[PATCH v3] clk: bcm2835: Round UART input clock up

[Stefan Wahren]

Hi Ivan,

Am 27.05.22 um 12:29 schrieb Ivan T. Ivanov:
> It was reported that RPi3[1] and RPi Zero 2W boards have issues with
> the Bluetooth. It turns out that when switching from initial to
> operation speed host and device no longer can talk each other because
> host uses incorrect UART baud rate.
>
> The UART driver used in this case is amba-pl011. Original fix, see
> below Github link[2], was inside pl011 module, but somehow it didn't
> look as the right place to fix. Beside that this original rounding
> function is not exactly perfect for all possible clock values. So I
> deiced to move the hack to the platform which actually need it.
>
> The UART clock is initialised to be as close to the requested
> frequency as possible without exceeding it. Now that there is a
> clock manager that returns the actual frequencies, an expected
> 48MHz clock is reported as 47999625. If the requested baud rate
> == requested clock/16, there is no headroom and the slight
> reduction in actual clock rate results in failure.
>
> If increasing a clock by less than 0.1% changes it from ..999..
> to ..000.., round it up.
i'm fine with this approach.
>
> [1] https://bugzilla.suse.com/show_bug.cgi?id=1188238
> [2] https://github.com/raspberrypi/linux/commit/ab3f1b39537f6d3825b8873006fbe2fc5ff057b7
>
> Cc: Phil Elwell <phil at raspberrypi.org>
> Signed-off-by: Ivan T. Ivanov <iivanov at suse.de>
> ---
>
> Changes since v2
> * Added more information in commit message
> * Changed hand crafted round function with the one form math.h
>
> Changes since v1
> Make bcm2835_clock_round() static to fix following warning
> when compiling for riscv:
> drivers/clk/bcm/clk-bcm2835.c:997:15: warning: no previous prototype for 'bcm2835_clock_round' [-Wmissing-prototypes]
> Reported-by: kernel test robot <lkp at intel.com>
>
>   drivers/clk/bcm/clk-bcm2835.c | 32 ++++++++++++++++++++++++++++++--
>   1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index 48a1eb9f2d55..cee59990a57b 100644
> --- a/drivers/clk/bcm/clk-bcm2835.c
> +++ b/drivers/clk/bcm/clk-bcm2835.c
> @@ -30,6 +30,7 @@
>   #include <linux/debugfs.h>
>   #include <linux/delay.h>
>   #include <linux/io.h>
> +#include <linux/math.h>
>   #include <linux/module.h>
>   #include <linux/of_device.h>
>   #include <linux/platform_device.h>
> @@ -502,6 +503,8 @@ struct bcm2835_clock_data {
>   	bool low_jitter;
>   
>   	u32 tcnt_mux;
> +
> +	bool round_up;
>   };
>   
>   struct bcm2835_gate_data {
> @@ -993,12 +996,31 @@ static long bcm2835_clock_rate_from_divisor(struct bcm2835_clock *clock,
>   	return temp;
>   }
>   
> +static unsigned long bcm2835_clock_round(unsigned long clk)
> +{
> +	unsigned long scaler;
> +
> +	scaler = 1;
> +	while (scaler * 100000 < clk)
> +		scaler *= 10;

I'm not sure about overflow behavior for this loop. But it's better to 
avoid any potential endless loops here.

Best regards

> +
> +	/*
> +	 * If increasing a clock by less than 0.1% changes it
> +	 * from ..999.. to ..000.., round up.
> +	 */
> +	if ((clk + scaler - 1) / scaler % 1000 == 0)
> +		clk = roundup(clk, scaler);
> +
> +	return clk;
> +}
> +
>   static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw,
>   					    unsigned long parent_rate)
>   {
>   	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
>   	struct bcm2835_cprman *cprman = clock->cprman;
>   	const struct bcm2835_clock_data *data = clock->data;
> +	unsigned long rate;
>   	u32 div;
>   
>   	if (data->int_bits == 0 && data->frac_bits == 0)
> @@ -1006,7 +1028,12 @@ static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw,
>   
>   	div = cprman_read(cprman, data->div_reg);
>   
> -	return bcm2835_clock_rate_from_divisor(clock, parent_rate, div);
> +	rate = bcm2835_clock_rate_from_divisor(clock, parent_rate, div);
> +
> +	if (data->round_up)
> +		rate = bcm2835_clock_round(rate);
> +
> +	return rate;
>   }
>   
>   static void bcm2835_clock_wait_busy(struct bcm2835_clock *clock)
> @@ -2143,7 +2170,8 @@ static const struct bcm2835_clk_desc clk_desc_array[] = {
>   		.div_reg = CM_UARTDIV,
>   		.int_bits = 10,
>   		.frac_bits = 12,
> -		.tcnt_mux = 28),
> +		.tcnt_mux = 28,
> +		.round_up = true),
>   
>   	/* TV encoder clock.  Only operating frequency is 108Mhz.  */
>   	[BCM2835_CLOCK_VEC]	= REGISTER_PER_CLK(