Re: [PATCH v3 1/5] clk: imx: lpcg-scu: SW workaround for errata (e10858)

[Abel Vesa <abel.vesa@linaro.org>]

On 24-10-27 20:00:07, Peng Fan (OSS) wrote:
> From: Peng Fan <peng.fan@nxp.com>
> 
> Back-to-back LPCG writes can be ignored by the LPCG register due to
> a HW bug. The writes need to be separated by at least 4 cycles of
> the gated clock. See https://www.nxp.com.cn/docs/en/errata/IMX8_1N94W.pdf
> 
> The workaround is implemented as follows:
> 1. For clocks running greater than or equal to 24MHz, a read
> followed by the write will provide sufficient delay.
> 2. For clocks running below 24MHz, add a delay of 4 clock cylces
> after the write to the LPCG register.
> 
> Fixes: 2f77296d3df9 ("clk: imx: add lpcg clock support")
> Signed-off-by: Peng Fan <peng.fan@nxp.com>

Reviewed-by: Abel Vesa <abel.vesa@linaro.org>

> ---
>  drivers/clk/imx/clk-lpcg-scu.c | 37 +++++++++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/clk/imx/clk-lpcg-scu.c b/drivers/clk/imx/clk-lpcg-scu.c
> index dd5abd09f3e206a5073767561b517d5b3320b28c..620afdf8dc03e9564bb074ca879cf778f7fc6419 100644
> --- a/drivers/clk/imx/clk-lpcg-scu.c
> +++ b/drivers/clk/imx/clk-lpcg-scu.c
> @@ -6,10 +6,12 @@
>  
>  #include <linux/bits.h>
>  #include <linux/clk-provider.h>
> +#include <linux/delay.h>
>  #include <linux/err.h>
>  #include <linux/io.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/units.h>
>  
>  #include "clk-scu.h"
>  
> @@ -41,6 +43,29 @@ struct clk_lpcg_scu {
>  
>  #define to_clk_lpcg_scu(_hw) container_of(_hw, struct clk_lpcg_scu, hw)
>  
> +/* e10858 -LPCG clock gating register synchronization errata */
> +static void lpcg_e10858_writel(unsigned long rate, void __iomem *reg, u32 val)
> +{
> +	writel(val, reg);
> +
> +	if (rate >= 24 * HZ_PER_MHZ || rate == 0) {
> +		/*
> +		 * The time taken to access the LPCG registers from the AP core
> +		 * through the interconnect is longer than the minimum delay
> +		 * of 4 clock cycles required by the errata.
> +		 * Adding a readl will provide sufficient delay to prevent
> +		 * back-to-back writes.
> +		 */
> +		readl(reg);
> +	} else {
> +		/*
> +		 * For clocks running below 24MHz, wait a minimum of
> +		 * 4 clock cycles.
> +		 */
> +		ndelay(4 * (DIV_ROUND_UP(1000 * HZ_PER_MHZ, rate)));
> +	}
> +}
> +
>  static int clk_lpcg_scu_enable(struct clk_hw *hw)
>  {
>  	struct clk_lpcg_scu *clk = to_clk_lpcg_scu(hw);
> @@ -57,7 +82,8 @@ static int clk_lpcg_scu_enable(struct clk_hw *hw)
>  		val |= CLK_GATE_SCU_LPCG_HW_SEL;
>  
>  	reg |= val << clk->bit_idx;
> -	writel(reg, clk->reg);
> +
> +	lpcg_e10858_writel(clk_hw_get_rate(hw), clk->reg, reg);
>  
>  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);
>  
> @@ -74,7 +100,7 @@ static void clk_lpcg_scu_disable(struct clk_hw *hw)
>  
>  	reg = readl_relaxed(clk->reg);
>  	reg &= ~(CLK_GATE_SCU_LPCG_MASK << clk->bit_idx);
> -	writel(reg, clk->reg);
> +	lpcg_e10858_writel(clk_hw_get_rate(hw), clk->reg, reg);
>  
>  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);
>  }
> @@ -145,13 +171,8 @@ static int __maybe_unused imx_clk_lpcg_scu_resume(struct device *dev)
>  {
>  	struct clk_lpcg_scu *clk = dev_get_drvdata(dev);
>  
> -	/*
> -	 * FIXME: Sometimes writes don't work unless the CPU issues
> -	 * them twice
> -	 */
> -
> -	writel(clk->state, clk->reg);
>  	writel(clk->state, clk->reg);
> +	lpcg_e10858_writel(0, clk->reg, clk->state);
>  	dev_dbg(dev, "restore lpcg state 0x%x\n", clk->state);
>  
>  	return 0;
> 
> -- 
> 2.37.1
>