3 files changed, 12 insertions, 0 deletions

diff --git a/drivers/mmc/host/mmc_spi.c b/drivers/mmc/host/mmc_spi.c
index 240608cc7ae9..a461017ce5ce 100644
--- a/drivers/mmc/host/mmc_spi.c
+++ b/drivers/mmc/host/mmc_spi.c
@@ -1313,6 +1313,12 @@ static int mmc_spi_probe(struct spi_device *spi)
         struct mmc_spi_host     *host;
         int                     status;
 
+        /* We rely on full duplex transfers, mostly to reduce
+         * per-transfer overheads (by making fewer transfers).
+         */
+        if (spi->master->flags & SPI_MASTER_HALF_DUPLEX)
+                return -EINVAL;
+
         /* MMC and SD specs only seem to care that sampling is on the
          * rising edge ... meaning SPI modes 0 or 3.  So either SPI mode
          * should be legit.  We'll use mode 0 since the steady state is 0,
diff --git a/drivers/spi/omap_uwire.c b/drivers/spi/omap_uwire.c
index aa90ddb37066..8980a5640bd9 100644
--- a/drivers/spi/omap_uwire.c
+++ b/drivers/spi/omap_uwire.c
@@ -514,6 +514,8 @@ static int __init uwire_probe(struct platform_device *pdev)
         /* the spi->mode bits understood by this driver: */
         master->mode_bits = SPI_CPOL | SPI_CPHA | SPI_CS_HIGH;
 
+        master->flags = SPI_MASTER_HALF_DUPLEX;
+
         master->bus_num = 2;    /* "official" */
         master->num_chipselect = 4;
         master->setup = uwire_setup;
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 743c933ac4e7..c47c4b4da97e 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -250,6 +250,10 @@ struct spi_master {
         /* spi_device.mode flags understood by this controller driver */
         u16                     mode_bits;
 
+        /* other constraints relevant to this driver */
+        u16                     flags;
+#define SPI_MASTER_HALF_DUPLEX  BIT(0)          /* can't do full duplex */
+
         /* Setup mode and clock, etc (spi driver may call many times).
          *
          * IMPORTANT:  this may be called when transfers to another