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Subject[RFC v3 3/7] dmaengine: Add dma_chan_is_in_use() function
Date
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 <hskinnemoen@atmel.com>
---
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 9d11f06..3076d80 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 9cd7ed9..5d9a3a2 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -175,6 +175,23 @@ 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.
--
1.5.3.8


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