Re: [PATCH v3 1/4] i2c: spacemit: configure ILCR for accurate SCL frequency

Alex Elder
2025-11-18T17:32:47Z

On 11/18/25 12:08 AM, Troy Mitchell wrote:
> The SpacemiT I2C controller's SCL (Serial Clock Line) frequency for
> master mode operations is determined by the ILCR (I2C Load Count Register).
> Previously, the driver relied on the hardware's reset default
> values for this register.
>
> The hardware's default ILCR values (SLV=0x156, FLV=0x5d) yield SCL
> frequencies lower than intended. For example, with the default
> 31.5 MHz input clock, these default settings result in an SCL
> frequency of approximately 93 kHz (standard mode) when targeting 100 kHz,
> and approximately 338 kHz (fast mode) when targeting 400 kHz.
> These frequencies are below the 100 kHz/400 kHz nominal speeds.
>
> This patch integrates the SCL frequency management into
> the Common Clock Framework (CCF). Specifically, the ILCR register,
> which acts as a frequency divider for the SCL clock, is now registered
> as a managed clock (scl_clk) within the CCF.
>
> This patch also cleans up unnecessary whitespace
> in the included header files.

I have a few comments below. Sorry I didn't comment on
earlier versions. > Reviewed-by: Yixun Lan <dlan@gentoo.org>
> Link: https://lore.kernel.org/all/176244506110.1925720.10807118665958896958.b4-ty@kernel.org/ [1]
> Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
> ---
> This patch was affected by the P1 Kconfig, which caused the maintainer
> to revert it.
> The current commit is a direct cherry-pick and reserves the original changelog.
> This note is to clarify for anyone who sees the cover letter marked as v2
> while the changelog entries reach v4.
> ---
> Changelog in v4:
> - initialize clk_init_data with {} so that init.flags is implicitly set to 0
> - minor cleanup and style fixes for better readability
> - remove unused spacemit_i2c_scl_clk_exclusive_put() cleanup callback
> - replace clk_set_rate_exclusive()/clk_rate_exclusive_put() pair with clk_set_rate()
> - simplify LCR LV field macros by using FIELD_GET/FIELD_MAX helpers
> - Link to v3: https://lore.kernel.org/all/20250814-k1-i2c-ilcr-v3-1-317723e74bcd@linux.spacemit.com/
>
> Changelog in v3:
> - use MASK macro in `recalc_rate` function
> - rename clock name
> - Link to v2: https://lore.kernel.org/r/20250718-k1-i2c-ilcr-v2-1-b4c68f13dcb1@linux.spacemit.com
>
> Changelog in v2:
> - Align line breaks.
> - Check `lv` in `clk_set_rate` function.
> - Force fast mode when SCL frequency is illegal or unavailable.
> - Change "linux/bits.h" to <linux/bits.h>
> - Kconfig: Add dependency on CCF.
> - Link to v1: https://lore.kernel.org/all/20250710-k1-i2c-ilcr-v1-1-188d1f460c7d@linux.spacemit.com/
> ---
> drivers/i2c/busses/Kconfig | 2 +-
> drivers/i2c/busses/i2c-k1.c | 159 ++++++++++++++++++++++++++++++++++++++++----
> 2 files changed, 146 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/i2c/busses/Kconfig b/drivers/i2c/busses/Kconfig
> index fd81e49638aaa161ae264a722e9e06adc7914cda..fedf5d31f9035b73a27a7f8a764bf5c26975d0e1 100644
> --- a/drivers/i2c/busses/Kconfig
> +++ b/drivers/i2c/busses/Kconfig
> @@ -798,7 +798,7 @@ config I2C_JZ4780
> config I2C_K1
> tristate "SpacemiT K1 I2C adapter"
> depends on ARCH_SPACEMIT || COMPILE_TEST
> - depends on OF
> + depends on OF && COMMON_CLK
> help
> This option enables support for the I2C interface on the SpacemiT K1
> platform.
> diff --git a/drivers/i2c/busses/i2c-k1.c b/drivers/i2c/busses/i2c-k1.c
> index 6b918770e612e098b8ad17418f420d87c94df166..e38a0ba71734ca602854c85672dcb61423453515 100644
> --- a/drivers/i2c/busses/i2c-k1.c
> +++ b/drivers/i2c/busses/i2c-k1.c
> @@ -4,18 +4,21 @@
> */
>
> #include <linux/bitfield.h>
> - #include <linux/clk.h>
> - #include <linux/i2c.h>
> - #include <linux/iopoll.h>
> - #include <linux/module.h>
> - #include <linux/of_address.h>
> - #include <linux/platform_device.h>
> +#include <linux/bits.h>
> +#include <linux/clk.h>
> +#include <linux/clk-provider.h>
> +#include <linux/i2c.h>
> +#include <linux/iopoll.h>
> +#include <linux/module.h>
> +#include <linux/of_address.h>
> +#include <linux/platform_device.h>
>
> /* spacemit i2c registers */
> #define SPACEMIT_ICR 0x0 /* Control register */
> #define SPACEMIT_ISR 0x4 /* Status register */
> #define SPACEMIT_IDBR 0xc /* Data buffer register */
> #define SPACEMIT_IRCR 0x18 /* Reset cycle counter */
> +#define SPACEMIT_ILCR 0x10 /* Load Count Register */
> #define SPACEMIT_IBMR 0x1c /* Bus monitor register */
>
> /* SPACEMIT_ICR register fields */
> @@ -87,6 +90,13 @@
> #define SPACEMIT_BMR_SDA BIT(0) /* SDA line level */
> #define SPACEMIT_BMR_SCL BIT(1) /* SCL line level */
>
> +#define SPACEMIT_LCR_LV_STANDARD_SHIFT 0
> +#define SPACEMIT_LCR_LV_FAST_SHIFT 9

Why do you need these SHIFT symbols? Just use the masks
and FIELD_GET() related macros. I'll provide examples below.

> +#define SPACEMIT_LCR_LV_STANDARD_MASK GENMASK(8, 0)
> +#define SPACEMIT_LCR_LV_FAST_MASK GENMASK(17, 9)
> +#define SPACEMIT_LCR_LV_STANDARD_MAX_VALUE FIELD_MAX(SPACEMIT_LCR_LV_STANDARD_MASK)
> +#define SPACEMIT_LCR_LV_FAST_MAX_VALUE FIELD_MAX(SPACEMIT_LCR_LV_FAST_MASK)
> +
> /* i2c bus recover timeout: us */
> #define SPACEMIT_I2C_BUS_BUSY_TIMEOUT 100000
>
> @@ -104,11 +114,20 @@ enum spacemit_i2c_state {
> SPACEMIT_STATE_WRITE,
> };
>
> +enum spacemit_i2c_mode {
> + SPACEMIT_MODE_STANDARD,
> + SPACEMIT_MODE_FAST
> +};

Will there ever be more than two i2c modes? If not, this
could simply be a Boolean.

> +
> /* i2c-spacemit driver's main struct */
> struct spacemit_i2c_dev {
> struct device *dev;
> struct i2c_adapter adapt;
>
> + struct clk_hw scl_clk_hw;
> + struct clk *scl_clk;
> + enum spacemit_i2c_mode mode;

Perhaps this could be:

bool fast_mode;

> +
> /* hardware resources */
> void __iomem *base;
> int irq;
> @@ -129,6 +148,79 @@ struct spacemit_i2c_dev {
> u32 status;
> };
>
> +static void spacemit_i2c_scl_clk_disable_unprepare(void *data)
> +{
> + struct spacemit_i2c_dev *i2c = data;
> +
> + clk_disable_unprepare(i2c->scl_clk);
> +}
> +
> +static int spacemit_i2c_clk_set_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long parent_rate)
> +{
> + struct spacemit_i2c_dev *i2c = container_of(hw, struct spacemit_i2c_dev, scl_clk_hw);
> + u32 lv, lcr, mask, shift, max_lv;
> +
> + lv = DIV_ROUND_UP(parent_rate, rate);

Would DIV_ROUND_CLOSEST() give a more accurate value?

> +
> + if (i2c->mode == SPACEMIT_MODE_STANDARD) {
> + mask = SPACEMIT_LCR_LV_STANDARD_MASK;
> + shift = SPACEMIT_LCR_LV_STANDARD_SHIFT;
> + max_lv = SPACEMIT_LCR_LV_STANDARD_MAX_VALUE;
> + } else if (i2c->mode == SPACEMIT_MODE_FAST) {
> + mask = SPACEMIT_LCR_LV_FAST_MASK;
> + shift = SPACEMIT_LCR_LV_FAST_SHIFT;
> + max_lv = SPACEMIT_LCR_LV_FAST_MAX_VALUE;
> + }
> +
> + if (!lv || lv > max_lv) {
> + dev_err(i2c->dev, "set scl clock failed: lv 0x%x", lv);
> + return -EINVAL;
> + }
> +
> + lcr = readl(i2c->base + SPACEMIT_ILCR);
> + lcr &= ~mask;
> + lcr |= lv << shift;
> + writel(lcr, i2c->base + SPACEMIT_ILCR);

FIELD_MODIFY(mask, &lcr, lv);

I suppose this might give you trouble because the mask isn't
constant at compile time, but anyway I think something like
this is simpler:

lv = DIV_ROUND_CLOSEST(parent_rate, rate);
lcr = readl(i2c->base + SPACEMIT_ILCR);
if (i2c->fast_mode)
FIELD_MODIFY(SPACEMIT_LCR_LV_FAST_MASK, &lcr, lv);
else
FIELD_MODIFY(SPACEMIT_LCR_LV_STANDARD_MASK, &lcr, lv);
writel(lcr, i2c->base + SPACEMIT_ILCR);

Note: I've never used FIELD_MODIFY(), but it looks like this is
how it's supposed to be used.

> + return 0;
> +}
> +
> +static long spacemit_i2c_clk_round_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long *parent_rate)
> +{
> + u32 lv, freq;
> +
> + lv = DIV_ROUND_UP(*parent_rate, rate);
> + freq = DIV_ROUND_UP(*parent_rate, lv);

Consider whether DIV_ROUND_CLOSEST() (in one or both of
these) provides a rate that is as close as possible to the
requested rate.

> +
> + return freq;
> +}
> +
> +static unsigned long spacemit_i2c_clk_recalc_rate(struct clk_hw *hw,
> + unsigned long parent_rate)
> +{
> + struct spacemit_i2c_dev *i2c = container_of(hw, struct spacemit_i2c_dev, scl_clk_hw);
> + u32 lcr, lv = 0;
> +
> + lcr = readl(i2c->base + SPACEMIT_ILCR);
> +
> + if (i2c->mode == SPACEMIT_MODE_STANDARD)
> + lv = FIELD_GET(SPACEMIT_LCR_LV_STANDARD_MASK, lcr);
> + else if (i2c->mode == SPACEMIT_MODE_FAST)
> + lv = FIELD_GET(SPACEMIT_LCR_LV_FAST_MASK, lcr);
> + else
> + return 0;

You shouldn't need the last else here. You can probably tell
by inspection that it will always be one mode or the other.
And a Boolean reinforces that.

> +
> + return DIV_ROUND_UP(parent_rate, lv);
> +}
> +
> +static const struct clk_ops spacemit_i2c_clk_ops = {
> + .set_rate = spacemit_i2c_clk_set_rate,
> + .round_rate = spacemit_i2c_clk_round_rate,
> + .recalc_rate = spacemit_i2c_clk_recalc_rate,
> +};
> +
> static void spacemit_i2c_enable(struct spacemit_i2c_dev *i2c)
> {
> u32 val;
> @@ -147,6 +239,26 @@ static void spacemit_i2c_disable(struct spacemit_i2c_dev *i2c)
> writel(val, i2c->base + SPACEMIT_ICR);
> }
>
> +static struct clk *spacemit_i2c_register_scl_clk(struct spacemit_i2c_dev *i2c,
> + struct clk *parent)
> +{
> + struct clk_init_data init = {};
> + char name[32];
> +
> + snprintf(name, sizeof(name), "%s_scl_clk", dev_name(i2c->dev));

What if dev_name(i2c->dev) is longer than 24? You should
be checking the return value here.

> +
> + init.name = name;
> + init.ops = &spacemit_i2c_clk_ops;
> + init.parent_data = (struct clk_parent_data[]) {
> + { .fw_name = "func" },
> + };
> + init.num_parents = 1;
> +
> + i2c->scl_clk_hw.init = &init;
> +
> + return devm_clk_register(i2c->dev, &i2c->scl_clk_hw);
> +}
> +
> static void spacemit_i2c_reset(struct spacemit_i2c_dev *i2c)
> {
> writel(SPACEMIT_CR_UR, i2c->base + SPACEMIT_ICR);
> @@ -246,7 +358,7 @@ static void spacemit_i2c_init(struct spacemit_i2c_dev *i2c)
> */
> val |= SPACEMIT_CR_DRFIE;
>
> - if (i2c->clock_freq == SPACEMIT_I2C_MAX_FAST_MODE_FREQ)
> + if (i2c->mode == SPACEMIT_MODE_FAST)
> val |= SPACEMIT_CR_MODE_FAST;
>
> /* disable response to general call */
> @@ -538,14 +650,15 @@ static int spacemit_i2c_probe(struct platform_device *pdev)
> dev_warn(dev, "failed to read clock-frequency property: %d\n", ret);

I don't think there's any need to warn when the "clock-frequency"
property is not found. It's an optional property, and the default
is specified in the binding to be 400 KHz.

> /* For now, this driver doesn't support high-speed. */
> - if (!i2c->clock_freq || i2c->clock_freq > SPACEMIT_I2C_MAX_FAST_MODE_FREQ) {
> - dev_warn(dev, "unsupported clock frequency %u; using %u\n",
> - i2c->clock_freq, SPACEMIT_I2C_MAX_FAST_MODE_FREQ);
> + if (i2c->clock_freq > SPACEMIT_I2C_MAX_STANDARD_MODE_FREQ &&
> + i2c->clock_freq <= SPACEMIT_I2C_MAX_FAST_MODE_FREQ) {
> + i2c->mode = SPACEMIT_MODE_FAST;
> + } else if (i2c->clock_freq && i2c->clock_freq <= SPACEMIT_I2C_MAX_STANDARD_MODE_FREQ) {
> + i2c->mode = SPACEMIT_MODE_STANDARD;
So this I2C driver will literally support *either* the standard
speed *or* the high speed frequency. Is this a necessary
restriction? If it is, perhaps the DT binding should be
clear that the speeds should be one of those supported
(because anything else will result in using a supported
speed, not the one provided).

(Sorry, these last two comments are not about your patch,
but about the driver that's already accepted.)

-Alex

> + } else {
> + dev_warn(i2c->dev, "invalid clock-frequency, fallback to fast mode");
> + i2c->mode = SPACEMIT_MODE_FAST;
> i2c->clock_freq = SPACEMIT_I2C_MAX_FAST_MODE_FREQ;
> - } else if (i2c->clock_freq < SPACEMIT_I2C_MAX_STANDARD_MODE_FREQ) {
> - dev_warn(dev, "unsupported clock frequency %u; using %u\n",
> - i2c->clock_freq, SPACEMIT_I2C_MAX_STANDARD_MODE_FREQ);
> - i2c->clock_freq = SPACEMIT_I2C_MAX_STANDARD_MODE_FREQ;
> }
>
> i2c->dev = &pdev->dev;
> @@ -567,10 +680,28 @@ static int spacemit_i2c_probe(struct platform_device *pdev)
> if (IS_ERR(clk))
> return dev_err_probe(dev, PTR_ERR(clk), "failed to enable func clock");
>
> + i2c->scl_clk = spacemit_i2c_register_scl_clk(i2c, clk);
> + if (IS_ERR(i2c->scl_clk))
> + return dev_err_probe(&pdev->dev, PTR_ERR(i2c->scl_clk),
> + "failed to register scl clock\n");
> +
> clk = devm_clk_get_enabled(dev, "bus");
> if (IS_ERR(clk))
> return dev_err_probe(dev, PTR_ERR(clk), "failed to enable bus clock");
>
> + ret = clk_set_rate(i2c->scl_clk, i2c->clock_freq);
> + if (ret)
> + return dev_err_probe(&pdev->dev, ret, "failed to set rate for SCL clock");
> +
> + ret = clk_prepare_enable(i2c->scl_clk);
> + if (ret)
> + return dev_err_probe(&pdev->dev, ret, "failed to prepare and enable clock");
> +
> + ret = devm_add_action_or_reset(dev, spacemit_i2c_scl_clk_disable_unprepare, i2c);
> + if (ret)
> + return dev_err_probe(&pdev->dev, ret,
> + "failed to register cleanup action for clk disable and unprepare");
> +
> spacemit_i2c_reset(i2c);
>
> i2c_set_adapdata(&i2c->adapt, i2c);
>