Re: [PATCH] clk: imx: imx6ul: change flexcan clock to support CiA bitrates

[Shawn Guo <shawnguo@kernel.org>]

On Wed, Mar 11, 2020 at 12:03:40PM +0000, Waibel Georg wrote:
> Setting a CAN bitrate of 800kbit/s fails with a bitrate error of 1.3% if the
> flexcan module is clocked at 30MHz (CAN_CLK_ROOT). This patch changes the clock
> frequency from 30MHz to 40MHz which allows to support all bitrates recommended

But code changes IMX6UL_CLK_PLL3_60M to IMX6UL_CLK_PLL3_80M?  Or
IMX6UL_CLK_CAN_SEL is different from CAN_CLK_ROOT in the context?

Shawn

> by CiA.
> 
> Background:
> CAN in Automation document 102 (CiA102) recommends the CAN bitrates 10, 20, 50,
> 125, 250, 500, 800 and 1000kbit/s.
> 
> With the flexcan serial clock at 30MHz (original value) setting some common
> bitrates ("ip link set canX type can bitrate <bitrate>") gives the following
> results:
> requested value / actually set value
> 5000: bitrate 5000 sample-point 0.708
> 10000: bitrate 10000 sample-point 0.866
> 20000: bitrate 20000 sample-point 0.866
> 40000: bitrate 40000 sample-point 0.866
> 50000: bitrate 50000 sample-point 0.866
> 80000: bitrate 80000 sample-point 0.866
> 100000: bitrate 100000 sample-point 0.866
> 125000: bitrate 125000 sample-point 0.875
> 250000: bitrate 250000 sample-point 0.866
> 400000: bitrate 400000 sample-point 0.866
> 500000: bitrate 500000 sample-point 0.866
> 666666: bitrate 666666 sample-point 0.800
> 800000: bitrate 789473 sample-point 0.789 !!!bitrate error 1.3%
> 1000000: bitrate 1000000 sample-point 0.733
> 
> With the flexcan serial clock at 40MHz (new value) we get this:
> 5000: no more possible
> 10000: bitrate 10000 sample-point 0.875
> 20000: bitrate 20000 sample-point 0.875
> 40000: bitrate 40000 sample-point 0.850
> 50000: bitrate 50000 sample-point 0.875
> 80000: bitrate 80000 sample-point 0.850
> 100000: bitrate 100000 sample-point 0.875
> 125000: bitrate 125000 sample-point 0.875
> 250000: bitrate 250000 sample-point 0.875
> 400000: bitrate 400000 sample-point 0.850
> 500000: bitrate 500000 sample-point 0.875
> 666666: bitrate 666666 sample-point 0.800
> 800000: bitrate 800000 sample-point 0.800
> 1000000: bitrate 1000000 sample-point 0.750
> 
> A drawback of the modification is that 5kbit/s is no more supported.
> 
> Setting the flexcan serial clock to 60MHz or 80MHz would produce similar
> results but with losing even more bitrates at the lower end.
> 
> Changing the flexcan serial clock to 40MHz might apply for other SoCs
> using the flaxcan module as well (e.g. imx6q/d/s..). But since I don't
> have such hardware to test I did not add this to the patch.
> 
> Signed-off-by: Georg Waibel <georg.waibel@wiedemann-group.de>
> ---
>  drivers/clk/imx/clk-imx6ul.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/imx/clk-imx6ul.c b/drivers/clk/imx/clk-imx6ul.c
> index dafc8806b03e..5dbb6a937732 100644
> --- a/drivers/clk/imx/clk-imx6ul.c
> +++ b/drivers/clk/imx/clk-imx6ul.c
> @@ -503,7 +503,7 @@ static void __init imx6ul_clocks_init(struct device_node *ccm_node)
>  		clk_prepare_enable(hws[IMX6UL_CLK_USBPHY2_GATE]->clk);
>  	}
>  
> -	clk_set_parent(hws[IMX6UL_CLK_CAN_SEL]->clk, hws[IMX6UL_CLK_PLL3_60M]->clk);
> +	clk_set_parent(hws[IMX6UL_CLK_CAN_SEL]->clk, hws[IMX6UL_CLK_PLL3_80M]->clk);
>  	if (clk_on_imx6ul())
>  		clk_set_parent(hws[IMX6UL_CLK_SIM_PRE_SEL]->clk, hws[IMX6UL_CLK_PLL3_USB_OTG]->clk);
>  	else if (clk_on_imx6ull())
> -- 
> 2.17.1
> 
> 
> 
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