[U-Boot] [PATCH RESEND-WITH-JUSTIFICATION] spi: soft_spi: Support NULL din/dout buffers

[Andrew Ruder]

This mirrors the conventions used in other SPI drivers (kirkwood,
davinci, atmel, et al) where the din/dout buffer can be NULL when the
received/transmitted data isn't important.  This reduces the need for
allocating additional buffers when write-only/read-only functionality is
needed.

In the din == NULL case, the received data is simply not stored.  In the
dout == NULL case, zeroes are transmitted.

Signed-off-by: Andrew Ruder <andrew.ruder at elecsyscorp.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj at jcrosoft.com>
Cc: Jagan Teki <jagannadh.teki at gmail.com>
---
So going through and cleaning up some of my trees and determining which
patches have not been taken.  A little more justification for this
patch - I've gone through all of the current SPI implementations and
have determined what (if any) support the driver has for half duplex
operation by giving NULL for one of the two buffers (din/dout).  This
greatly reduces the need for temporary buffers for writing or reading
large amounts of half-duplex data to devices over SPI.  Here's the list:

altera_spi.c          - SUPPORTS NULL din or dout
andes_spi.c           - REQUIRES NULL din or dout
armada100_spi.c       - SUPPORTS NULL din or dout
atmel_spi.c           - SUPPORTS NULL din or dout
bfin_spi6xx.c         - SUPPORTS NULL din or dout
bfin_spi.c            - SUPPORTS NULL din or dout
cf_qspi.c             - SUPPORTS NULL din or dout
cf_spi.c              - SUPPORTS NULL din or dout
davinci_spi.c         - SUPPORTS NULL din or dout
exynos_spi.c          - SUPPORTS NULL din or dout
fdt_spi.c-tegra20_sf  - SUPPORTS NULL din or dout
fdt_spi.c-tegra20_sl  - SUPPORTS NULL din or dout
fdt_spi.c-tegra114    - SUPPORTS NULL din or dout
fsl_espi.c            - SUPPORTS NULL din (NOT dout)
ftssp010_spi.c        - SUPPORTS NULL din or dout
ich.c                 - SUPPORTS NULL din or dout
kirkwood_spi.c        - SUPPORTS NULL din or dout
mpc52xx_spi.c         - NO SUPPORT
mpc8xxx_spi.c         - NO SUPPORT
mxc_spi.c             - NO SUPPORT
mxs_spi.c             - REQUIRES NULL din or dout
oc_tiny_spi.c         - SUPPORTS NULL din or dout
omap3_spi.c           - SUPPORTS NULL din or dout
sandbox_spi.c         - SUPPORTS NULL din or dout
sh_qspi.c             - SUPPORTS NULL din or dout
sh_spi.c              - SUPPORTS NULL din or dout
soft_spi.c            - NO SUPPORT
ti_qspi.c             - SUPPORTS NULL din or dout
xilinx_spi.c          - SUPPORTS NULL din or dout
zynq_spi.c            - SUPPORTS NULL din or dout

Furthermore, this would not affect any board already using temporary
buffers and it would continue to work as usual.  The *only* obscure
corner case that this will not work is if NULL (0x0) is a valid buffer
address in your system and you are transferring SPI to/from it.  This
seems very unlikely and would probably break in other ways from library
functions that check their arguments.

 drivers/spi/soft_spi.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/spi/soft_spi.c b/drivers/spi/soft_spi.c
index 5d22351..5fdd091 100644
--- a/drivers/spi/soft_spi.c
+++ b/drivers/spi/soft_spi.c
@@ -137,9 +137,15 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 		 * Check if it is time to work on a new byte.
 		 */
 		if((j % 8) == 0) {
-			tmpdout = *txd++;
+			if (txd) {
+				tmpdout = *txd++;
+			} else {
+				tmpdout = 0;
+			}
 			if(j != 0) {
-				*rxd++ = tmpdin;
+				if (rxd) {
+					*rxd++ = tmpdin;
+				}
 			}
 			tmpdin  = 0;
 		}
@@ -164,9 +170,11 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 	 * bits over to left-justify them.  Then store the last byte
 	 * read in.
 	 */
-	if((bitlen % 8) != 0)
-		tmpdin <<= 8 - (bitlen % 8);
-	*rxd++ = tmpdin;
+	if (rxd) {
+		if((bitlen % 8) != 0)
+			tmpdin <<= 8 - (bitlen % 8);
+		*rxd++ = tmpdin;
+	}
 
 	if (flags & SPI_XFER_END)
 		spi_cs_deactivate(slave);
-- 
1.9.0.rc3.12.gbc97e2d