mtd-nand: davinci: Correct 4-bit error correction

On TI's DA830/OMAP-L137, DA850/OMAP-L138 and DM365, after
setting the 4BITECC_ADD_CALC_START bit in the NAND Flash
control register to 1 and before waiting for the NAND Flash
status register to be equal to 1, 2 or 3, we have to wait
till the ECC HW goes to correction state. Without this wait,
ECC correction calculations will not be proper.

This has been tested on DA830/OMAP-L137, DA850/OMAP-L138,
DM355 and DM365 EVMs.
Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Acked-by: Sneha Narnakaje <nsnehaprabha@ti.com>

drivers/mtd/nand/davinci_nand.c

@@ -310,6 +310,7 @@ static int nand_davinci_correct_4bit(struct mtd_info *mtd,
 	unsigned short ecc10[8];
 	unsigned short *ecc16;
 	u32 syndrome[4];
+	u32 ecc_state;
 	unsigned num_errors, corrected;
 
 	/* All bytes 0xff?  It's an erased page; ignore its ECC. */
@@ -360,6 +361,21 @@ compare:
 	 */
 	davinci_nand_writel(info, NANDFCR_OFFSET,
 			davinci_nand_readl(info, NANDFCR_OFFSET) | BIT(13));
+
+	/*
+	 * ECC_STATE field reads 0x3 (Error correction complete) immediately
+	 * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately
+	 * begin trying to poll for the state, you may fall right out of your
+	 * loop without any of the correction calculations having taken place.
+	 * The recommendation from the hardware team is to wait till ECC_STATE
+	 * reads less than 4, which means ECC HW has entered correction state.
+	 */
+	do {
+		ecc_state = (davinci_nand_readl(info,
+				NANDFSR_OFFSET) >> 8) & 0x0f;
+		cpu_relax();
+	} while (ecc_state < 4);
+
 	for (;;) {
 		u32	fsr = davinci_nand_readl(info, NANDFSR_OFFSET);