[PATCH 3.16 132/212] clk: renesas: mstp: ensure register writes complete

[Ben Hutchings]

3.16.44-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Brandt <chris.brandt@xxxxxxxxxxx>

commit f59de563358eb9351b7f8f0ba2d3be2ebb70b93d upstream.

When there is no status bit, it is possible for the clock enable/disable
operation to have not completed by the time the driver code resumes
execution. This is due to the fact that write operations are sometimes
queued and delayed internally. Doing a read ensures the write operations
has completed.

Fixes: b6face404f38 ("ARM: shmobile: r7s72100: add essential clock nodes to dtsi")
Signed-off-by: Chris Brandt <chris.brandt@xxxxxxxxxxx>
Reviewed-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - Use barrier() instead of barrier_data()
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/clk/shmobile/clk-mstp.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/clk/shmobile/clk-mstp.c
+++ b/drivers/clk/shmobile/clk-mstp.c
@@ -85,6 +85,12 @@ static int cpg_mstp_clock_endisable(stru
 		value |= bitmask;
 	cpg_mstp_write(group, value, group->smstpcr);
 
+	if (!group->mstpsr) {
+		/* dummy read to ensure write has completed */
+		cpg_mstp_read(group, group->smstpcr);
+		barrier();
+	}
+
 	spin_unlock_irqrestore(&group->lock, flags);
 
 	if (!enable || !group->mstpsr)