linux-kernel - [PATCH v1] i2c: designware: save the preset value of DW_IC_SDA

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Date:   Sat, 27 Aug 2016 15:39:30 +0800
From:   Zhuo-hao Lee <zhuo-hao.lee@...el.com>
To:     jarkko.nikula@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        mika.westerberg@...ux.intel.com, wsa@...-dreams.de,
        linux-i2c@...r.kernel.org
Cc:     zhuo-hao.lee@...el.com, linux-kernel@...r.kernel.org
Subject: [PATCH v1] i2c: designware: save the preset value of DW_IC_SDA_HOLD

There are several ways to set the SDA hold time for i2c controller,
including: Device Tree, built-in device properties and ACPI. However,
if the SDA hold time is not specified by above method, we should
read the value, where it is preset by firmware, and save it to
sda_hold_time. This is needed because when i2c controller enters
runtime suspend, the DW_IC_SDA_HOLD value will be reset to chipset
default value. And during runtime resume, i2c_dw_init will be called
to reconfigure i2c controller. If sda_hold_time is zero, the chipset
default hold time will be used, that will be too short for some
platforms. Therefore, to have a better tolerance, the DW_IC_SDA_HOLD
value should be kept by sda_hold_time.

Signed-off-by: Zhuo-hao Lee <zhuo-hao.lee@...el.com>
---
 drivers/i2c/busses/i2c-designware-core.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-core.c b/drivers/i2c/busses/i2c-designware-core.c
index c6922b8..fcd973d 100644
--- a/drivers/i2c/busses/i2c-designware-core.c
+++ b/drivers/i2c/busses/i2c-designware-core.c
@@ -367,13 +367,17 @@ int i2c_dw_init(struct dw_i2c_dev *dev)
 	dev_dbg(dev->dev, "Fast-mode HCNT:LCNT = %d:%d\n", hcnt, lcnt);

 	/* Configure SDA Hold Time if required */
-	if (dev->sda_hold_time) {
-		reg = dw_readl(dev, DW_IC_COMP_VERSION);
-		if (reg >= DW_IC_SDA_HOLD_MIN_VERS)
+	reg = dw_readl(dev, DW_IC_COMP_VERSION);
+	if (reg >= DW_IC_SDA_HOLD_MIN_VERS) {
+		if (dev->sda_hold_time) {
 			dw_writel(dev, dev->sda_hold_time, DW_IC_SDA_HOLD);
-		else
-			dev_warn(dev->dev,
-				"Hardware too old to adjust SDA hold time.");
+		} else {
+			/* Keep previous hold time setting if no one set it */
+			dev->sda_hold_time = dw_readl(dev, DW_IC_SDA_HOLD);
+		}
+	} else {
+		dev_warn(dev->dev,
+			"Hardware too old to adjust SDA hold time.\n");
 	}

 	/* Configure Tx/Rx FIFO threshold levels */
-- 
1.9.1