[PATCH v2 2/2] i2c: meson: Use 50% duty cycle for I2C clock

Lucas Tanure
2022-04-09T16:46:26Z

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 | 70 +++++++++++++++++++++++++++++-----
1 file changed, 60 insertions(+), 10 deletions(-)

diff --git a/drivers/i2c/busses/i2c-meson.c b/drivers/i2c/busses/i2c-meson.c
index 4b4a5b2d77ab..50dab123380a 100644
--- a/drivers/i2c/busses/i2c-meson.c
+++ b/drivers/i2c/busses/i2c-meson.c
@@ -65,10 +65,6 @@ enum {
STATE_WRITE,
};

-struct meson_i2c_data {
- unsigned char div_factor;
-};
-
/**
* struct meson_i2c - Meson I2C device private data
*
@@ -109,6 +105,10 @@ struct meson_i2c {
const struct meson_i2c_data *data;
};

+struct meson_i2c_data {
+ void (*set_clk_div)(struct meson_i2c *i2c, unsigned int freq);
+};
+
static void meson_i2c_set_mask(struct meson_i2c *i2c, int reg, u32 mask,
u32 val)
{
@@ -137,14 +137,62 @@ static void meson_i2c_add_token(struct meson_i2c *i2c, int token)
i2c->num_tokens++;
}

-static void meson_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
+static void meson_gxbb_axg_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
+{
+ unsigned long clk_rate = clk_get_rate(i2c->clk);
+ unsigned int div_h, div_l;
+
+ /* 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
+ */
+ if (freq <= I2C_MAX_STANDARD_MODE_FREQ) {
+ 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 {
+ 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_h > GENMASK(11, 0)) {
+ dev_err(i2c->dev, "requested bus frequency too low\n");
+ 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_h & GENMASK(9, 0)));
+
+ meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
+ 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,
+ FIELD_PREP(REG_SLV_SCL_LOW_MASK, div_l));
+
+ /* 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, divh %u, divl %u\n", __func__,
+ clk_rate, freq, div_h, div_l);
+}
+
+static void meson6_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
{
unsigned long clk_rate = clk_get_rate(i2c->clk);
unsigned int div;

div = DIV_ROUND_UP(clk_rate, freq);
div -= FILTER_DELAY;
- div = DIV_ROUND_UP(div, i2c->data->div_factor);
+ div = DIV_ROUND_UP(div, 4);

/* clock divider has 12 bits */
if (div > GENMASK(11, 0)) {
@@ -472,7 +520,9 @@ static int meson_i2c_probe(struct platform_device *pdev)
meson_i2c_set_mask(i2c, REG_SLAVE_ADDR,
REG_SLV_SDA_FILTER_MASK | REG_SLV_SCL_FILTER_MASK, 0);

- meson_i2c_set_clk_div(i2c, timings.bus_freq_hz);
+ if (!i2c->data->set_clk_div)
+ return -EINVAL;
+ i2c->data->set_clk_div(i2c, timings.bus_freq_hz);

ret = i2c_add_adapter(&i2c->adap);
if (ret < 0) {
@@ -494,15 +544,15 @@ static int meson_i2c_remove(struct platform_device *pdev)
}

static const struct meson_i2c_data i2c_meson6_data = {
- .div_factor = 4,
+ .set_clk_div = meson6_i2c_set_clk_div,
};

static const struct meson_i2c_data i2c_gxbb_data = {
- .div_factor = 4,
+ .set_clk_div = meson_gxbb_axg_i2c_set_clk_div,
};

static const struct meson_i2c_data i2c_axg_data = {
- .div_factor = 3,
+ .set_clk_div = meson_gxbb_axg_i2c_set_clk_div,
};

static const struct of_device_id meson_i2c_match[] = {
--
2.35.1