Input: ads7846 - add dummy command register clearing cycle

On STM32MP135F with XPT2046 touch controller attached to SPI bus, it has
been observed that the touch controller locks up after Linux kernel has
finished booting. Adding a dummy cycle on the SPI bus seems to mitigate
the lock up.

The XPTEK XPT2046 controller seems to be an identical clone of TI TSC2046,
the datasheet seems to be a clone of the TI part as well, text seem to be
word to word identical, except all the pictures have been drawn again.

This touch controller is present e.g. on WaveShare 3.2inch RPi LCD (B)
panel, the DTO provided by WaveShare uses 50 kHz SPI clock for this
touch controller, which is unusually low and possibly might have been
used as some sort of workaround for an issue. The SPI LCD on the same
bus uses 16 MHz clock.

SPI bus DT properties spi-cs-setup-delay-ns, spi-cs-hold-delay-ns,
spi-cs-inactive-delay-ns, spi-rx-delay-us, spi-tx-delay-us set to
range of 500ns..5us seem to have no impact on the behavior of the
touch controller, the lock up always occurs. The STM32MP13xx SPI
controller users GPIO control for the nCS pins.

Since the dummy cycle happens after the controller has been put into
power down mode and both ADC and REF regulators have been disabled,
the cycle should have no impact on the configuration of the controller,
i.e. it should be a NOP.

It is unclear whether this problem is specific to this cloned XPT2046
controller, or whether this is also present on TSC2046. A test on
either TSC2046 or ADS7846 would be very welcome.

Signed-off-by: Marek Vasut <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/touchscreen/ads7846.c

···
 
 	req->xfer[5].rx_buf = &req->scratch;
 	req->xfer[5].len = 2;
-	CS_CHANGE(req->xfer[5]);
 	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;
+	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);
 
 	mutex_lock(&ts->lock);
 	ads7846_stop(ts);