i2c-algo-bit: Implement a 50/50 SCL duty cycle

The original i2c-algo-bit implementation uses a 33/66 SCL duty cycle
when bits are being written on the bus. While the I2C specification
doesn't forbid it, this prevents us from driving the I2C bus to its
max speed, limiting us to 66 kbps max on standard I2C busses.

Implementing a 50/50 duty cycle instead lets us max out the bandwidth
up to the theoretical max of 100 kbps on standard I2C busses. This is
particularly important when large amounts of data need to be transfered
over the bus, as is the case with some TV adapters when the firmware is
being uploaded.

In fact this change even allows, at least in theory, fast-mode I2C
support at 125, 166 and 250 kbps. There's no way to reach the
theoretical max of 400 kbps with this implementation. But I don't
think we want to put efforts in that direction anyway: software-driven
I2C is very CPU-intensive and bad for latency.

Other timing changes:
* Don't set SDA high explicitly on error, we're going to issue a stop
  condition before we leave anyway.
* If an error occurs when sending the slave address, yield the CPU
  before retrying, and remove the additional delay after the new start
  condition.

Signed-off-by: Jean Delvare <khali@linux-fr.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/i2c-algo-bit.h b/include/linux/i2c-algo-bit.h
index d91dab8..9ee0f80 100644
--- a/include/linux/i2c-algo-bit.h
+++ b/include/linux/i2c-algo-bit.h
@@ -38,8 +38,10 @@ struct i2c_algo_bit_data {
 	int  (*getscl) (void *data);
 
 	/* local settings */
-	int udelay;		/* half-clock-cycle time in microsecs */
-				/* i.e. clock is (500 / udelay) KHz */
+	int udelay;		/* half clock cycle time in us,
+				   minimum 2 us for fast-mode I2C,
+				   minimum 5 us for standard-mode I2C and SMBus,
+				   maximum 50 us for SMBus */
 	int timeout;		/* in jiffies */
 };