i2c: davinci: Increase module clock frequency

I2C controller used in Keystone SoC has an undocumented peculiarity which
results in SDA-SCL margins being dependent on module clock. Driving high
capacity bus near its limits can result in STOP condition sometimes being
understood as REPEATED-START by slaves (or NACK instead of ACK, etc...).
Driving the module with higher clocks increases the margin between SDA and SCL
transitions, making the operations with higher bus rates more robust. Therefore,
target the module clock to 12MHz instead of 7MHz, still staying within
the specification limits.

Before the change STOP timing looked like this on 400kHz:

SDA   ----------+          +----
                 \        /
                  \      /
                   +----+
                       (1)
SCL   --+          +------------
         \        /
          \      /
           +----+
               (2)

While only point (1) signals STOP, point (2) could be incorrectly recognized as
repeated-START (almost no margin between SDA and SCL transitions).

After the change there is at least 600ns margin measured between SCL fall and
SDA fall during STOP generation:

SDA   ------+          +----
             \        /
              \      /
               +----+

SCL   --+          +--------
         \        /
          \      /
           +----+
           ->|    |<- 600ns
                ->|   |<- tSUSTO

So called tSUSTO (setup time for STOP condition) is still slightly higher than
600ns, so no problem here.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/i2c/busses/i2c-davinci.c b/drivers/i2c/busses/i2c-davinci.c
index c5628a4..a8bdcb5 100644
--- a/drivers/i2c/busses/i2c-davinci.c
+++ b/drivers/i2c/busses/i2c-davinci.c
@@ -202,8 +202,15 @@ static void i2c_davinci_calc_clk_dividers(struct davinci_i2c_dev *dev)
 	 * d is always 6 on Keystone I2C controller
 	 */
 
-	/* get minimum of 7 MHz clock, but max of 12 MHz */
-	psc = (input_clock / 7000000) - 1;
+	/*
+	 * Both Davinci and current Keystone User Guides recommend a value
+	 * between 7MHz and 12MHz. In reality 7MHz module clock doesn't
+	 * always produce enough margin between SDA and SCL transitions.
+	 * Measurements show that the higher the module clock is, the
+	 * bigger is the margin, providing more reliable communication.
+	 * So we better target for 12MHz.
+	 */
+	psc = (input_clock / 12000000) - 1;
 	if ((input_clock / (psc + 1)) > 12000000)
 		psc++;	/* better to run under spec than over */
 	d = (psc >= 2) ? 5 : 7 - psc;