mmc: sdhci-of-esdhc: avoid clock glitch when frequency is changing

The eSDHC_PRSSTAT[SDSTB] bit indicates whether the internal card clock is
stable. This bit is for the host driver to poll clock status when changing
the clock frequency. It is recommended to clear eSDHC_SYSCTL[SDCLKEN]
to remove glitch on the card clock when the frequency is changing. This
patch is to disable SDCLKEN bit before changing frequency and enable it
after SDSTB bit is set.

Signed-off-by: Yangbo Lu <[e-mail]>
Acked-by: Adrian Hunter <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/sdhci-esdhc.h

···
  * eSDHC register definition
  */
 
+/* Present State Register */
+#define ESDHC_PRSSTAT			0x24
+#define ESDHC_CLOCK_STABLE		0x00000008
+
 /* Protocol Control Register */
 #define ESDHC_PROCTL			0x28
 #define ESDHC_CTRL_4BITBUS		(0x1 << 1)
···
 #define ESDHC_CLOCK_MASK		0x0000fff0
 #define ESDHC_PREDIV_SHIFT		8
 #define ESDHC_DIVIDER_SHIFT		4
+#define ESDHC_CLOCK_SDCLKEN		0x00000008
 #define ESDHC_CLOCK_PEREN		0x00000004
 #define ESDHC_CLOCK_HCKEN		0x00000002
 #define ESDHC_CLOCK_IPGEN		0x00000001

drivers/mmc/host/sdhci-of-esdhc.c

···
 	struct sdhci_esdhc *esdhc = sdhci_pltfm_priv(pltfm_host);
 	int pre_div = 1;
 	int div = 1;
+	u32 timeout;
 	u32 temp;
 
 	host->mmc->actual_clock = 0;
···
 	}
 
 	temp = sdhci_readl(host, ESDHC_SYSTEM_CONTROL);
-	temp &= ~(ESDHC_CLOCK_IPGEN | ESDHC_CLOCK_HCKEN | ESDHC_CLOCK_PEREN
-		| ESDHC_CLOCK_MASK);
+	temp &= ~(ESDHC_CLOCK_SDCLKEN | ESDHC_CLOCK_IPGEN | ESDHC_CLOCK_HCKEN |
+		  ESDHC_CLOCK_PEREN | ESDHC_CLOCK_MASK);
 	sdhci_writel(host, temp, ESDHC_SYSTEM_CONTROL);
 
 	while (host->max_clk / pre_div / 16 > clock && pre_div < 256)
···
 		| (div << ESDHC_DIVIDER_SHIFT)
 		| (pre_div << ESDHC_PREDIV_SHIFT));
 	sdhci_writel(host, temp, ESDHC_SYSTEM_CONTROL);
-	mdelay(1);
+
+	/* Wait max 20 ms */
+	timeout = 20;
+	while (!(sdhci_readl(host, ESDHC_PRSSTAT) & ESDHC_CLOCK_STABLE)) {
+		if (timeout == 0) {
+			pr_err("%s: Internal clock never stabilised.\n",
+				mmc_hostname(host->mmc));
+			return;
+		}
+		timeout--;
+		mdelay(1);
+	}
+
+	temp |= ESDHC_CLOCK_SDCLKEN;
+	sdhci_writel(host, temp, ESDHC_SYSTEM_CONTROL);
 }
 
 static void esdhc_pltfm_set_bus_width(struct sdhci_host *host, int width)