[PATCH] Input: ads7846 - don't use scratch for tx_buf when clearing register

[Kris Bahnsen <kris@embeddedTS.com>]

The workaround for XPT2046 clears the command register, giving the
touchscreen controller a NOP. The change incorrectly re-uses the
req->scratch variable which is used as rx_buf for xfer[5], so by
the time xfer[6] occurs, the contents of req->scratch may not be
0. It was found that the touchscreen controller can end up in
a completely unresponsive state due to it being given a command
the driver does not expect.

Instead, rely on the spi_transfer behavior of tx_buf being NULL to
transmit all 0 bits, moving the 3 bytes to a single message.

This change was tested on real TSC2046 and ADS7843 controllers,
but not the XPT2046 the workaround was originally created for.
Confirming that the original modification to clear the command
register does not impact either real controller.

Fixes: 781a07da9bb94 ("Input: ads7846 - add dummy command register clearing cycle")
Cc: stable@vger.kernel.org
Co-developed-by: Mark Featherston <mark@embeddedTS.com>
Signed-off-by: Mark Featherston <mark@embeddedTS.com>
Signed-off-by: Kris Bahnsen <kris@embeddedTS.com>
---
 drivers/input/touchscreen/ads7846.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 4b39f7212d35c..599793d27129e 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -327,7 +327,7 @@ struct ser_req {
 	u8			ref_off;
 	u16			scratch;
 	struct spi_message	msg;
-	struct spi_transfer	xfer[8];
+	struct spi_transfer	xfer[7];
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
@@ -403,16 +403,11 @@ static int ads7846_read12_ser(struct device *dev, unsigned command)
 	spi_message_add_tail(&req->xfer[5], &req->msg);
 
 	/* clear the command register */
-	req->scratch = 0;
-	req->xfer[6].tx_buf = &req->scratch;
-	req->xfer[6].len = 1;
+	req->xfer[6].rx_buf = &req->scratch;
+	req->xfer[6].len = 3;
+	CS_CHANGE(req->xfer[6]);
 	spi_message_add_tail(&req->xfer[6], &req->msg);
 
-	req->xfer[7].rx_buf = &req->scratch;
-	req->xfer[7].len = 2;
-	CS_CHANGE(req->xfer[7]);
-	spi_message_add_tail(&req->xfer[7], &req->msg);
-
 	scoped_guard(mutex, &ts->lock) {
 		ads7846_stop(ts);
 		status = spi_sync(spi, &req->msg);

base-commit: dd6c438c3e64a5ff0b5d7e78f7f9be547803ef1b
-- 
2.34.1