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Date: Wed, 28 Feb 2024 14:17:22 -0500
From: Frank Li <Frank.li@nxp.com>
To: "Peng Fan (OSS)" <peng.fan@oss.nxp.com>
Cc: abelvesa@kernel.org, mturquette@baylibre.com, sboyd@kernel.org,
	shawnguo@kernel.org, s.hauer@pengutronix.de, kernel@pengutronix.de,
	festevam@gmail.com, imx@lists.linux.dev, linux-clk@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	Peng Fan <peng.fan@nxp.com>
Subject: Re: [PATCH] clk: imx: lpcg-scu: SW workaround for errata (e10858)
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On Wed, Feb 28, 2024 at 04:26:49PM +0800, 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.
> 
> 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.
> 
> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> ---

Fixes tag? 

Frank
 
> 
>  This is to upstream a downstream patch that goes through many releases
>  in several years. It maybe hard to trigger, but to address potential
>  issues, let's add this workaround.
> 
>  drivers/clk/imx/clk-lpcg-scu.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clk/imx/clk-lpcg-scu.c b/drivers/clk/imx/clk-lpcg-scu.c
> index dd5abd09f3e2..b30d0f8b5bca 100644
> --- a/drivers/clk/imx/clk-lpcg-scu.c
> +++ b/drivers/clk/imx/clk-lpcg-scu.c
> @@ -6,6 +6,7 @@
>  
>  #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>
> @@ -41,6 +42,31 @@ 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 do_lpcg_workaround(u32 rate, void __iomem *reg, u32 val)
> +{
> +	writel(val, reg);
> +
> +	if (rate >= 24000000 || rate == 0) {
> +		u32 reg1;
> +
> +		/*
> +		 * 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.
> +		 */
> +		reg1 = readl(reg);
> +	} else {
> +		/*
> +		 * For clocks running below 24MHz, wait a minimum of
> +		 * 4 clock cycles.
> +		 */
> +		ndelay(4 * (DIV_ROUND_UP(1000000000, rate)));
> +	}
> +}
> +
>  static int clk_lpcg_scu_enable(struct clk_hw *hw)
>  {
>  	struct clk_lpcg_scu *clk = to_clk_lpcg_scu(hw);
> @@ -57,7 +83,8 @@ 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);
> +
> +	do_lpcg_workaround(clk_hw_get_rate(hw), clk->reg, reg);
>  
>  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);
>  
> @@ -74,7 +101,7 @@ 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);
> +	do_lpcg_workaround(clk_hw_get_rate(hw), clk->reg, reg);
>  
>  	spin_unlock_irqrestore(&imx_lpcg_scu_lock, flags);
>  }
> @@ -151,7 +178,7 @@ static int __maybe_unused imx_clk_lpcg_scu_resume(struct device *dev)
>  	 */
>  
>  	writel(clk->state, clk->reg);
> -	writel(clk->state, clk->reg);
> +	do_lpcg_workaround(0, clk->reg, clk->state);
>  	dev_dbg(dev, "restore lpcg state 0x%x\n", clk->state);
>  
>  	return 0;
> -- 
> 2.37.1
>