_mem_alloc(struct vb2_buffer *vb)
> @@ -1391,6 +1413,10 @@ static void __fill_vb2_buffer(struct vb2_buffer *vb,
> const struct v4l2_buffer *b static int __prepare_mmap(struct vb2_buffer
> *vb, const struct v4l2_buffer *b) {
>  	__fill_vb2_buffer(vb, b, vb->v4l2_planes);
> +
> +	if (!(b->flags & V4L2_BUF_FLAG_NO_CACHE_SYNC))
> +		__mem_prepare_planes(vb);
> +
>  	return call_vb_qop(vb, buf_prepare, vb);
>  }
> 
> @@ -1476,6 +1502,11 @@ static int __prepare_userptr(struct vb2_buffer *vb,
>  			dprintk(1, "buffer initialization failed\n");
>  			goto err;
>  		}
> +
> +		/* This is new buffer memory --- always synchronise cache. */
> +		__mem_prepare_planes(vb);
> +	} else if (!(b->flags & V4L2_BUF_FLAG_NO_CACHE_SYNC)) {
> +		__mem_prepare_planes(vb);
>  	}
> 
>  	ret = call_vb_qop(vb, buf_prepare, vb);
> @@ -1601,6 +1632,11 @@ static int __prepare_dmabuf(struct vb2_buffer *vb,
> const struct v4l2_buffer *b) dprintk(1, "buffer initialization failed\n");
>  			goto err;
>  		}
> +
> +		/* This is new buffer memory --- always synchronise cache. */
> +		__mem_prepare_planes(vb);
> +	} else if (!(b->flags & V4L2_BUF_FLAG_NO_CACHE_SYNC)) {
> +		__mem_prepare_planes(vb);
>  	}
> 
>  	ret = call_vb_qop(vb, buf_prepare, vb);
> @@ -1624,7 +1660,6 @@ err:
>  static void __enqueue_in_driver(struct vb2_buffer *vb)
>  {
>  	struct vb2_queue *q = vb->vb2_queue;
> -	unsigned int plane;
> 
>  	vb->state = VB2_BUF_STATE_ACTIVE;
>  	atomic_inc(&q->owned_by_drv_count);
> @@ -1691,10 +1726,6 @@ static int __buf_prepare(struct vb2_buffer *vb, const
> struct v4l2_buffer *b) return ret;
>  	}
> 
> -	/* sync buffers */
> -	for (plane = 0; plane < vb->num_planes; ++plane)
> -		call_void_memop(vb, prepare, vb->planes[plane].mem_priv);
> -
>  	vb->state = VB2_BUF_STATE_PREPARED;
> 
>  	return 0;
> @@ -2078,7 +2109,7 @@ EXPORT_SYMBOL_GPL(vb2_wait_for_all_buffers);
>  /**
>   * __vb2_dqbuf() - bring back the buffer to the DEQUEUED state
>   */
> -static void __vb2_dqbuf(struct vb2_buffer *vb)
> +static void __vb2_dqbuf(struct vb2_buffer *vb, bool no_cache_sync)
>  {
>  	struct vb2_queue *q = vb->vb2_queue;
>  	unsigned int plane;
> @@ -2089,9 +2120,8 @@ static void __vb2_dqbuf(struct vb2_buffer *vb)
> 
>  	vb->state = VB2_BUF_STATE_DEQUEUED;
> 
> -	/* sync buffers */
> -	for (plane = 0; plane < vb->num_planes; plane++)
> -		call_void_memop(vb, finish, vb->planes[plane].mem_priv);
> +	if (!no_cache_sync)
> +		__mem_finish_planes(vb);
> 
>  	/* unmap DMABUF buffer */
>  	if (q->memory == V4L2_MEMORY_DMABUF)
> @@ -2143,7 +2173,7 @@ static int vb2_internal_dqbuf(struct vb2_queue *q,
> struct v4l2_buffer *b, bool n vb->v4l2_buf.flags & V4L2_BUF_FLAG_LAST)
>  		q->last_buffer_dequeued = true;
>  	/* go back to dequeued state */
> -	__vb2_dqbuf(vb);
> +	__vb2_dqbuf(vb, b->flags & V4L2_BUF_FLAG_NO_CACHE_SYNC);
> 
>  	dprintk(1, "dqbuf of buffer %d, with state %d\n",
>  			vb->v4l2_buf.index, vb->state);
> @@ -2246,7 +2276,7 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
>  			vb->state = VB2_BUF_STATE_PREPARED;
>  			call_void_vb_qop(vb, buf_finish, vb);
>  		}
> -		__vb2_dqbuf(vb);
> +		__vb2_dqbuf(vb, false);
>  	}
>  }
> 
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 4f7f7ae..a825bd5 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -59,10 +59,14 @@ struct vb2_threadio_data;
>   *		dmabuf.
>   * @unmap_dmabuf: releases access control to the dmabuf - allocator is
> notified
> *		  that this driver is done using the dmabuf for now.
> - * @prepare:	called every time the buffer is passed from userspace to the
> - *		driver, useful for cache synchronisation, optional.
> - * @finish:	called every time the buffer is passed back from the driver
> - *		to the userspace, also optional.
> + * @prepare:	Called on the plane when the buffer ownership is passed from
> + *		the user space to the kernel and the plane must be cache
> + *		syncronised. The V4L2_BUF_FLAG_NO_CACHE_SYNC buffer flag may

s/syncronised/synchronised/

> + *		be used to skip this call. Optional.

I would say "The call might be skipped by the videobuf core when the 
V4L2_BUF_FLAG_NO_CACHE_SYNC buffer flag is set". Your wording could mean that 
the memops implementation is free to skip cache synchronization internally if 
the flag is set.

If the core skips the prepare and finish operations based on the 
V4L2_BUF_FLAG_NO_CACHE_SYNC flag then those operations must not perform 
anything else than cache synchronization. That's fine with the current 
implementation, but it should be documented as such, and I'd rename the 
operations to sync_for_cpu and sync_for dev (or something similar).

> + * @finish:	Called on the plane when the buffer ownership is passed from
> + *		the kernel to the user space and the plane must be cache
> + *		syncronised. The V4L2_BUF_FLAG_NO_CACHE_SYNC buffer flag may
> + *		be used to skip this call. Optional.

Same comment here.

>   * @vaddr:	return a kernel virtual address to a given memory buffer
>   *		associated with the passed private structure or NULL if no
>   *		such mapping exists.

-- 
Regards,

Laurent Pinchart