spi: add spi_master flag word

Add a new spi_master.flags word listing constraints relevant to that
controller.  Define the first constraint bit: a half duplex restriction.
Include that constraint in the OMAP1 MicroWire controller driver.

Have the mmc_spi host be the first customer of this flag.  Its coding
relies heavily on full duplex transfers, so it must fail when the
underlying controller driver won't perform them.

(The spi_write_then_read routine could use it too: use the
temporarily-withdrawn full-duplex speedup unless this flag is set, in
which case the existing code applies.  Similarly, any spi_master
implementing only SPI_3WIRE should set the flag.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 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,