linux-kernel - Re: [PATCH 2/2] mmc: sdhci-esdhc-imx: optimize clock loopback selection with dummy pad support

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Message-ID: <405271f5-3ae9-4f64-a433-043fb3194834@intel.com>
Date: Wed, 28 May 2025 13:34:54 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: <ziniu.wang_1@....com>, <haibo.chen@....com>, <ulf.hansson@...aro.org>,
	<linux-mmc@...r.kernel.org>
CC: <shawnguo@...nel.org>, <s.hauer@...gutronix.de>, <kernel@...gutronix.de>,
	<festevam@...il.com>, <imx@...ts.linux.dev>, <s32@....com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mmc: sdhci-esdhc-imx: optimize clock loopback
 selection with dummy pad support

On 21/05/2025 05:55, ziniu.wang_1@....com wrote:
> From: Luke Wang <ziniu.wang_1@....com>
> 
> For legacy platforms without dummy pad:
> When clock <= 100MHz: Set ESDHC_MIX_CTRL_FBCLK_SEL to 0 (external clock
> pad loopback) for better bus clock proximity.
> When clock > 100MHz: Set ESDHC_MIX_CTRL_FBCLK_SEL to 1 (internal clock
> loopback) to avoid signal reflection noise at high frequency.
> 
> For i.MX94/95 with dummy pad support:
> Keep ESDHC_MIX_CTRL_FBCLK_SEL at 0 for all speed mode. Hardware
> automatically substitutes clock pad loopback with dummy pad loopback
> when available, eliminating signal reflections while preserving better
> bus clock proximity.
> 
> Signed-off-by: Luke Wang <ziniu.wang_1@....com>

Acked-by: Adrian Hunter <adrian.hunter@...el.com>

> ---
>  drivers/mmc/host/sdhci-esdhc-imx.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/mmc/host/sdhci-esdhc-imx.c b/drivers/mmc/host/sdhci-esdhc-imx.c
> index c448a53530a5..5f1c45b2bd5d 100644
> --- a/drivers/mmc/host/sdhci-esdhc-imx.c
> +++ b/drivers/mmc/host/sdhci-esdhc-imx.c
> @@ -212,6 +212,9 @@
>  /* The IP does not have GPIO CD wake capabilities */
>  #define ESDHC_FLAG_SKIP_CD_WAKE		BIT(18)
>  
> +/* the controller has dummy pad for clock loopback */
> +#define ESDHC_FLAG_DUMMY_PAD		BIT(19)
> +
>  #define ESDHC_AUTO_TUNING_WINDOW	3
>  
>  enum wp_types {
> @@ -348,6 +351,15 @@ static struct esdhc_soc_data usdhc_imx8mm_data = {
>  	.quirks = SDHCI_QUIRK_NO_LED,
>  };
>  
> +static struct esdhc_soc_data usdhc_imx95_data = {
> +	.flags = ESDHC_FLAG_USDHC | ESDHC_FLAG_MAN_TUNING
> +			| ESDHC_FLAG_HAVE_CAP1 | ESDHC_FLAG_HS200
> +			| ESDHC_FLAG_HS400 | ESDHC_FLAG_HS400_ES
> +			| ESDHC_FLAG_STATE_LOST_IN_LPMODE
> +			| ESDHC_FLAG_DUMMY_PAD,
> +	.quirks = SDHCI_QUIRK_NO_LED,
> +};
> +
>  struct pltfm_imx_data {
>  	u32 scratchpad;
>  	struct pinctrl *pinctrl;
> @@ -392,6 +404,8 @@ static const struct of_device_id imx_esdhc_dt_ids[] = {
>  	{ .compatible = "fsl,imx7ulp-usdhc", .data = &usdhc_imx7ulp_data, },
>  	{ .compatible = "fsl,imx8qxp-usdhc", .data = &usdhc_imx8qxp_data, },
>  	{ .compatible = "fsl,imx8mm-usdhc", .data = &usdhc_imx8mm_data, },
> +	{ .compatible = "fsl,imx94-usdhc", .data = &usdhc_imx95_data, },
> +	{ .compatible = "fsl,imx95-usdhc", .data = &usdhc_imx95_data, },
>  	{ .compatible = "fsl,imxrt1050-usdhc", .data = &usdhc_imxrt1050_data, },
>  	{ .compatible = "nxp,s32g2-usdhc", .data = &usdhc_s32g2_data, },
>  	{ /* sentinel */ }
> @@ -1424,9 +1438,10 @@ static void esdhc_set_uhs_signaling(struct sdhci_host *host, unsigned timing)
>  		break;
>  	}
>  
> -	if (timing == MMC_TIMING_UHS_SDR104 ||
> -	    timing == MMC_TIMING_MMC_HS200 ||
> -	    timing == MMC_TIMING_MMC_HS400)
> +	if (!(imx_data->socdata->flags & ESDHC_FLAG_DUMMY_PAD) &&
> +	    (timing == MMC_TIMING_UHS_SDR104 ||
> +	     timing == MMC_TIMING_MMC_HS200 ||
> +	     timing == MMC_TIMING_MMC_HS400))
>  		m |= ESDHC_MIX_CTRL_FBCLK_SEL;
>  	else
>  		m &= ~ESDHC_MIX_CTRL_FBCLK_SEL;
> @@ -1678,7 +1693,9 @@ static void sdhc_esdhc_tuning_restore(struct sdhci_host *host)
>  		writel(reg, host->ioaddr + ESDHC_TUNING_CTRL);
>  
>  		reg = readl(host->ioaddr + ESDHC_MIX_CTRL);
> -		reg |= ESDHC_MIX_CTRL_SMPCLK_SEL | ESDHC_MIX_CTRL_FBCLK_SEL;
> +		reg |= ESDHC_MIX_CTRL_SMPCLK_SEL;
> +		if (!(imx_data->socdata->flags & ESDHC_FLAG_DUMMY_PAD))
> +			reg |= ESDHC_MIX_CTRL_FBCLK_SEL;
>  		writel(reg, host->ioaddr + ESDHC_MIX_CTRL);
>  
>  		writel(FIELD_PREP(ESDHC_TUNE_CTRL_STATUS_DLY_CELL_SET_PRE_MASK,