gianfar: Add macros for stepping through BDs

This code is based strongly on code from Dai Haruki <Dai.Haruki@freescale.com>.

The gianfar Buffer Descriptors are arranged in a circular array, the end of
which is denoted by setting the "WRAP" bit in the descriptor.  However, the
software knows the end of the ring because it knows how many descriptors are
there.  Rather than check each descriptor for whether the WRAP bit is set,
use pointer math to determine where the next BD is.  This is also useful for
when we want to look at BDs other than the very next one (for Scatter-Gather).

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/net/gianfar.h b/drivers/net/gianfar.h
index 1bdb50c..1ebf7ac 100644
--- a/drivers/net/gianfar.h
+++ b/drivers/net/gianfar.h
@@ -196,6 +196,12 @@ extern const char gfar_driver_version[];
 #define DEFAULT_TXIC mk_ic_value(DEFAULT_TXCOUNT, DEFAULT_TXTIME)
 #define DEFAULT_RXIC mk_ic_value(DEFAULT_RXCOUNT, DEFAULT_RXTIME)
 
+#define skip_bd(bdp, stride, base, ring_size) ({ \
+	typeof(bdp) new_bd = (bdp) + (stride); \
+	(new_bd >= (base) + (ring_size)) ? (new_bd - (ring_size)) : new_bd; })
+
+#define next_bd(bdp, base, ring_size) skip_bd(bdp, 1, base, ring_size)
+
 #define RCTRL_PAL_MASK		0x001f0000
 #define RCTRL_VLEX		0x00002000
 #define RCTRL_FILREN		0x00001000