From: Frode Isaksen <fisaksen@baylibre.com>
Date: Fri, 17 Mar 2017 15:41:10 +0000 (+0100)
Subject: spi: davinci: add comment about dummy tx buffer usage
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=1234e8398fe03267bf2e353e36825dbd0abc2fc6;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git

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>
---

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;
 		}