[PATCH 2/3] net: ag71xx: remove MMIO read-back drain from register writes

[Rosen Penev]

ag71xx_wr(), ag71xx_sb() and ag71xx_cb() issue a synchronous
ioread32() after every iowrite32() under the assumption that the
write must be flushed to the device before any subsequent operation.

On the ath79 on-chip MMIO bus the uncached __raw_writel used by
iowrite32() is non-posted - the write completes before the next
instruction executes.  The extra ioread32() therefore serves no
ordering purpose and costs at least one uncached bus round-trip
per call.

Remove the read-back and the associated comments, leaving the
barrier that iowrite32() already provides.

Small iperf3 improvement:

Before:

[ ID][Role] Interval           Transfer     Bitrate         Retr
[  5][TX-C]   0.00-10.00  sec   343 MBytes   288 Mbits/sec   32            sender
[  5][TX-C]   0.00-10.00  sec   341 MBytes   286 Mbits/sec                  receiver
[  7][RX-C]   0.00-10.00  sec   183 MBytes   153 Mbits/sec    0            sender
[  7][RX-C]   0.00-10.00  sec   183 MBytes   153 Mbits/sec                  receiver

After:

[ ID][Role] Interval           Transfer     Bitrate         Retr
[  5][TX-C]   0.00-10.00  sec   349 MBytes   292 Mbits/sec   28            sender
[  5][TX-C]   0.00-10.00  sec   346 MBytes   291 Mbits/sec                  receiver
[  7][RX-C]   0.00-10.00  sec   180 MBytes   151 Mbits/sec    0            sender
[  7][RX-C]   0.00-10.00  sec   180 MBytes   151 Mbits/sec                  receiver

Assisted-by: opencode:big-pickle
Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
---
 drivers/net/ethernet/atheros/ag71xx.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/atheros/ag71xx.c b/drivers/net/ethernet/atheros/ag71xx.c
index 1def2ad4c5ce..77f8e75e98ac 100644
--- a/drivers/net/ethernet/atheros/ag71xx.c
+++ b/drivers/net/ethernet/atheros/ag71xx.c
@@ -397,11 +397,13 @@ static bool ag71xx_is(struct ag71xx *ag, enum ag71xx_type type)
 	return ag->dcfg->type == type;
 }
 
+/* ath79 on-chip MMIO bus is non-posted - iowrite32/iowrite32be completes
+ * before the next instruction executes. No read-back drain is needed.
+ */
+
 static void ag71xx_wr(struct ag71xx *ag, unsigned int reg, u32 value)
 {
 	iowrite32(value, ag->mac_base + reg);
-	/* flush write */
-	(void)ioread32(ag->mac_base + reg);
 }
 
 static u32 ag71xx_rr(struct ag71xx *ag, unsigned int reg)
@@ -411,22 +413,16 @@ static u32 ag71xx_rr(struct ag71xx *ag, unsigned int reg)
 
 static void ag71xx_sb(struct ag71xx *ag, unsigned int reg, u32 mask)
 {
-	void __iomem *r;
+	void __iomem *r = ag->mac_base + reg;
 
-	r = ag->mac_base + reg;
 	iowrite32(ioread32(r) | mask, r);
-	/* flush write */
-	(void)ioread32(r);
 }
 
 static void ag71xx_cb(struct ag71xx *ag, unsigned int reg, u32 mask)
 {
-	void __iomem *r;
+	void __iomem *r = ag->mac_base + reg;
 
-	r = ag->mac_base + reg;
 	iowrite32(ioread32(r) & ~mask, r);
-	/* flush write */
-	(void)ioread32(r);
 }
 
 static void ag71xx_int_enable(struct ag71xx *ag, u32 ints)
-- 
2.54.0