[PATCH 2/3] i2c: meson: Use 50% duty cycle for I2C clock

Lucas Tanure
2022-03-26T10:23:15Z

The duty cycle of 33% is less than the required
by the I2C specs for the LOW period of the SCL
clock.

Move the duty cyle to 50% for 100Khz or lower
clocks, and (40% High SCL / 60% Low SCL) duty
cycle for clocks above 100Khz.

Signed-off-by: Lucas Tanure <tanure@linux.com>
---
drivers/i2c/busses/i2c-meson.c | 45 +++++++++++++++++++++++++---------
1 file changed, 33 insertions(+), 12 deletions(-)

diff --git a/drivers/i2c/busses/i2c-meson.c b/drivers/i2c/busses/i2c-meson.c
index 4b4a5b2d77ab..b913ba20f06e 100644
--- a/drivers/i2c/busses/i2c-meson.c
+++ b/drivers/i2c/busses/i2c-meson.c
@@ -140,29 +140,50 @@ static void meson_i2c_add_token(struct meson_i2c *i2c, int token)
static void meson_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
{
unsigned long clk_rate = clk_get_rate(i2c->clk);
- unsigned int div;
+ unsigned int div_h, div_l;

- div = DIV_ROUND_UP(clk_rate, freq);
- div -= FILTER_DELAY;
- div = DIV_ROUND_UP(div, i2c->data->div_factor);
+ if (freq <= 100000) {
+ div_h = DIV_ROUND_UP(clk_rate, freq);
+ div_l = DIV_ROUND_UP(div_h, 4);
+ div_h = DIV_ROUND_UP(div_h, 2) - FILTER_DELAY;
+ } else {
+ /* According to I2C-BUS Spec 2.1, in FAST-MODE, the minimum LOW period is 1.3uS, and
+ * minimum HIGH is least 0.6us.
+ * For 400000 freq, the period is 2.5us. To keep within the specs, give 40% of period to
+ * HIGH and 60% to LOW. This means HIGH at 1.0us and LOW 1.5us.
+ * The same applies for Fast-mode plus, where LOW is 0.5us and HIGH is 0.26us.
+ * Duty = H/(H + L) = 2/5
+ */
+ div_h = DIV_ROUND_UP(clk_rate * 2, freq * 5) - FILTER_DELAY;
+ div_l = DIV_ROUND_UP(clk_rate * 3, freq * 5 * 2);
+ }

/* clock divider has 12 bits */
- if (div > GENMASK(11, 0)) {
+ if (div_h > GENMASK(11, 0)) {
dev_err(i2c->dev, "requested bus frequency too low\n");
- div = GENMASK(11, 0);
+ div_h = GENMASK(11, 0);
+ }
+ if (div_l > GENMASK(11, 0)) {
+ dev_err(i2c->dev, "requested bus frequency too low\n");
+ div_l = GENMASK(11, 0);
}

meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIV_MASK,
- FIELD_PREP(REG_CTRL_CLKDIV_MASK, div & GENMASK(9, 0)));
+ FIELD_PREP(REG_CTRL_CLKDIV_MASK, div_h & GENMASK(9, 0)));

meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
- FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div >> 10));
+ FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div_h >> 10));
+
+
+ /* set SCL low delay */
+ meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_MASK,
+ (div_l << REG_SLV_SCL_LOW_SHIFT) & REG_SLV_SCL_LOW_MASK);

- /* Disable HIGH/LOW mode */
- meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, 0);
+ /* Enable HIGH/LOW mode */
+ meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, REG_SLV_SCL_LOW_EN);

- dev_dbg(i2c->dev, "%s: clk %lu, freq %u, div %u\n", __func__,
- clk_rate, freq, div);
+ dev_dbg(i2c->dev, "%s: clk %lu, freq %u, divh %u, divl %u\n", __func__,
+ clk_rate, freq, div_h, div_l);
}

static void meson_i2c_get_data(struct meson_i2c *i2c, char *buf, int len)
--
2.35.1