spi: davinci: add comment about dummy tx buffer usage

Add explanation about using the the rx buffer as the
dummy tx buffer.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/spi/spi-davinci.c b/drivers/spi/spi-davinci.c
index f37bbdd18d61..595acdcfc7d0 100644
--- a/drivers/spi/spi-davinci.c
+++ b/drivers/spi/spi-davinci.c
@@ -660,7 +660,11 @@ static int davinci_spi_bufs(struct spi_device *spi, struct spi_transfer *t)
                         goto err_desc;
 
                 if (!t->tx_buf) {
-                        /* use rx buffer as dummy tx buffer */
+                        /* To avoid errors when doing rx-only transfers with
+                         * many SG entries (> 20), use the rx buffer as the
+                         * dummy tx buffer so that dma reloads are done at the
+                         * same time for rx and tx.
+                         */
                         t->tx_sg.sgl = t->rx_sg.sgl;
                         t->tx_sg.nents = t->rx_sg.nents;
                 }