mtd: spi-nor: core: do 2 byte reads for SR and FSR in DTR mode

Some controllers, like the cadence qspi controller, have trouble reading
only 1 byte in DTR mode. So, do 2 byte reads for SR and FSR commands in
DTR mode, and then discard the second byte.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201005153138.6437-8-p.yadav@ti.com

drivers/mtd/spi-nor/core.c

@@ -370,7 +370,7 @@ int spi_nor_write_disable(struct spi_nor *nor)
  * spi_nor_read_sr() - Read the Status Register.
  * @nor:	pointer to 'struct spi_nor'.
  * @sr:		pointer to a DMA-able buffer where the value of the
- *              Status Register will be written.
+ *              Status Register will be written. Should be at least 2 bytes.
  *
  * Return: 0 on success, -errno otherwise.
  */
@@ -388,6 +388,11 @@ static int spi_nor_read_sr(struct spi_nor *nor, u8 *sr)
 		if (nor->reg_proto == SNOR_PROTO_8_8_8_DTR) {
 			op.addr.nbytes = nor->params->rdsr_addr_nbytes;
 			op.dummy.nbytes = nor->params->rdsr_dummy;
+			/*
+			 * We don't want to read only one byte in DTR mode. So,
+			 * read 2 and then discard the second byte.
+			 */
+			op.data.nbytes = 2;
 		}
 
 		spi_nor_spimem_setup_op(nor, &op, nor->reg_proto);
@@ -408,7 +413,8 @@ static int spi_nor_read_sr(struct spi_nor *nor, u8 *sr)
  * spi_nor_read_fsr() - Read the Flag Status Register.
  * @nor:	pointer to 'struct spi_nor'
  * @fsr:	pointer to a DMA-able buffer where the value of the
- *              Flag Status Register will be written.
+ *              Flag Status Register will be written. Should be at least 2
+ *              bytes.
  *
  * Return: 0 on success, -errno otherwise.
  */
@@ -426,6 +432,11 @@ static int spi_nor_read_fsr(struct spi_nor *nor, u8 *fsr)
 		if (nor->reg_proto == SNOR_PROTO_8_8_8_DTR) {
 			op.addr.nbytes = nor->params->rdsr_addr_nbytes;
 			op.dummy.nbytes = nor->params->rdsr_dummy;
+			/*
+			 * We don't want to read only one byte in DTR mode. So,
+			 * read 2 and then discard the second byte.
+			 */
+			op.data.nbytes = 2;
 		}
 
 		spi_nor_spimem_setup_op(nor, &op, nor->reg_proto);