async_tx: make async_tx channel switching opt-in

The majority of drivers in drivers/dma/ will never establish cross
channel operation chains and do not need the extra overhead in struct
dma_async_tx_descriptor.  Make channel switching opt-in by default.

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Ira Snyder <iws@ovro.caltech.edu>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: Saeed Bishara <saeed@marvell.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/Kconfig

@@ -46,7 +46,7 @@ config INTEL_MID_DMAC
 
 	  If unsure, say N.
 
-config ASYNC_TX_DISABLE_CHANNEL_SWITCH
+config ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	bool
 
 config AMBA_PL08X
@@ -62,7 +62,6 @@ config INTEL_IOATDMA
 	depends on PCI && X86
 	select DMA_ENGINE
 	select DCA
-	select ASYNC_TX_DISABLE_CHANNEL_SWITCH
 	select ASYNC_TX_DISABLE_PQ_VAL_DMA
 	select ASYNC_TX_DISABLE_XOR_VAL_DMA
 	help
@@ -77,6 +76,7 @@ config INTEL_IOP_ADMA
 	tristate "Intel IOP ADMA support"
 	depends on ARCH_IOP32X || ARCH_IOP33X || ARCH_IOP13XX
 	select DMA_ENGINE
+	select ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	help
 	  Enable support for the Intel(R) IOP Series RAID engines.
 
@@ -101,6 +101,7 @@ config FSL_DMA
 	tristate "Freescale Elo and Elo Plus DMA support"
 	depends on FSL_SOC
 	select DMA_ENGINE
+	select ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	---help---
 	  Enable support for the Freescale Elo and Elo Plus DMA controllers.
 	  The Elo is the DMA controller on some 82xx and 83xx parts, and the
@@ -117,6 +118,7 @@ config MV_XOR
 	bool "Marvell XOR engine support"
 	depends on PLAT_ORION
 	select DMA_ENGINE
+	select ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	---help---
 	  Enable support for the Marvell XOR engine.
 
@@ -174,6 +176,7 @@ config AMCC_PPC440SPE_ADMA
 	depends on 440SPe || 440SP
 	select DMA_ENGINE
 	select ARCH_HAS_ASYNC_TX_FIND_CHANNEL
+	select ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	help
 	  Enable support for the AMCC PPC440SPe RAID engines.
 

drivers/dma/dmaengine.c

@@ -706,7 +706,7 @@ int dma_async_device_register(struct dma_device *device)
 	BUG_ON(!device->dev);
 
 	/* note: this only matters in the
-	 * CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH=y case
+	 * CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH=n case
 	 */
 	if (device_has_all_tx_types(device))
 		dma_cap_set(DMA_ASYNC_TX, device->cap_mask);
@@ -980,7 +980,7 @@ void dma_async_tx_descriptor_init(struct dma_async_tx_descriptor *tx,
 	struct dma_chan *chan)
 {
 	tx->chan = chan;
-	#ifndef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+	#ifdef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	spin_lock_init(&tx->lock);
 	#endif
 }

include/linux/dmaengine.h

@@ -321,14 +321,14 @@ struct dma_async_tx_descriptor {
 	dma_cookie_t (*tx_submit)(struct dma_async_tx_descriptor *tx);
 	dma_async_tx_callback callback;
 	void *callback_param;
-#ifndef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+#ifdef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	struct dma_async_tx_descriptor *next;
 	struct dma_async_tx_descriptor *parent;
 	spinlock_t lock;
 #endif
 };
 
-#ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+#ifndef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
 static inline void txd_lock(struct dma_async_tx_descriptor *txd)
 {
 }
@@ -656,11 +656,11 @@ static inline void net_dmaengine_put(void)
 #ifdef CONFIG_ASYNC_TX_DMA
 #define async_dmaengine_get()	dmaengine_get()
 #define async_dmaengine_put()	dmaengine_put()
-#ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+#ifndef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
 #define async_dma_find_channel(type) dma_find_channel(DMA_ASYNC_TX)
 #else
 #define async_dma_find_channel(type) dma_find_channel(type)
-#endif /* CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH */
+#endif /* CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH */
 #else
 static inline void async_dmaengine_get(void)
 {