[PATCH] ARM: imx6: Fix procedure to switch the parent of LDB_DI_CLK

Dirk Behme
2014-05-19T07:22:03Z

Hi Fabio and Ranjani,

On 09.04.2014 13:55, Fabio Estevam wrote:
> From: Fabio Estevam <fabio.estevam@freescale.com>
>
> Due to incorrect placement of the clock gate cell in the ldb_di[x]_clk tree,
> the glitchy parent mux of ldb_di[x]_clk can cause a glitch to enter the
> ldb_di_ipu_div divider. If the divider gets locked up, no ldb_di[x]_clk is
> generated, and the LVDS display will hang when the ipu_di_clk is sourced from
> ldb_di_clk.
>
> To fix the problem, both the new and current parent of the ldb_di_clk should
> be disabled before the switch. This patch ensures that correct steps are
> followed when ldb_di_clk parent is switched in the beginning of boot.
>
> Signed-off-by: Ranjani Vaidyanathan <Ranjani.Vaidyanathan@freescale.com>
> Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
> ---
> arch/arm/mach-imx/clk-imx6q.c | 125 ++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 121 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm/mach-imx/clk-imx6q.c b/arch/arm/mach-imx/clk-imx6q.c
> index 20ad0d1..3ee45f4 100644
> --- a/arch/arm/mach-imx/clk-imx6q.c
> +++ b/arch/arm/mach-imx/clk-imx6q.c
> @@ -140,6 +140,123 @@ static struct clk_div_table video_div_table[] = {
> { /* sentinel */ }
> };
>
> +static void init_ldb_clks(enum mx6q_clks new_parent)
> +{
> + struct device_node *np;
> + static void __iomem *ccm_base;
> + unsigned int reg;
> +
> + np = of_find_compatible_node(NULL, NULL, "fsl,imx6q-ccm");
> + ccm_base = of_iomap(np, 0);
> + WARN_ON(!ccm_base);
> +
> + /*
> + * Need to follow a strict procedure when changing the LDB
> + * clock, else we can introduce a glitch. Things to keep in
> + * mind:
> + * 1. The current and new parent clocks must be disabled.
> + * 2. The default clock for ldb_dio_clk is mmdc_ch1 which has
> + * no CG bit.
> + * 3. In the RTL implementation of the LDB_DI_CLK_SEL mux
> + * the top four options are in one mux and the PLL3 option along
> + * with another option is in the second mux. There is third mux
> + * used to decide between the first and second mux.
> + * The code below switches the parent to the bottom mux first
> + * and then manipulates the top mux. This ensures that no glitch
> + * will enter the divider.
> + *
> + * Need to disable MMDC_CH1 clock manually as there is no CG bit
> + * for this clock. The only way to disable this clock is to move
> + * it topll3_sw_clk and then to disable pll3_sw_clk
> + * Make sure periph2_clk2_sel is set to pll3_sw_clk
> + */
> + reg = readl_relaxed(ccm_base + 0x18);
> + reg &= ~(1 << 20);
> + writel_relaxed(reg, ccm_base + 0x18);
> +
> + /* Set MMDC_CH1 mask bit */
> + reg = readl_relaxed(ccm_base + 0x4);
> + reg |= 1 << 16;
> + writel_relaxed(reg, ccm_base + 0x4);

Just a hopefully simple question: Why do you mask MMDC_CH1 *after*
switching to pll3_sw_clk above? Why not mask first, and then switch
periph2_clk2_sel to pll3_sw_clk?

Thanks

Dirk