Re: [PATCH v2] i2c: bcm2835: Model Divider in CCF

Annaliese McDermond
2019-05-16T05:37:03Z

I’m just following up on this since I haven’t heard anything since I submitted the
v2 patch a week ago. Wolfram, does this look like a sane approach? Stefan,
were my changes in v2 acceptable?

I’m happy to go back to the drawing board to change things if there’s a better
answer to the problem.

Thanks for everyone’s patience with me trying to work through the proper
process.

--
Annaliese McDermond
nh6z@nh6z.net

> On May 8, 2019, at 12:12 AM, Annaliese McDermond <nh6z@nh6z.net> wrote:
>
> Model the I2C bus clock divider as a part of the Core Clock Framework.
> Primarily this removes the clk_get_rate() call from each transfer.
> This call causes problems for slave drivers that themselves have
> internal clock components that are controlled by an I2C interface.
> When the slave's internal clock component is prepared, the prepare
> lock is obtained, and it makes calls to the I2C subsystem to
> command the hardware to activate the clock. In order to perform
> the I2C transfer, this driver sets the divider, which requires
> it to get the parent clock rate, which it does with clk_get_rate().
> Unfortunately, this function will try to take the clock prepare
> lock, which is already held by the slave's internal clock calls
> creating a deadlock.
>
> Modeling the divider in the CCF natively removes this dependency
> and the divider value is only set upon changing the bus clock
> frequency or changes in the parent clock that cascade down to this
> divisor. This obviates the need to set the divider with every
> transfer and avoids the deadlock described above. It also should
> provide better clock debugging and save a few cycles on each
> transfer due to not having to recalcuate the divider value.
>
> Signed-off-by: Annaliese McDermond <nh6z@nh6z.net>
> ---
> Changes in v2:
> - Remove empty whitespace between declarations
> - Preserve comment regarding register rounding
> - Consolodate declarations and initial assignments
> - Return proper error pointer from bcm2835_i2c_register_div()
> - Properly handle return value from bcm2835_i2c_register_div()
> in bcm2835_i2c_probe()
> - Refactor divider calulation code into separate function
> - Use clk_set_rate_exclusive() to ensure the clock is not
> modified by other consumers
> - Deregister clock in bcm2835_i2c_remove() to properly clean up
> after ourselves
>
> drivers/i2c/busses/i2c-bcm2835.c | 144 ++++++++++++++++++++++++-------
> 1 file changed, 113 insertions(+), 31 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
> index d2fbb4bb4a43..e66e569b8f09 100644
> --- a/drivers/i2c/busses/i2c-bcm2835.c
> +++ b/drivers/i2c/busses/i2c-bcm2835.c
> @@ -4,6 +4,8 @@
> */
>
> #include <linux/clk.h>
> +#include <linux/clkdev.h>
> +#include <linux/clk-provider.h>
> #include <linux/completion.h>
> #include <linux/err.h>
> #include <linux/i2c.h>
> @@ -51,9 +53,7 @@
> struct bcm2835_i2c_dev {
> struct device *dev;
> void __iomem *regs;
> - struct clk *clk;
> int irq;
> - u32 bus_clk_rate;
> struct i2c_adapter adapter;
> struct completion completion;
> struct i2c_msg *curr_msg;
> @@ -74,12 +74,17 @@ static inline u32 bcm2835_i2c_readl(struct bcm2835_i2c_dev *i2c_dev, u32 reg)
> return readl(i2c_dev->regs + reg);
> }
>
> -static int bcm2835_i2c_set_divider(struct bcm2835_i2c_dev *i2c_dev)
> +#define to_clk_bcm2835_i2c(_hw) container_of(_hw, struct clk_bcm2835_i2c, hw)
> +struct clk_bcm2835_i2c {
> + struct clk_hw hw;
> + struct bcm2835_i2c_dev *i2c_dev;
> +};
> +
> +static int clk_bcm2835_i2c_calc_divider(unsigned long rate,
> + unsigned long parent_rate)
> {
> - u32 divider, redl, fedl;
> + u32 divider = DIV_ROUND_UP(parent_rate, rate);
>
> - divider = DIV_ROUND_UP(clk_get_rate(i2c_dev->clk),
> - i2c_dev->bus_clk_rate);
> /*
> * Per the datasheet, the register is always interpreted as an even
> * number, by rounding down. In other words, the LSB is ignored. So,
> @@ -88,12 +93,23 @@ static int bcm2835_i2c_set_divider(struct bcm2835_i2c_dev *i2c_dev)
> if (divider & 1)
> divider++;
> if ((divider < BCM2835_I2C_CDIV_MIN) ||
> - (divider > BCM2835_I2C_CDIV_MAX)) {
> - dev_err_ratelimited(i2c_dev->dev, "Invalid clock-frequency\n");
> + (divider > BCM2835_I2C_CDIV_MAX))
> return -EINVAL;
> - }
>
> - bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DIV, divider);
> + return divider;
> +}
> +
> +static int clk_bcm2835_i2c_set_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long parent_rate)
> +{
> + struct clk_bcm2835_i2c *div = to_clk_bcm2835_i2c(hw);
> + u32 redl, fedl;
> + u32 divider = clk_bcm2835_i2c_calc_divider(rate, parent_rate);
> +
> + if (divider == -EINVAL)
> + return -EINVAL;
> +
> + bcm2835_i2c_writel(div->i2c_dev, BCM2835_I2C_DIV, divider);
>
> /*
> * Number of core clocks to wait after falling edge before
> @@ -108,12 +124,60 @@ static int bcm2835_i2c_set_divider(struct bcm2835_i2c_dev *i2c_dev)
> */
> redl = max(divider / 4, 1u);
>
> - bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DEL,
> + bcm2835_i2c_writel(div->i2c_dev, BCM2835_I2C_DEL,
> (fedl << BCM2835_I2C_FEDL_SHIFT) |
> (redl << BCM2835_I2C_REDL_SHIFT));
> return 0;
> }
>
> +static long clk_bcm2835_i2c_round_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long *parent_rate)
> +{
> + u32 divider = clk_bcm2835_i2c_calc_divider(rate, *parent_rate);
> +
> + return DIV_ROUND_UP(*parent_rate, divider);
> +}
> +
> +static unsigned long clk_bcm2835_i2c_recalc_rate(struct clk_hw *hw,
> + unsigned long parent_rate)
> +{
> + struct clk_bcm2835_i2c *div = to_clk_bcm2835_i2c(hw);
> + u32 divider = bcm2835_i2c_readl(div->i2c_dev, BCM2835_I2C_DIV);
> +
> + return DIV_ROUND_UP(parent_rate, divider);
> +}
> +
> +static const struct clk_ops clk_bcm2835_i2c_ops = {
> + .set_rate = clk_bcm2835_i2c_set_rate,
> + .round_rate = clk_bcm2835_i2c_round_rate,
> + .recalc_rate = clk_bcm2835_i2c_recalc_rate,
> +};
> +
> +static struct clk *bcm2835_i2c_register_div(struct device *dev,
> + const char *mclk_name,
> + struct bcm2835_i2c_dev *i2c_dev)
> +{
> + struct clk_init_data init;
> + struct clk_bcm2835_i2c *priv;
> + const char *devname = dev_name(dev);
> +
> + init.ops = &clk_bcm2835_i2c_ops;
> + init.name = "bcm2835-i2c";
> + init.parent_names = (const char* []) { mclk_name };
> + init.num_parents = 1;
> + init.flags = 0;
> +
> + priv = devm_kzalloc(dev, sizeof(struct clk_bcm2835_i2c), GFP_KERNEL);
> + if (priv == NULL)
> + return ERR_PTR(-ENOMEM);
> +
> + priv->hw.init = &init;
> + priv->i2c_dev = i2c_dev;
> +
> + clk_hw_register_clkdev(&priv->hw, init.name, devname);
> + return devm_clk_register(dev, &priv->hw);
> +}
> +
> static void bcm2835_fill_txfifo(struct bcm2835_i2c_dev *i2c_dev)
> {
> u32 val;
> @@ -271,7 +335,7 @@ static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
> {
> struct bcm2835_i2c_dev *i2c_dev = i2c_get_adapdata(adap);
> unsigned long time_left;
> - int i, ret;
> + int i;
>
> for (i = 0; i < (num - 1); i++)
> if (msgs[i].flags & I2C_M_RD) {
> @@ -280,10 +344,6 @@ static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
> return -EOPNOTSUPP;
> }
>
> - ret = bcm2835_i2c_set_divider(i2c_dev);
> - if (ret)
> - return ret;
> -
> i2c_dev->curr_msg = msgs;
> i2c_dev->num_msgs = num;
> reinit_completion(&i2c_dev->completion);
> @@ -338,6 +398,9 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
> struct resource *mem, *irq;
> int ret;
> struct i2c_adapter *adap;
> + const char *mclk_name;
> + struct clk *bus_clk;
> + u32 bus_clk_rate;
>
> i2c_dev = devm_kzalloc(&pdev->dev, sizeof(*i2c_dev), GFP_KERNEL);
> if (!i2c_dev)
> @@ -351,21 +414,6 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
> if (IS_ERR(i2c_dev->regs))
> return PTR_ERR(i2c_dev->regs);
>
> - i2c_dev->clk = devm_clk_get(&pdev->dev, NULL);
> - if (IS_ERR(i2c_dev->clk)) {
> - if (PTR_ERR(i2c_dev->clk) != -EPROBE_DEFER)
> - dev_err(&pdev->dev, "Could not get clock\n");
> - return PTR_ERR(i2c_dev->clk);
> - }
> -
> - ret = of_property_read_u32(pdev->dev.of_node, "clock-frequency",
> - &i2c_dev->bus_clk_rate);
> - if (ret < 0) {
> - dev_warn(&pdev->dev,
> - "Could not read clock-frequency property\n");
> - i2c_dev->bus_clk_rate = 100000;
> - }
> -
> irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
> if (!irq) {
> dev_err(&pdev->dev, "No IRQ resource\n");
> @@ -380,6 +428,35 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
> return -ENODEV;
> }
>
> + mclk_name = of_clk_get_parent_name(pdev->dev.of_node, 0);
> +
> + bus_clk = bcm2835_i2c_register_div(&pdev->dev, mclk_name, i2c_dev);
> +
> + if (IS_ERR(bus_clk)) {
> + dev_err(&pdev->dev, "Could not register clock\n");
> + return PTR_ERR(bus_clk);
> + }
> +
> + ret = of_property_read_u32(pdev->dev.of_node, "clock-frequency",
> + &bus_clk_rate);
> + if (ret < 0) {
> + dev_warn(&pdev->dev,
> + "Could not read clock-frequency property\n");
> + bus_clk_rate = 100000;
> + }
> +
> + ret = clk_set_rate_exclusive(bus_clk, bus_clk_rate);
> + if (ret < 0) {
> + dev_err(&pdev->dev, "Could not set clock frequency\n");
> + return ret;
> + }
> +
> + ret = clk_prepare_enable(bus_clk);
> + if (ret) {
> + dev_err(&pdev->dev, "Couldn't prepare clock");
> + return ret;
> + }
> +
> adap = &i2c_dev->adapter;
> i2c_set_adapdata(adap, i2c_dev);
> adap->owner = THIS_MODULE;
> @@ -402,6 +479,11 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
> static int bcm2835_i2c_remove(struct platform_device *pdev)
> {
> struct bcm2835_i2c_dev *i2c_dev = platform_get_drvdata(pdev);
> + struct clk *bus_clk = devm_clk_get(i2c_dev->dev, "bcm2835-i2c");
> +
> + clk_rate_exclusive_put(bus_clk);
> + clk_disable_unprepare(bus_clk);
> + devm_clk_unregister(i2c_dev->dev, bus_clk);
>
> free_irq(i2c_dev->irq, i2c_dev);
> i2c_del_adapter(&i2c_dev->adapter);
> --
> 2.19.1