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

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Message-ID: <20250211214645.469279-1-erick.shepherd@ni.com>
Date: Tue, 11 Feb 2025 15:46:45 -0600
From: Erick Shepherd <erick.shepherd@...com>
To: <linux-kernel@...r.kernel.org>
CC: <linux-mmc@...r.kernel.org>, <ulf.hansson@...aro.org>,
        <adrian.hunter@...el.com>, <gratian.crisan@...rson.com>,
        Erick Shepherd
	<erick.shepherd@...com>,
        Kyle Roeschley <kyle.roeschley@...com>,
        Brad Mouring
	<brad.mouring@...com>
Subject: [PATCH] mmc: sdhci: Disable SD card clock before changing parameters

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>
---
 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);
-- 
2.43.0