[PATCH v7 1/1] i2c: lpi2c: use clk notifier for rate changes

[Andi Shyti]

Hi Alexander,

On Tue, Nov 07, 2023 at 03:12:01PM +0100, Alexander Stein wrote:
> From: Alexander Sverdlin <alexander.sverdlin at siemens.com>
> 
> Instead of repeatedly calling clk_get_rate for each transfer, register
> a clock notifier to update the cached divider value each time the clock
> rate actually changes.
> There is also a lockdep splat if a I2C based clock provider needs to
> access the i2c bus while in recalc_rate(). "prepare_lock" is about to
> be locked recursively.
> 
> Fixes: a55fa9d0e42e ("i2c: imx-lpi2c: add low power i2c bus driver")

What's the exact fix here? Is it the lockdep warning? Is it
actually causing a real deadlock?

> Signed-off-by: Alexander Sverdlin <alexander.sverdlin at siemens.com>
> Reviewed-by: Alexander Stein <alexander.stein at ew.tq-group.com>
> Signed-off-by: Alexander Stein <alexander.stein at ew.tq-group.com>
> ---
> Changes in v7:
> * Use dev_err_probe
> * Reworked/Shortened the commit message
>  It is not about saving CPU cycles, but to avoid locking the clk subsystem
>  upon each transfer.
> 
>  drivers/i2c/busses/i2c-imx-lpi2c.c | 40 +++++++++++++++++++++++++++++-
>  1 file changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx-lpi2c.c b/drivers/i2c/busses/i2c-imx-lpi2c.c
> index 678b30e90492a..3070e605fd8ff 100644
> --- a/drivers/i2c/busses/i2c-imx-lpi2c.c
> +++ b/drivers/i2c/busses/i2c-imx-lpi2c.c
> @@ -5,6 +5,7 @@
>   * Copyright 2016 Freescale Semiconductor, Inc.
>   */
>  
> +#include <linux/atomic.h>
>  #include <linux/clk.h>
>  #include <linux/completion.h>
>  #include <linux/delay.h>
> @@ -99,6 +100,8 @@ struct lpi2c_imx_struct {
>  	__u8			*rx_buf;
>  	__u8			*tx_buf;
>  	struct completion	complete;
> +	struct notifier_block	clk_change_nb;
> +	atomic_t		rate_per;
>  	unsigned int		msglen;
>  	unsigned int		delivered;
>  	unsigned int		block_data;
> @@ -197,6 +200,20 @@ static void lpi2c_imx_stop(struct lpi2c_imx_struct *lpi2c_imx)
>  	} while (1);
>  }
>  
> +static int lpi2c_imx_clk_change_cb(struct notifier_block *nb,
> +				   unsigned long action, void *data)
> +{
> +	struct clk_notifier_data *ndata = data;
> +	struct lpi2c_imx_struct *lpi2c_imx = container_of(nb,
> +							  struct lpi2c_imx_struct,
> +							  clk_change_nb);
> +
> +	if (action & POST_RATE_CHANGE)
> +		atomic_set(&lpi2c_imx->rate_per, ndata->new_rate);
> +
> +	return NOTIFY_OK;
> +}
> +
>  /* CLKLO = I2C_CLK_RATIO * CLKHI, SETHOLD = CLKHI, DATAVD = CLKHI/2 */
>  static int lpi2c_imx_config(struct lpi2c_imx_struct *lpi2c_imx)
>  {
> @@ -207,7 +224,7 @@ static int lpi2c_imx_config(struct lpi2c_imx_struct *lpi2c_imx)
>  
>  	lpi2c_imx_set_mode(lpi2c_imx);
>  
> -	clk_rate = clk_get_rate(lpi2c_imx->clks[0].clk);
> +	clk_rate = atomic_read(&lpi2c_imx->rate_per);
>  	if (!clk_rate)
>  		return -EINVAL;

Doesn't seem like EINVAL, defined as "Invalid argument", is the
correct number here. As we are failing to read the clock rate, do
you think EIO is better?

>  
> @@ -590,6 +607,27 @@ static int lpi2c_imx_probe(struct platform_device *pdev)
>  	if (ret)
>  		return ret;
>  
> +	lpi2c_imx->clk_change_nb.notifier_call = lpi2c_imx_clk_change_cb;
> +	ret = devm_clk_notifier_register(&pdev->dev, lpi2c_imx->clks[0].clk,
> +					 &lpi2c_imx->clk_change_nb);
> +	if (ret)
> +		return dev_err_probe(&pdev->dev, ret,
> +				     "can't register peripheral clock notifier\n");

can't we fall back to how things were instead of failing the
probe? But I'm not sure it would remove the lockdep warning,
though. I can live with it.

> +	/*
> +	 * Lock the clock rate to avoid rate change between clk_get_rate() and
> +	 * atomic_set()
> +	 */
> +	ret = clk_rate_exclusive_get(lpi2c_imx->clks[0].clk);
> +	if (ret)
> +		return dev_err_probe(&pdev->dev, ret,
> +				     "can't lock I2C peripheral clock rate\n");
> +
> +	atomic_set(&lpi2c_imx->rate_per, clk_get_rate(lpi2c_imx->clks[0].clk));
> +	clk_rate_exclusive_put(lpi2c_imx->clks[0].clk);
> +	if (!atomic_read(&lpi2c_imx->rate_per))
> +		return dev_err_probe(&pdev->dev, -EINVAL,
> +				     "can't get I2C peripheral clock rate\n");
> +

Not a bad patch, would be nice if all the above was provided by
the library so that other drivers can use it.

Andi

>  	pm_runtime_set_autosuspend_delay(&pdev->dev, I2C_PM_TIMEOUT);
>  	pm_runtime_use_autosuspend(&pdev->dev);
>  	pm_runtime_get_noresume(&pdev->dev);
> -- 
> 2.34.1
>