[PATCH] Prevent submission of empty read requests in linux_spi

[Michael Karcher]

The submission of zero-sized read requests in a write-only transaction
fails at least for omap2_mcspi drivers and is pointless in general.

Even with this patch, zero-sized write requests might be submitted for
read-only transactions, but for most SPI chips, there always is a command
transferred (written) before reading data, so there is no such thing as
a pure-read transaction.

Signed-off-by: Michael Karcher 
---
 linux_spi.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/linux_spi.c b/linux_spi.c
index d994389..3d1ca3e 100644
--- a/linux_spi.c
+++ b/linux_spi.c
@@ -114,6 +114,7 @@ static int linux_spi_send_command(struct flashctx *flash, unsigned int writecnt,
    			  const unsigned char *txbuf,
    			  unsigned char *rxbuf)
 {
+	unsigned iocontrol_code;
    struct spi_ioc_transfer msg[2] = {
    	{
    		.tx_buf = (uint64_t)(ptrdiff_t)txbuf,
@@ -128,7 +129,14 @@ static int linux_spi_send_command(struct flashctx *flash, unsigned int writecnt,
    if (fd == -1)
    	return -1;

-	if (ioctl(fd, SPI_IOC_MESSAGE(2), msg) == -1) {
+	/* Just submit the first (write) request in case there is nothing
+	   to read. Otherwise submit both requests */
+	if (readcnt == 0)
+		iocontrol_code = SPI_IOC_MESSAGE(1);
+	else
+		iocontrol_code = SPI_IOC_MESSAGE(2);
+
+	if (ioctl(fd, iocontrol_code, msg) == -1) {
    	msg_cerr("%s: ioctl: %s\n", __func__, strerror(errno));
    	return -1;
    }