spi: Remove SPI_BUFSIZ restriction on spi_write_then_read()

In order to avoid constantly allocating and deallocating there is a fixed
buffer which spi_write_then_read() uses for transfers, with an early error
check to ensure that the transfer fits within the buffer. This limits the
size of transfers to this size, currently max(32, SMP_CACHE_BYTES).

Since we can dynamically allocate and in fact already have a fallback
to do so when there is contention for the fixed buffer remove this
restriction and instead dynamically allocate a suitably sized buffer if
the transfer won't fit.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 11 insertions, 13 deletions

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index f1217ae59f3d..22c71e238fdf 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1545,12 +1545,18 @@ int spi_write_then_read(struct spi_device *spi,
         struct spi_transfer     x[2];
         u8                      *local_buf;
 
-        /* Use preallocated DMA-safe buffer.  We can't avoid copying here,
-         * (as a pure convenience thing), but we can keep heap costs
-         * out of the hot path ...
+        /* Use preallocated DMA-safe buffer if we can.  We can't avoid
+         * copying here, (as a pure convenience thing), but we can
+         * keep heap costs out of the hot path unless someone else is
+         * using the pre-allocated buffer or the transfer is too large.
          */
-        if ((n_tx + n_rx) > SPI_BUFSIZ)
-                return -EINVAL;
+        if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
+                local_buf = kmalloc(max(SPI_BUFSIZ, n_tx + n_rx), GFP_KERNEL);
+                if (!local_buf)
+                        return -ENOMEM;
+        } else {
+                local_buf = buf;
+        }
 
         spi_message_init(&message);
         memset(x, 0, sizeof x);
@@ -1563,14 +1569,6 @@ int spi_write_then_read(struct spi_device *spi,
                 spi_message_add_tail(&x[1], &message);
         }
 
-        /* ... unless someone else is using the pre-allocated buffer */
-        if (!mutex_trylock(&lock)) {
-                local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
-                if (!local_buf)
-                        return -ENOMEM;
-        } else
-                local_buf = buf;
-
         memcpy(local_buf, txbuf, n_tx);
         x[0].tx_buf = local_buf;
         x[1].rx_buf = local_buf + n_tx;