spi: spi_write_then_read() bugfixes

The "simplify spi_write_then_read()" patch included two regressions from
the 2.6.27 behaviors:

 - The data it wrote out during the (full duplex) read side
   of the transfer was not zeroed.

 - It fails completely on half duplex hardware, such as
   Microwire and most "3-wire" SPI variants.

So, revert that patch.  A revised version should be submitted at some
point, which can get the speedup on standard hardware (full duplex)
without breaking on less-capable half-duplex stuff.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: <stable@kernel.org>		[2.6.28.x, 2.6.29.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 643908b..8eba98c 100644 (file)
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -658,7 +658,7 @@ int spi_write_then_read(struct spi_device *spi,
 
        int                     status;
        struct spi_message      message;
-       struct spi_transfer     x;
+       struct spi_transfer     x[2];
        u8                      *local_buf;
 
        /* Use preallocated DMA-safe buffer.  We can't avoid copying here,
@@ -669,9 +669,15 @@ int spi_write_then_read(struct spi_device *spi,
                return -EINVAL;
 
        spi_message_init(&message);
-       memset(&x, 0, sizeof x);
-       x.len = n_tx + n_rx;
-       spi_message_add_tail(&x, &message);
+       memset(x, 0, sizeof x);
+       if (n_tx) {
+               x[0].len = n_tx;
+               spi_message_add_tail(&x[0], &message);
+       }
+       if (n_rx) {
+               x[1].len = n_rx;
+               spi_message_add_tail(&x[1], &message);
+       }
 
        /* ... unless someone else is using the pre-allocated buffer */
        if (!mutex_trylock(&lock)) {
@@ -682,15 +688,15 @@ int spi_write_then_read(struct spi_device *spi,
                local_buf = buf;
 
        memcpy(local_buf, txbuf, n_tx);
-       x.tx_buf = local_buf;
-       x.rx_buf = local_buf;
+       x[0].tx_buf = local_buf;
+       x[1].rx_buf = local_buf + n_tx;
 
        /* do the i/o */
        status = spi_sync(spi, &message);
        if (status == 0)
-               memcpy(rxbuf, x.rx_buf + n_tx, n_rx);
+               memcpy(rxbuf, x[1].rx_buf, n_rx);
 
-       if (x.tx_buf == buf)
+       if (x[0].tx_buf == buf)
                mutex_unlock(&lock);
        else
                kfree(local_buf);