mtd: nand: pxa3xx: Support command buffer #3

Some newer controllers support a fourth command buffer. This additional
command buffer allows to set an arbitrary length count, using the
NDCB3.NDLENCNT field, to perform non-standard length operations
such as the ONFI parameter page read.
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: NDaniel Mack <zonque@gmail.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/nand/pxa3xx_nand.c

@@ -197,6 +197,7 @@ struct pxa3xx_nand_info {
 	uint32_t		ndcb0;
 	uint32_t		ndcb1;
 	uint32_t		ndcb2;
+	uint32_t		ndcb3;
 };
 
 static bool use_dma = 1;
@@ -493,9 +494,22 @@ static irqreturn_t pxa3xx_nand_irq(int irq, void *devid)
 		nand_writel(info, NDSR, NDSR_WRCMDREQ);
 		status &= ~NDSR_WRCMDREQ;
 		info->state = STATE_CMD_HANDLE;
+
+		/*
+		 * Command buffer registers NDCB{0-2} (and optionally NDCB3)
+		 * must be loaded by writing directly either 12 or 16
+		 * bytes directly to NDCB0, four bytes at a time.
+		 *
+		 * Direct write access to NDCB1, NDCB2 and NDCB3 is ignored
+		 * but each NDCBx register can be read.
+		 */
 		nand_writel(info, NDCB0, info->ndcb0);
 		nand_writel(info, NDCB0, info->ndcb1);
 		nand_writel(info, NDCB0, info->ndcb2);
+
+		/* NDCB3 register is available in NFCv2 (Armada 370/XP SoC) */
+		if (info->variant == PXA3XX_NAND_VARIANT_ARMADA370)
+			nand_writel(info, NDCB0, info->ndcb3);
 	}
 
 	/* clear NDSR to let the controller exit the IRQ */
@@ -554,6 +568,7 @@ static int prepare_command_pool(struct pxa3xx_nand_info *info, int command,
 	default:
 		info->ndcb1 = 0;
 		info->ndcb2 = 0;
+		info->ndcb3 = 0;
 		break;
 	}