[PATCH] spi: spi-qpic-snand: write the feature value before executing SET_FEATURE

[Stanislaw Pal <kuncy7@gmail.com>]

qcom_spi_send_cmdaddr() programs NAND_FLASH_CMD/NAND_EXEC_CMD and submits
the descriptors, which makes the controller execute the command
immediately. For SPINAND_SET_FEATURE the value to be written is only
placed into NAND_FLASH_FEATURES afterwards, by qcom_spi_io_op(), in a
second submission - so the chip is programmed with whatever that register
happened to hold from a previous operation, and the intended value is only
applied by the *next* SET_FEATURE.

Measured on a TP-Link Archer AX55 v1 (IPQ5018, ESMT F50L1G41LB): writing
0x40 to the configuration register (0xb0) leaves the chip at 0x00, and the
subsequent write of 0x00 leaves it at 0x40 - every write lands one
operation late.

This stayed unnoticed until v6.18 added SPI-NAND OTP support together
with OTP entries for ESMT chips. spinand_otp_rw() enables OTP mode,
reads, and disables it again, and mtd_otp_nvmem_add() does this during
MTD registration. With the off-by-one, the "disable" write actually
applies the previously requested value, so CFG_OTP_ENABLE ends up set:
the chip stays in OTP mode, every subsequent array read returns the OTP
area instead of the array (UBI reports an empty device) and all writes
fail with -EIO because the OTP area is write protected. On this board
that makes the whole flash unusable and the device unbootable.

Write the feature value into NAND_FLASH_FEATURES as part of the same
transaction, before NAND_EXEC_CMD. While at it, copy only the bytes the
operation actually carries - the previous code dereferenced a 4-byte
pointer on a one-byte buffer (spinand->scratchbuf).

With this patch the flash contents read back bit-identical to a
known-good dump of the same board taken under the vendor firmware
(md5-verified across partitions), and writes work.

Fixes: 7304d1909080 ("spi: spi-qpic: add driver for QCOM SPI NAND flash Interface")
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Pal <kuncy7@gmail.com>
---
--- a/drivers/spi/spi-qpic-snand.c
+++ b/drivers/spi/spi-qpic-snand.c
@@ -1358,6 +1358,22 @@
 	snandc->regs->addr0 = cpu_to_le32(op->addr.val);
 	snandc->regs->addr1 = cpu_to_le32(0);
 
+	/*
+	 * The feature value has to reach NAND_FLASH_FEATURES before the
+	 * command is executed, otherwise the controller programs the chip
+	 * with whatever the register happened to hold from a previous
+	 * operation.
+	 */
+	if (opcode == SPINAND_SET_FEATURE) {
+		u32 ftr = 0;
+
+		memcpy(&ftr, op->data.buf.out,
+		       min_t(size_t, op->data.nbytes, sizeof(ftr)));
+		snandc->regs->flash_feature = cpu_to_le32(ftr);
+		qcom_write_reg_dma(snandc, &snandc->regs->flash_feature,
+				   NAND_FLASH_FEATURES, 1, NAND_BAM_NEXT_SGL);
+	}
+
 	qcom_write_reg_dma(snandc, &snandc->regs->cmd, NAND_FLASH_CMD, 3, NAND_BAM_NEXT_SGL);
 	qcom_write_reg_dma(snandc, &snandc->regs->exec, NAND_EXEC_CMD, 1, NAND_BAM_NEXT_SGL);
 
@@ -1395,10 +1411,8 @@
 		copy_ftr = true;
 		break;
 	case SPINAND_SET_FEATURE:
-		snandc->regs->flash_feature = cpu_to_le32(*(u32 *)op->data.buf.out);
-		qcom_write_reg_dma(snandc, &snandc->regs->flash_feature,
-				   NAND_FLASH_FEATURES, 1, NAND_BAM_NEXT_SGL);
-		break;
+		/* fully handled by qcom_spi_send_cmdaddr() */
+		return 0;
 	case SPINAND_PROGRAM_EXECUTE:
 	case SPINAND_WRITE_EN:
 	case SPINAND_RESET: