There are several cases where current clock configuration algorithm produces not optimal results: - truncation in "clk" calculation leads to the fact that actual BUS frequency will be always higher than spec except two exact module frequences 8MHz and 12MHz in the whole 7-12MHz range of permitted frequences - driver configures SCL HIGH to LOW ratio always 1 to 1 and this doesn't work well in 400kHz case, namely minimum time of LOW state (according to I2C Spec 2.1) 1.3us will not be fulfilled. HIGH to LOW ratio 1 to 2 would be more approriate here.
Signed-off-by: Michael Lawnick <[email protected]> Signed-off-by: Alexander Sverdlin <[email protected]> --- Tested on custom Keystone II SoC-based board with complicated I2C network working on the edge of its HW specs (signal quality). In this case 1 to 2 HIGH to LOW relation really improves the reliability with 400kHz bus rate. drivers/i2c/busses/i2c-davinci.c | 27 ++++++++++++++++++++++++--- 1 files changed, 24 insertions(+), 3 deletions(-) diff --git a/drivers/i2c/busses/i2c-davinci.c b/drivers/i2c/busses/i2c-davinci.c index e35c9df..4a110af 100644 --- a/drivers/i2c/busses/i2c-davinci.c +++ b/drivers/i2c/busses/i2c-davinci.c @@ -211,9 +211,30 @@ static void i2c_davinci_calc_clk_dividers(struct davinci_i2c_dev *dev) psc++; /* better to run under spec than over */ d = (psc >= 2) ? 5 : 7 - psc; - clk = ((input_clock / (psc + 1)) / (pdata->bus_freq * 1000)) - (d << 1); - clkh = clk >> 1; - clkl = clk - clkh; + clk = ((input_clock / (psc + 1)) / (pdata->bus_freq * 1000)); + /* Avoid driving the bus too fast because of rounding errors above */ + if (input_clock / (psc + 1) / clk > pdata->bus_freq * 1000) + clk++; + /* + * According to I2C-BUS Spec 2.1, in FAST-MODE LOW period should be at + * least 1.3uS, which is not the case with 50% duty cycle. Driving HIGH + * to LOW ratio as 1 to 2 is more safe. + */ + if (pdata->bus_freq > 100) + clkl = (clk << 1) / 3; + else + clkl = (clk >> 1); + /* + * It's not always possible to have 1 to 2 ratio when d=7, so fall back + * to minimal possible clkh in this case. + */ + if (clk >= clkl + d) { + clkh = clk - clkl - d; + clkl -= d; + } else { + clkh = 0; + clkl = clk - (d << 1); + } davinci_i2c_write_reg(dev, DAVINCI_I2C_PSC_REG, psc); davinci_i2c_write_reg(dev, DAVINCI_I2C_CLKH_REG, clkh); -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
