ARM: at91: pm: fix DDR recalibration when resuming from backup and self-refresh

On SAMA7G5, when resuming from backup and self-refresh, the bootloader
performs DDR PHY recalibration by restoring the value of ZQ0SR0 (stored
in RAM by Linux before going to backup and self-refresh). It has been
discovered that the current procedure doesn't work for all possible values
that might go to ZQ0SR0 due to hardware bug. The workaround to this is to
avoid storing some values in ZQ0SR0. Thus Linux will read the ZQ0SR0
register and cache its value in RAM after processing it (using
modified_gray_code array). The bootloader will restore the processed value.

Fixes: d2d4716 ("ARM: at91: pm: save ddr phy calibration data to securam")
Suggested-by: Frederic Schumacher <[email protected]>
Signed-off-by: Claudiu Beznea <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: claudiubeznea

arch/arm/mach-at91/pm.c

@@ -541,9 +541,41 @@ extern u32 at91_pm_suspend_in_sram_sz;
 
 static int at91_suspend_finish(unsigned long val)
 {
+	unsigned char modified_gray_code[] = {
+		0x00, 0x01, 0x02, 0x03, 0x06, 0x07, 0x04, 0x05, 0x0c, 0x0d,
+		0x0e, 0x0f, 0x0a, 0x0b, 0x08, 0x09, 0x18, 0x19, 0x1a, 0x1b,
+		0x1e, 0x1f, 0x1c, 0x1d, 0x14, 0x15, 0x16, 0x17, 0x12, 0x13,
+		0x10, 0x11,
+	};
+	unsigned int tmp, index;
 	int i;
 
 	if (soc_pm.data.mode == AT91_PM_BACKUP && soc_pm.data.ramc_phy) {
+		/*
+		 * Bootloader will perform DDR recalibration and will try to
+		 * restore the ZQ0SR0 with the value saved here. But the
+		 * calibration is buggy and restoring some values from ZQ0SR0
+		 * is forbidden and risky thus we need to provide processed
+		 * values for these (modified gray code values).
+		 */
+		tmp = readl(soc_pm.data.ramc_phy + DDR3PHY_ZQ0SR0);
+
+		/* Store pull-down output impedance select. */
+		index = (tmp >> DDR3PHY_ZQ0SR0_PDO_OFF) & 0x1f;
+		soc_pm.bu->ddr_phy_calibration[0] = modified_gray_code[index];
+
+		/* Store pull-up output impedance select. */
+		index = (tmp >> DDR3PHY_ZQ0SR0_PUO_OFF) & 0x1f;
+		soc_pm.bu->ddr_phy_calibration[0] |= modified_gray_code[index];
+
+		/* Store pull-down on-die termination impedance select. */
+		index = (tmp >> DDR3PHY_ZQ0SR0_PDODT_OFF) & 0x1f;
+		soc_pm.bu->ddr_phy_calibration[0] |= modified_gray_code[index];
+
+		/* Store pull-up on-die termination impedance select. */
+		index = (tmp >> DDR3PHY_ZQ0SRO_PUODT_OFF) & 0x1f;
+		soc_pm.bu->ddr_phy_calibration[0] |= modified_gray_code[index];
+
 		/*
 		 * The 1st 8 words of memory might get corrupted in the process
 		 * of DDR PHY recalibration; it is saved here in securam and it
@@ -1066,10 +1098,6 @@ static int __init at91_pm_backup_init(void)
 		of_scan_flat_dt(at91_pm_backup_scan_memcs, &located);
 		if (!located)
 			goto securam_fail;
-
-		/* DDR3PHY_ZQ0SR0 */
-		soc_pm.bu->ddr_phy_calibration[0] = readl(soc_pm.data.ramc_phy +
-							  0x188);
 	}
 
 	return 0;

include/soc/at91/sama7-ddr.h

@@ -38,6 +38,10 @@
 #define		DDR3PHY_DSGCR_ODTPDD_ODT0	(1 << 20)	/* ODT[0] Power Down Driver */
 
 #define DDR3PHY_ZQ0SR0				(0x188)		/* ZQ status register 0 */
+#define DDR3PHY_ZQ0SR0_PDO_OFF			(0)		/* Pull-down output impedance select offset */
+#define DDR3PHY_ZQ0SR0_PUO_OFF			(5)		/* Pull-up output impedance select offset */
+#define DDR3PHY_ZQ0SR0_PDODT_OFF		(10)		/* Pull-down on-die termination impedance select offset */
+#define DDR3PHY_ZQ0SRO_PUODT_OFF		(15)		/* Pull-up on-die termination impedance select offset */
 
 #define	DDR3PHY_DX0DLLCR			(0x1CC)		/* DDR3PHY DATX8 DLL Control Register */
 #define	DDR3PHY_DX1DLLCR			(0x20C)		/* DDR3PHY DATX8 DLL Control Register */