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Date:	Thu, 20 Dec 2012 10:50:17 -0600
From:	Rob Clark <robdclark@...il.com>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	DRI Development <dri-devel@...ts.freedesktop.org>,
	linaro-mm-sig@...ts.linaro.org, linux-media@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Aaron Plattner <aplattner@...dia.com>
Subject: Re: [PATCH] [RFC] dma-buf: implement vmap refcounting in the
 interface logic

On Thu, Dec 20, 2012 at 7:14 AM, Daniel Vetter <daniel.vetter@...ll.ch> wrote:
> All drivers which implement this need to have some sort of refcount to
> allow concurrent vmap usage. Hence implement this in the dma-buf core.
>
> To protect against concurrent calls we need a lock, which potentially
> causes new funny locking inversions. But this shouldn't be a problem
> for exporters with statically allocated backing storage, and more
> dynamic drivers have decent issues already anyway.
>
> Inspired by some refactoring patches from Aaron Plattner, who
> implemented the same idea, but only for drm/prime drivers.
>
> v2: Check in dma_buf_release that no dangling vmaps are left.
> Suggested by Aaron Plattner. We might want to do similar checks for
> attachments, but that's for another patch. Also fix up ERR_PTR return
> for vmap.
>
> v3: Check whether the passed-in vmap address matches with the cached
> one for vunmap. Eventually we might want to remove that parameter -
> compared to the kmap functions there's no need for the vaddr for
> unmapping.  Suggested by Chris Wilson.
>
> v4: Fix a brown-paper-bag bug spotted by Aaron Plattner.

yeah, I think the locking does worry me a bit but hopefully should be
able to do something better when reservations land

Signed-off-by: Rob Clark <rob@...com>

> Cc: Aaron Plattner <aplattner@...dia.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
>  Documentation/dma-buf-sharing.txt |  6 +++++-
>  drivers/base/dma-buf.c            | 43 ++++++++++++++++++++++++++++++++++-----
>  include/linux/dma-buf.h           |  4 +++-
>  3 files changed, 46 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> index 0188903..4966b1b 100644
> --- a/Documentation/dma-buf-sharing.txt
> +++ b/Documentation/dma-buf-sharing.txt
> @@ -302,7 +302,11 @@ Access to a dma_buf from the kernel context involves three steps:
>        void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
>
>     The vmap call can fail if there is no vmap support in the exporter, or if it
> -   runs out of vmalloc space. Fallback to kmap should be implemented.
> +   runs out of vmalloc space. Fallback to kmap should be implemented. Note that
> +   the dma-buf layer keeps a reference count for all vmap access and calls down
> +   into the exporter's vmap function only when no vmapping exists, and only
> +   unmaps it once. Protection against concurrent vmap/vunmap calls is provided
> +   by taking the dma_buf->lock mutex.
>
>  3. Finish access
>
> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> index a3f79c4..26b68de 100644
> --- a/drivers/base/dma-buf.c
> +++ b/drivers/base/dma-buf.c
> @@ -39,6 +39,8 @@ static int dma_buf_release(struct inode *inode, struct file *file)
>
>         dmabuf = file->private_data;
>
> +       BUG_ON(dmabuf->vmapping_counter);
> +
>         dmabuf->ops->release(dmabuf);
>         kfree(dmabuf);
>         return 0;
> @@ -482,12 +484,34 @@ EXPORT_SYMBOL_GPL(dma_buf_mmap);
>   */
>  void *dma_buf_vmap(struct dma_buf *dmabuf)
>  {
> +       void *ptr;
> +
>         if (WARN_ON(!dmabuf))
>                 return NULL;
>
> -       if (dmabuf->ops->vmap)
> -               return dmabuf->ops->vmap(dmabuf);
> -       return NULL;
> +       if (!dmabuf->ops->vmap)
> +               return NULL;
> +
> +       mutex_lock(&dmabuf->lock);
> +       if (dmabuf->vmapping_counter) {
> +               dmabuf->vmapping_counter++;
> +               BUG_ON(!dmabuf->vmap_ptr);
> +               ptr = dmabuf->vmap_ptr;
> +               goto out_unlock;
> +       }
> +
> +       BUG_ON(dmabuf->vmap_ptr);
> +
> +       ptr = dmabuf->ops->vmap(dmabuf);
> +       if (IS_ERR_OR_NULL(ptr))
> +               goto out_unlock;
> +
> +       dmabuf->vmap_ptr = ptr;
> +       dmabuf->vmapping_counter = 1;
> +
> +out_unlock:
> +       mutex_unlock(&dmabuf->lock);
> +       return ptr;
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_vmap);
>
> @@ -501,7 +525,16 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
>         if (WARN_ON(!dmabuf))
>                 return;
>
> -       if (dmabuf->ops->vunmap)
> -               dmabuf->ops->vunmap(dmabuf, vaddr);
> +       BUG_ON(!dmabuf->vmap_ptr);
> +       BUG_ON(dmabuf->vmapping_counter == 0);
> +       BUG_ON(dmabuf->vmap_ptr != vaddr);
> +
> +       mutex_lock(&dmabuf->lock);
> +       if (--dmabuf->vmapping_counter == 0) {
> +               if (dmabuf->ops->vunmap)
> +                       dmabuf->ops->vunmap(dmabuf, vaddr);
> +               dmabuf->vmap_ptr = NULL;
> +       }
> +       mutex_unlock(&dmabuf->lock);
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_vunmap);
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index bd2e52c..e3bf2f6 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -119,8 +119,10 @@ struct dma_buf {
>         struct file *file;
>         struct list_head attachments;
>         const struct dma_buf_ops *ops;
> -       /* mutex to serialize list manipulation and attach/detach */
> +       /* mutex to serialize list manipulation, attach/detach and vmap/unmap */
>         struct mutex lock;
> +       unsigned vmapping_counter;
> +       void *vmap_ptr;
>         void *priv;
>  };
>
> --
> 1.7.11.7
>
> --
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