linux-kernel - Re: [PATCH] mmc: sdhci: Disable SD card clock before changing parameters

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Message-ID: <f76517de-9d59-4cf7-a7f5-b19ee4b2b3b2@intel.com>
Date: Thu, 6 Mar 2025 10:12:34 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Erick Shepherd <erick.shepherd@...com>, <linux-kernel@...r.kernel.org>
CC: <linux-mmc@...r.kernel.org>, <ulf.hansson@...aro.org>,
	<gratian.crisan@...rson.com>, Kyle Roeschley <kyle.roeschley@...com>, "Brad
 Mouring" <brad.mouring@...com>
Subject: Re: [PATCH] mmc: sdhci: Disable SD card clock before changing
 parameters

On 11/02/25 23:46, Erick Shepherd wrote:
> Per the SD Host Controller Simplified Specification v4.20 §3.2.3, change
> the SD card clock parameters only after first disabling the external card
> clock. Doing this fixes a spurious clock pulse on Baytrail and Apollo Lake
> SD controllers which otherwise breaks voltage switching with a specific
> Swissbit SD card.
> 
> Signed-off-by: Kyle Roeschley <kyle.roeschley@...com>
> Signed-off-by: Brad Mouring <brad.mouring@...com>
> Signed-off-by: Erick Shepherd <erick.shepherd@...com>

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

> ---
>  drivers/mmc/host/sdhci.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
> index f4a7733a8ad2..5f91b44891f9 100644
> --- a/drivers/mmc/host/sdhci.c
> +++ b/drivers/mmc/host/sdhci.c
> @@ -2065,10 +2065,15 @@ void sdhci_set_clock(struct sdhci_host *host, unsigned int clock)
>  
>  	host->mmc->actual_clock = 0;
>  
> -	sdhci_writew(host, 0, SDHCI_CLOCK_CONTROL);
> +	clk = sdhci_readw(host, SDHCI_CLOCK_CONTROL);
> +	if (clk & SDHCI_CLOCK_CARD_EN)
> +		sdhci_writew(host, clk & ~SDHCI_CLOCK_CARD_EN,
> +			SDHCI_CLOCK_CONTROL);
>  
> -	if (clock == 0)
> +	if (clock == 0) {
> +		sdhci_writew(host, 0, SDHCI_CLOCK_CONTROL);
>  		return;
> +	}
>  
>  	clk = sdhci_calc_clk(host, clock, &host->mmc->actual_clock);
>  	sdhci_enable_clk(host, clk);