RE: [PATCH v4 2/6] dmaengine: Add dma_chan_is_in_use() function

[Sosnowski, Maciej]

> ---------- Original message ----------
> From: Haavard Skinnemoen <haavard.skinnemoen@xxxxxxxxx>
> Date: Jun 26, 2008 3:23 PM
> Subject: [PATCH v4 2/6] dmaengine: Add dma_chan_is_in_use() function
> To: Dan Williams <dan.j.williams@xxxxxxxxx>, Pierre Ossman
> <drzeus-list@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-embedded@xxxxxxxxxxxxxxx,
> kernel@xxxxxxxxxxxxxx, shannon.nelson@xxxxxxxxx, David Brownell
> <david-b@xxxxxxxxxxx>, Haavard Skinnemoen
> <haavard.skinnemoen@xxxxxxxxx>
> 
> 
> This moves the code checking if a DMA channel is in use from
> show_in_use() into an inline helper function, dma_is_in_use(). DMA
> controllers can use this in order to give clients exclusive access to
> channels (usually necessary when setting up slave DMA.)
> 
> I have to admit that I don't really understand the channel refcounting
> logic at all... dma_chan_get() simply increments a per-cpu value. How
> can we be sure that whatever CPU calls dma_chan_is_in_use() sees the
> same value?
> 
> Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@xxxxxxxxx>
> ---
>  drivers/dma/dmaengine.c   |   12 +-----------
>  include/linux/dmaengine.h |   17 +++++++++++++++++
>  2 files changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index a57c337..ad8d811 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -105,17 +105,7 @@ static ssize_t show_bytes_transferred(struct
> device *dev, struct device_attribut
>  static ssize_t show_in_use(struct device *dev, struct
> device_attribute *attr, char *buf)
>  {
>        struct dma_chan *chan = to_dma_chan(dev);
> -       int in_use = 0;
> -
> -       if (unlikely(chan->slow_ref) &&
> -               atomic_read(&chan->refcount.refcount) > 1)
> -               in_use = 1;
> -       else {
> -               if (local_read(&(per_cpu_ptr(chan->local,
> -                       get_cpu())->refcount)) > 0)
> -                       in_use = 1;
> -               put_cpu();
> -       }
> +       int in_use = dma_chan_is_in_use(chan);
> 
>        return sprintf(buf, "%d\n", in_use);
>  }
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index cffb95f..4b602d3 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -180,6 +180,23 @@ static inline void dma_chan_put(struct dma_chan
*chan)
>        }
>  }
> 
> +static inline bool dma_chan_is_in_use(struct dma_chan *chan)
> +{
> +       bool in_use = false;
> +
> +       if (unlikely(chan->slow_ref) &&
> +               atomic_read(&chan->refcount.refcount) > 1)
> +               in_use = true;
> +       else {
> +               if (local_read(&(per_cpu_ptr(chan->local,
> +                       get_cpu())->refcount)) > 0)
> +                       in_use = true;
> +               put_cpu();
> +       }
> +
> +       return in_use;
> +}
> +
>  /*
>  * typedef dma_event_callback - function pointer to a DMA event
callback
>  * For each channel added to the system this routine is called for
each
> client. --
> 1.5.5.4

Acked-by: Maciej Sosnowski <maciej.sosnowski@xxxxxxxxx>

Regards,
Maciej
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