dmaengine: add interface of dma_get_slave_channel

Suggested by Arnd, add dma_get_slave_channel interface
Dma host driver could get specific channel specificied by request line, rather than filter.

host example:
static struct dma_chan *xx_of_dma_simple_xlate(struct of_phandle_args *dma_spec,
		struct of_dma *ofdma)
{
	struct xx_dma_dev *d = ofdma->of_dma_data;
	unsigned int request = dma_spec->args[0];

	if (request > d->dma_requests)
		return NULL;

	return dma_get_slave_channel(&(d->chans[request].vc.chan));
}

probe:
of_dma_controller_register((&op->dev)->of_node, xx_of_dma_simple_xlate, d);

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/dmaengine.c

@@ -503,6 +503,32 @@ static struct dma_chan *private_candidate(const dma_cap_mask_t *mask,
 	return NULL;
 }
 
+/**
+ * dma_request_channel - try to get specific channel exclusively
+ * @chan: target channel
+ */
+struct dma_chan *dma_get_slave_channel(struct dma_chan *chan)
+{
+	int err = -EBUSY;
+
+	/* lock against __dma_request_channel */
+	mutex_lock(&dma_list_mutex);
+
+	if (chan->client_count == 0)
+		err = dma_chan_get(chan);
+	else
+		chan = NULL;
+
+	mutex_unlock(&dma_list_mutex);
+
+	if (err)
+		pr_debug("%s: failed to get %s: (%d)\n",
+			__func__, dma_chan_name(chan), err);
+
+	return chan;
+}
+EXPORT_SYMBOL_GPL(dma_get_slave_channel);
+
 /**
  * dma_request_channel - try to allocate an exclusive channel
  * @mask: capabilities that the channel must satisfy

include/linux/dmaengine.h

@@ -995,6 +995,7 @@ int dma_async_device_register(struct dma_device *device);
 void dma_async_device_unregister(struct dma_device *device);
 void dma_run_dependencies(struct dma_async_tx_descriptor *tx);
 struct dma_chan *dma_find_channel(enum dma_transaction_type tx_type);
+struct dma_chan *dma_get_slave_channel(struct dma_chan *chan);
 struct dma_chan *net_dma_find_channel(void);
 #define dma_request_channel(mask, x, y) __dma_request_channel(&(mask), x, y)
 #define dma_request_slave_channel_compat(mask, x, y, dev, name) \