[PATCH] i2c: designware: use enable on resume instead initialization

[lucas.de.marchi-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org]

From: Fabio Mello <fabio.mello-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>

According to documentation and tests, initialization is not
necessary on module resume, since the controller keeps its state
between disable/enable. Change the target address is also allowed.

So, this patch replaces the initialization on module resume with a
simple enable, and removes the (non required anymore) enables and
disables.

Signed-off-by: Fabio Mello <fabio.mello-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Signed-off-by: Lucas De Marchi <lucas.demarchi-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
---

These pictures explain a little more the consequence of letting the
enable+disable in the code:

	http://pub.politreco.com/paste/TEK0011-before.jpg
	http://pub.politreco.com/paste/TEK0007-after.jpg

The yellow line is a GPIO toggle in userspace to mark when we start and finish
the i2c transactions.  The blue line is the SCL in that i2c bus. Take a look on
the huge pauses we have between any 2 transactions.  These pauses are removed
with this patch and we are able to read our sensor's values in 950usec rather
than 5.24msec we had before.  We are testing this using a Minnowboard Max that
has a designware i2c controller.

There's this comment in the code suggesting the designware controller might
have problems if we don't disable it after each transfer.  We left a stress
code running for 3 days to check if anything bad would happen.  This is the
test code using a PCA9685 (just because it was the easiest device we had
available to check read+write commands):

	http://pub.politreco.com/paste/test-i2c.c

 drivers/i2c/busses/i2c-designware-core.c    | 19 ++++---------------
 drivers/i2c/busses/i2c-designware-platdrv.c |  2 +-
 2 files changed, 5 insertions(+), 16 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-core.c b/drivers/i2c/busses/i2c-designware-core.c
index 6e25c01..b386c10 100644
--- a/drivers/i2c/busses/i2c-designware-core.c
+++ b/drivers/i2c/busses/i2c-designware-core.c
@@ -375,8 +375,12 @@ int i2c_dw_init(struct dw_i2c_dev *dev)
 	/* configure the i2c master */
 	dw_writel(dev, dev->master_cfg , DW_IC_CON);
 
+	/* Enable the adapter */
+	__i2c_dw_enable(dev, true);
+
 	if (dev->release_lock)
 		dev->release_lock(dev);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(i2c_dw_init);
@@ -405,9 +409,6 @@ static void i2c_dw_xfer_init(struct dw_i2c_dev *dev)
 	struct i2c_msg *msgs = dev->msgs;
 	u32 ic_con, ic_tar = 0;
 
-	/* Disable the adapter */
-	__i2c_dw_enable(dev, false);
-
 	/* if the slave address is ten bit address, enable 10BITADDR */
 	ic_con = dw_readl(dev, DW_IC_CON);
 	if (msgs[dev->msg_write_idx].flags & I2C_M_TEN) {
@@ -434,9 +435,6 @@ static void i2c_dw_xfer_init(struct dw_i2c_dev *dev)
 	/* enforce disabled interrupts (due to HW issues) */
 	i2c_dw_disable_int(dev);
 
-	/* Enable the adapter */
-	__i2c_dw_enable(dev, true);
-
 	/* Clear and enable interrupts */
 	i2c_dw_clear_int(dev);
 	dw_writel(dev, DW_IC_INTR_DEFAULT_MASK, DW_IC_INTR_MASK);
@@ -665,15 +663,6 @@ i2c_dw_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], int num)
 		goto done;
 	}
 
-	/*
-	 * We must disable the adapter before unlocking the &dev->lock mutex
-	 * below. Otherwise the hardware might continue generating interrupts
-	 * which in turn causes a race condition with the following transfer.
-	 * Needs some more investigation if the additional interrupts are
-	 * a hardware bug or this driver doesn't handle them correctly yet.
-	 */
-	__i2c_dw_enable(dev, false);
-
 	if (dev->msg_err) {
 		ret = dev->msg_err;
 		goto done;
diff --git a/drivers/i2c/busses/i2c-designware-platdrv.c b/drivers/i2c/busses/i2c-designware-platdrv.c
index c270f5f..0598695 100644
--- a/drivers/i2c/busses/i2c-designware-platdrv.c
+++ b/drivers/i2c/busses/i2c-designware-platdrv.c
@@ -320,7 +320,7 @@ static int dw_i2c_resume(struct device *dev)
 	clk_prepare_enable(i_dev->clk);
 
 	if (!i_dev->pm_runtime_disabled)
-		i2c_dw_init(i_dev);
+		i2c_dw_enable(i_dev);
 
 	return 0;
 }
-- 
2.4.2