[RFC PATCH] mtd: spi-nor-core: Set dummy buswidth equal to data buswidth

[Ashok Reddy Soma]

In current implementation dummy buswidth is set equal to address
buswidth. In case of quad spi (mode 1-1-4), where address width is 1
the dummy bytes will be calculated to 1(8 dummy cycles) and dummy
buswidth is set to 1. Due to this, the controller driver will introduce
8 dummy cycles on data line(D0) during read operation.

But since we are using 4 data lines in case of qspi, we need to change
this dummy bus width to 4. This will make dummy bytes to 4 inplace of 1.
This will be taken care in controller driver by dividing with dummy
buswidth again as in below code, which makes dummy cycles to 8 as
earlier.

dummy_cycles = op->dummy.nbytes * 8 / op->dummy.buswidth;

https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/spi/zynqmp_gqspi.c#L511

So with this change dummy cycles will be on all data lines(D0-D3) and it
is taken care for all the configurations(single, dual, quad and octal).

SPI experts, please advice if this change is good. What is the reason
we are using dummy bus width as address bus width so far ?

I have tested this change on all xilinx platforms with single, quad and
octal configurations. It works perfectly fine.

I would appreciate if anyone can test on your board and give feedback.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma at xilinx.com>
---

 drivers/mtd/spi/spi-nor-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/spi/spi-nor-core.c b/drivers/mtd/spi/spi-nor-core.c
index a70fbda4bb..6849da9113 100644
--- a/drivers/mtd/spi/spi-nor-core.c
+++ b/drivers/mtd/spi/spi-nor-core.c
@@ -251,7 +251,7 @@ static void spi_nor_setup_op(const struct spi_nor *nor,
 		op->addr.buswidth = spi_nor_get_protocol_addr_nbits(proto);
 
 	if (op->dummy.nbytes)
-		op->dummy.buswidth = spi_nor_get_protocol_addr_nbits(proto);
+		op->dummy.buswidth = spi_nor_get_protocol_data_nbits(proto);
 
 	if (op->data.nbytes)
 		op->data.buswidth = spi_nor_get_protocol_data_nbits(proto);
-- 
2.17.1