dma: add channel request API that supports deferred probe

dma_request_slave_channel() simply returns NULL whenever DMA channel
lookup fails. Lookup could fail for two distinct reasons:

a) No DMA specification exists for the channel name.
   This includes situations where no DMA specifications exist at all, or
   other general lookup problems.

b) A DMA specification does exist, yet the driver for that channel is not
   yet registered.

Case (b) should trigger deferred probe in client drivers. However, since
they have no way to differentiate the two situations, it cannot.

Implement new function dma_request_slave_channel_reason(), which performs
identically to dma_request_slave_channel(), except that it returns an
error-pointer rather than NULL, which allows callers to detect when
deferred probe should occur.

Eventually, all drivers should be converted to this new API, the old API
removed, and the new API renamed to the more desirable name. This patch
doesn't convert the existing API and all drivers in one go, since some
drivers call dma_request_slave_channel() then dma_request_channel() if
that fails. That would require either modifying dma_request_channel() in
the same way, or adding extra error-handling code to all affected
drivers, and there are close to 100 drivers using the other API, rather
than just the 15-20 or so that use dma_request_slave_channel(), which
might be tenable in a single patch.

acpi_dma_request_slave_chan_by_name() doesn't currently implement
deferred probe. It should, but this will be addressed later.

Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/dmaengine.c

@@ -540,6 +540,8 @@ EXPORT_SYMBOL_GPL(dma_get_slave_channel);
  * @mask: capabilities that the channel must satisfy
  * @fn: optional callback to disposition available channels
  * @fn_param: opaque parameter to pass to dma_filter_fn
+ *
+ * Returns pointer to appropriate DMA channel on success or NULL.
  */
 struct dma_chan *__dma_request_channel(const dma_cap_mask_t *mask,
 				       dma_filter_fn fn, void *fn_param)
@@ -591,18 +593,43 @@ EXPORT_SYMBOL_GPL(__dma_request_channel);
  * dma_request_slave_channel - try to allocate an exclusive slave channel
  * @dev:	pointer to client device structure
  * @name:	slave channel name
+ *
+ * Returns pointer to appropriate DMA channel on success or an error pointer.
  */
-struct dma_chan *dma_request_slave_channel(struct device *dev, const char *name)
+struct dma_chan *dma_request_slave_channel_reason(struct device *dev,
+						  const char *name)
 {
+	struct dma_chan *chan;
+
 	/* If device-tree is present get slave info from here */
 	if (dev->of_node)
 		return of_dma_request_slave_channel(dev->of_node, name);
 
 	/* If device was enumerated by ACPI get slave info from here */
-	if (ACPI_HANDLE(dev))
-		return acpi_dma_request_slave_chan_by_name(dev, name);
+	if (ACPI_HANDLE(dev)) {
+		chan = acpi_dma_request_slave_chan_by_name(dev, name);
+		if (chan)
+			return chan;
+	}
 
-	return NULL;
+	return ERR_PTR(-ENODEV);
+}
+EXPORT_SYMBOL_GPL(dma_request_slave_channel_reason);
+
+/**
+ * dma_request_slave_channel - try to allocate an exclusive slave channel
+ * @dev:	pointer to client device structure
+ * @name:	slave channel name
+ *
+ * Returns pointer to appropriate DMA channel on success or NULL.
+ */
+struct dma_chan *dma_request_slave_channel(struct device *dev,
+					   const char *name)
+{
+	struct dma_chan *ch = dma_request_slave_channel_reason(dev, name);
+	if (IS_ERR(ch))
+		return NULL;
+	return ch;
 }
 EXPORT_SYMBOL_GPL(dma_request_slave_channel);
 

drivers/dma/of-dma.c

@@ -143,7 +143,7 @@ static int of_dma_match_channel(struct device_node *np, const char *name,
  * @np:		device node to get DMA request from
  * @name:	name of desired channel
  *
- * Returns pointer to appropriate dma channel on success or NULL on error.
+ * Returns pointer to appropriate DMA channel on success or an error pointer.
  */
 struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 					      const char *name)
@@ -152,17 +152,18 @@ struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 	struct of_dma		*ofdma;
 	struct dma_chan		*chan;
 	int			count, i;
+	int			ret_no_channel = -ENODEV;
 
 	if (!np || !name) {
 		pr_err("%s: not enough information provided\n", __func__);
-		return NULL;
+		return ERR_PTR(-ENODEV);
 	}
 
 	count = of_property_count_strings(np, "dma-names");
 	if (count < 0) {
 		pr_err("%s: dma-names property of node '%s' missing or empty\n",
 			__func__, np->full_name);
-		return NULL;
+		return ERR_PTR(-ENODEV);
 	}
 
 	for (i = 0; i < count; i++) {
@@ -172,10 +173,12 @@ struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 		mutex_lock(&of_dma_lock);
 		ofdma = of_dma_find_controller(&dma_spec);
 
-		if (ofdma)
+		if (ofdma) {
 			chan = ofdma->of_dma_xlate(&dma_spec, ofdma);
-		else
+		} else {
+			ret_no_channel = -EPROBE_DEFER;
 			chan = NULL;
+		}
 
 		mutex_unlock(&of_dma_lock);
 
@@ -185,7 +188,7 @@ struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 			return chan;
 	}
 
-	return NULL;
+	return ERR_PTR(ret_no_channel);
 }
 
 /**

include/linux/dmaengine.h

@@ -22,6 +22,7 @@
 #define LINUX_DMAENGINE_H
 
 #include <linux/device.h>
+#include <linux/err.h>
 #include <linux/uio.h>
 #include <linux/bug.h>
 #include <linux/scatterlist.h>
@@ -1040,6 +1041,8 @@ enum dma_status dma_wait_for_async_tx(struct dma_async_tx_descriptor *tx);
 void dma_issue_pending_all(void);
 struct dma_chan *__dma_request_channel(const dma_cap_mask_t *mask,
 					dma_filter_fn fn, void *fn_param);
+struct dma_chan *dma_request_slave_channel_reason(struct device *dev,
+						  const char *name);
 struct dma_chan *dma_request_slave_channel(struct device *dev, const char *name);
 void dma_release_channel(struct dma_chan *chan);
 #else
@@ -1063,6 +1066,11 @@ static inline struct dma_chan *__dma_request_channel(const dma_cap_mask_t *mask,
 {
 	return NULL;
 }
+static inline struct dma_chan *dma_request_slave_channel_reason(
+					struct device *dev, const char *name)
+{
+	return ERR_PTR(-ENODEV);
+}
 static inline struct dma_chan *dma_request_slave_channel(struct device *dev,
 							 const char *name)
 {