mtd: rawnand: qcom: set BAM mode only if not set already

BAM is DMA controller on QCOM ipq platforms, BAM mode on NAND driver
is set by writing BAM_MODE_EN bit on NAND_CTRL register.

NAND_CTRL is an operational register and in BAM mode operational
registers are read only.

So, before enabling BAM mode by writing the NAND_CTRL register, check
if BAM mode was already enabled by the bootloader, and enable BAM mode
only if it is not enabled already.

Signed-off-by: Sivaprakash Murugesan <sivaprak@codeaurora.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1591948696-16015-3-git-send-email-sivaprak@codeaurora.org

drivers/mtd/nand/raw/qcom_nandc.c

@@ -2784,7 +2784,16 @@ static int qcom_nandc_setup(struct qcom_nand_controller *nandc)
 	/* enable ADM or BAM DMA */
 	if (nandc->props->is_bam) {
 		nand_ctrl = nandc_read(nandc, NAND_CTRL);
-		nandc_write(nandc, NAND_CTRL, nand_ctrl | BAM_MODE_EN);
+
+		/*
+		 *NAND_CTRL is an operational registers, and CPU
+		 * access to operational registers are read only
+		 * in BAM mode. So update the NAND_CTRL register
+		 * only if it is not in BAM mode. In most cases BAM
+		 * mode will be enabled in bootloader
+		 */
+		if (!(nand_ctrl & BAM_MODE_EN))
+			nandc_write(nandc, NAND_CTRL, nand_ctrl | BAM_MODE_EN);
 	} else {
 		nandc_write(nandc, NAND_FLASH_CHIP_SELECT, DM_EN);
 	}