scsi: vmbus: Add function to report available ring buffer to write in total ring size percentage

Netvsc has a function to calculate how much ring buffer in percentage is
available to write. This function is also useful for storvsc and other
vmbus devices.

Define a similar function in vmbus to be used by other vmbus devices.

Signed-off-by: Long Li <longli@microsoft.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/hv/ring_buffer.c

@@ -227,6 +227,8 @@ int hv_ringbuffer_init(struct hv_ring_buffer_info *ring_info,
 	ring_info->ring_buffer->feature_bits.value = 1;
 
 	ring_info->ring_size = page_cnt << PAGE_SHIFT;
+	ring_info->ring_size_div10_reciprocal =
+		reciprocal_value(ring_info->ring_size / 10);
 	ring_info->ring_datasize = ring_info->ring_size -
 		sizeof(struct hv_ring_buffer);
 

include/linux/hyperv.h

@@ -35,6 +35,7 @@
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
 #include <linux/interrupt.h>
+#include <linux/reciprocal_div.h>
 
 #define MAX_PAGE_BUFFER_COUNT				32
 #define MAX_MULTIPAGE_BUFFER_COUNT			32 /* 128K */
@@ -120,6 +121,7 @@ struct hv_ring_buffer {
 struct hv_ring_buffer_info {
 	struct hv_ring_buffer *ring_buffer;
 	u32 ring_size;			/* Include the shared header */
+	struct reciprocal_value ring_size_div10_reciprocal;
 	spinlock_t ring_lock;
 
 	u32 ring_datasize;		/* < ring_size */
@@ -154,6 +156,16 @@ static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi)
 	return write;
 }
 
+static inline u32 hv_get_avail_to_write_percent(
+		const struct hv_ring_buffer_info *rbi)
+{
+	u32 avail_write = hv_get_bytes_to_write(rbi);
+
+	return reciprocal_divide(
+			(avail_write  << 3) + (avail_write << 1),
+			rbi->ring_size_div10_reciprocal);
+}
+
 /*
  * VMBUS version is 32 bit entity broken up into
  * two 16 bit quantities: major_number. minor_number.