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spi: davinci: add comment about dummy tx buffer usage

Message ID 1489765270-17950-1-git-send-email-fisaksen@baylibre.com (mailing list archive)
State Accepted
Commit 1234e8398fe03267bf2e353e36825dbd0abc2fc6
Headers show

Commit Message

Frode Isaksen March 17, 2017, 3:41 p.m. UTC 
Add explanation about using the the rx buffer as the
dummy tx buffer.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
---
 drivers/spi/spi-davinci.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
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Patch

diff --git a/drivers/spi/spi-davinci.c b/drivers/spi/spi-davinci.c
index f37bbdd..595acdc 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;
 		}