mmc: core: make the mmc queue depth configurable

This patch make driver able to set the queue depth and keep the default
vaule to be '64'.

Change-Id: I54cd065d6536c170d1615a6f9d7071d479bfeb84
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>

drivers/mmc/core/Kconfig

@@ -89,3 +89,14 @@ config MMC_CRYPTO
 	  Enabling this makes it possible for the kernel to use the crypto
 	  capabilities of the MMC device (if present) to perform crypto
 	  operations on data being transferred to/from the device.
+
+config MMC_QUEUE_DEPTH
+	int "MMC Queue Depth number"
+	range 1 64
+	depends on ROCKCHIP_MINI_KERNEL
+	default 64
+	help
+	  Set a depth num for mmc queue, user can set it lower to decrease
+	  queue number to save memory, suggest set to NR_CPUS.
+
+	  If unsure, say 64 here.

drivers/mmc/core/queue.c

@@ -419,7 +419,11 @@ static inline bool mmc_merge_capable(struct mmc_host *host)
 }
 
 /* Set queue depth to get a reasonable value for q->nr_requests */
+#ifdef CONFIG_MMC_QUEUE_DEPTH
+#define MMC_QUEUE_DEPTH CONFIG_MMC_QUEUE_DEPTH
+#else
 #define MMC_QUEUE_DEPTH 64
+#endif
 
 /**
  * mmc_init_queue - initialise a queue structure.