[U-Boot] [PATCH] i.MX6: Ensure AHB clock is 132MHz in low freq boot mode

[Stefano Babic <sbabic@denx.de>]

Hi Anson,

On 06/01/2014 22:31, Anson Huang wrote:
> For low freq boot mode(ARM boot up with 396MHz), ROM
> will not set AHB clock to 132MHz, and the reset value of
> AHB divider is incorrect which will lead to wrong AHB
> rate, need to correct it. To enable low freq boot mode,
> need to set BOOT_CFG2[2] to high, tested on i.MX6Q/DL
> SabreSD board and i.MX6SL EVK board.
> 
> Signed-off-by: Anson Huang <b20788@freescale.com>
> ---
>  arch/arm/cpu/armv7/mx6/soc.c |   20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/arch/arm/cpu/armv7/mx6/soc.c b/arch/arm/cpu/armv7/mx6/soc.c
> index a390296..edf3139 100644
> --- a/arch/arm/cpu/armv7/mx6/soc.c
> +++ b/arch/arm/cpu/armv7/mx6/soc.c
> @@ -131,10 +131,30 @@ static void imx_set_wdog_powerdown(bool enable)
>  	writew(enable, &wdog2->wmcr);
>  }
>  
> +static void set_ahb_rate(u32 val)
> +{
> +	struct mxc_ccm_reg *mxc_ccm = (struct mxc_ccm_reg *)CCM_BASE_ADDR;
> +	u32 reg, div;
> +
> +	div = get_periph_clk() / val - 1;
> +	reg = readl(&mxc_ccm->cbcdr);
> +
> +	writel((reg & (~MXC_CCM_CBCDR_AHB_PODF_MASK)) |
> +		(div << MXC_CCM_CBCDR_AHB_PODF_OFFSET), &mxc_ccm->cbcdr);
> +}
> +
>  int arch_cpu_init(void)
>  {
>  	init_aips();
>  
> +	/*
> +	 * When low freq boot is enabled, ROM will not set AHB
> +	 * freq, so we need to ensure AHB freq is 132MHz in such
> +	 * scenario.
> +	 */
> +	if (mxc_get_clock(MXC_ARM_CLK) == 396000000)
> +		set_ahb_rate(132000000);
> +

Are we sure that mxc_get_clock() returns *exactly* 396000000 ?

As far as I see, this depends firstly from MXC_HCLK, that can be
configurable with CONFIG_SYS_MX6_HCLK and could have a different value
from the default 24000000. I do not know if there is cases where
CONFIG_SYS_MX6_HCLK is useful, no boards at the moment are setting it.

But the test requires that the computed frequency is exactly 396Mhz. Is
this reliable or should we simply test if frequency is lower of a
defined threshold ?

Best regards,
Stefano Babic


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