power: reset: at91-poweroff: switch to slow clock before shutdown

The SAMA5D2 NRST input signal is resynchronized with the SLCK clock and it
can take up to 2 SLCK cycles (about 90us) for the internal reset to be
effective. During this delay, the VDDCORE current consumption may still be
high (application-dependent) with the VDDCORE regulator already OFF. Under
such conditions, VDDCORE may operate below its operating range leading to
potential register corruption.

To prevent such situation, it is recommended to decrease significantly the
power consumption of the device once the voltage regulator is  turned-off.
This can be achieved by operating the device at a much lower low frequency.

To solve this switch the master clock to slock clock just before writing
shutdown command to shutdown controller.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Suggested-by: Patrice Vilchez <patrice.vilchez@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

arch/arm/mach-at91/pm_suspend.S

@@ -149,6 +149,14 @@ exit_suspend:
 ENDPROC(at91_pm_suspend_in_sram)
 
 ENTRY(at91_backup_mode)
+	/* Switch the master clock source to slow clock. */
+	ldr	pmc, .pmc_base
+	ldr	tmp1, [pmc, #AT91_PMC_MCKR]
+	bic	tmp1, tmp1, #AT91_PMC_CSS
+	str	tmp1, [pmc, #AT91_PMC_MCKR]
+
+	wait_mckrdy
+
 	/*BUMEN*/
 	ldr	r0, .sfr
 	mov	tmp1, #0x1

drivers/power/reset/at91-sama5d2_shdwc.c

@@ -19,6 +19,7 @@
  */
 
 #include <linux/clk.h>
+#include <linux/clk/at91_pmc.h>
 #include <linux/io.h>
 #include <linux/module.h>
 #include <linux/of.h>
@@ -70,6 +71,7 @@ struct shdwc_config {
 struct shdwc {
 	const struct shdwc_config *cfg;
 	void __iomem *at91_shdwc_base;
+	void __iomem *pmc_base;
 };
 
 /*
@@ -108,6 +110,12 @@ static void __init at91_wakeup_status(struct platform_device *pdev)
 
 static void at91_poweroff(void)
 {
+	/* Switch the master clock source to slow clock. */
+	writel(readl(at91_shdwc->pmc_base + AT91_PMC_MCKR) & ~AT91_PMC_CSS,
+	       at91_shdwc->pmc_base + AT91_PMC_MCKR);
+	while (!(readl(at91_shdwc->pmc_base + AT91_PMC_SR) & AT91_PMC_MCKRDY))
+		;
+
 	writel(AT91_SHDW_KEY | AT91_SHDW_SHDW,
 	       at91_shdwc->at91_shdwc_base + AT91_SHDW_CR);
 }
@@ -123,6 +131,16 @@ static void at91_lpddr_poweroff(void)
 
 		/* Power down SDRAM0 */
 		"	str	%1, [%0, #" __stringify(AT91_DDRSDRC_LPR) "]\n\t"
+
+		/* Switch the master clock source to slow clock. */
+		"	ldr	r6, [%4, #" __stringify(AT91_PMC_MCKR) "]\n\t"
+		"	bic	r6, r6,  #" __stringify(AT91_PMC_CSS) "\n\t"
+		"	str	r6, [%4, #" __stringify(AT91_PMC_MCKR) "]\n\t"
+		/* Wait for clock switch. */
+		"1:	ldr	r6, [%4, #" __stringify(AT91_PMC_SR) "]\n\t"
+		"	tst	r6, #"	    __stringify(AT91_PMC_MCKRDY) "\n\t"
+		"	beq	1b\n\t"
+
 		/* Shutdown CPU */
 		"	str	%3, [%2, #" __stringify(AT91_SHDW_CR) "]\n\t"
 
@@ -131,7 +149,8 @@ static void at91_lpddr_poweroff(void)
 		: "r" (mpddrc_base),
 		  "r" cpu_to_le32(AT91_DDRSDRC_LPDDR2_PWOFF),
 		  "r" (at91_shdwc->at91_shdwc_base),
-		  "r" cpu_to_le32(AT91_SHDW_KEY | AT91_SHDW_SHDW)
+		  "r" cpu_to_le32(AT91_SHDW_KEY | AT91_SHDW_SHDW),
+		  "r" (at91_shdwc->pmc_base)
 		: "r6");
 }
 
@@ -276,26 +295,53 @@ static int __init at91_shdwc_probe(struct platform_device *pdev)
 
 	at91_shdwc_dt_configure(pdev);
 
-	pm_power_off = at91_poweroff;
+	np = of_find_compatible_node(NULL, NULL, "atmel,sama5d2-pmc");
+	if (!np) {
+		ret = -ENODEV;
+		goto clk_disable;
+	}
+
+	at91_shdwc->pmc_base = of_iomap(np, 0);
+	of_node_put(np);
+
+	if (!at91_shdwc->pmc_base) {
+		ret = -ENOMEM;
+		goto clk_disable;
+	}
 
 	np = of_find_compatible_node(NULL, NULL, "atmel,sama5d3-ddramc");
-	if (!np)
-		return 0;
+	if (!np) {
+		ret = -ENODEV;
+		goto unmap;
+	}
 
 	mpddrc_base = of_iomap(np, 0);
 	of_node_put(np);
 
-	if (!mpddrc_base)
-		return 0;
+	if (!mpddrc_base) {
+		ret = -ENOMEM;
+		goto unmap;
+	}
+
+	pm_power_off = at91_poweroff;
 
 	ddr_type = readl(mpddrc_base + AT91_DDRSDRC_MDR) & AT91_DDRSDRC_MD;
 	if ((ddr_type == AT91_DDRSDRC_MD_LPDDR2) ||
-	    (ddr_type == AT91_DDRSDRC_MD_LPDDR3))
+	    (ddr_type == AT91_DDRSDRC_MD_LPDDR3)) {
 		pm_power_off = at91_lpddr_poweroff;
-	else
+	} else {
 		iounmap(mpddrc_base);
+		mpddrc_base = NULL;
+	}
 
 	return 0;
+
+unmap:
+	iounmap(at91_shdwc->pmc_base);
+clk_disable:
+	clk_disable_unprepare(sclk);
+
+	return ret;
 }
 
 static int __exit at91_shdwc_remove(struct platform_device *pdev)
@@ -310,6 +356,10 @@ static int __exit at91_shdwc_remove(struct platform_device *pdev)
 	writel(0, shdw->at91_shdwc_base + AT91_SHDW_MR);
 	writel(0, shdw->at91_shdwc_base + AT91_SHDW_WUIR);
 
+	if (mpddrc_base)
+		iounmap(mpddrc_base);
+	iounmap(shdw->pmc_base);
+
 	clk_disable_unprepare(sclk);
 
 	return 0;