[PATCH v3 2/2] iio: adc: ad_sigma_delta: fix clear_pending_event for registerless devices

[Radu Sabau via B4 Relay <devnull+radu.sabau.analog.com@kernel.org>]

From: Radu Sabau <radu.sabau@analog.com>

ad_sigma_delta_clear_pending_event() falls through to the status register
read path for devices with has_registers = false and no rdy_gpiod. For
such devices, ad_sd_read_reg() skips the address byte entirely and clocks
raw MISO bytes with no address phase — making it byte-for-byte identical
to reading conversion data. If a pending conversion result is present,
this partially consumes it and corrupts the data stream for the subsequent
ad_sd_read_reg() call in ad_sigma_delta_single_conversion().

Furthermore, with num_resetclks = 0 on these devices, data_read_len
evaluates to 0. If the clocked byte has bit 7 clear, pending_event is set
and the code attempts memset(data + 2, 0xff, 0 - 1), overflowing to
SIZE_MAX and corrupting the heap.

Fix by returning 0 immediately when neither rdy_gpiod nor has_registers
is set. This is safe because the IRQ is requested with IRQF_NO_AUTOEN and
IRQ_DISABLE_UNLAZY, which keeps the hardware IRQ line unmasked even while
software-disabled. Any falling edge from a completed conversion is latched
by the IRQ controller. When ad_sd_enable_irq() is subsequently called the
latched edge fires immediately, and the existing ad_sd_read_reg() call in
ad_sigma_delta_single_conversion() reads the complete stale result from
the beginning with no prior partial clock corruption.

Signed-off-by: Radu Sabau <radu.sabau@analog.com>
---
 drivers/iio/adc/ad_sigma_delta.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index a33a7e8c264f..88dcaa910b2d 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -262,11 +262,16 @@ static int ad_sigma_delta_clear_pending_event(struct ad_sigma_delta *sigma_delta
 
 	/*
 	 * Read R̅D̅Y̅ pin (if possible) or status register to check if there is an
-	 * old event.
+	 * old event. For devices with neither an RDY GPIO nor registers,
+	 * ad_sd_read_reg() transmits no address byte and clocks raw MISO bytes,
+	 * which is indistinguishable from reading conversion data and would
+	 * partially consume a pending result. Skip the check for such devices;
+	 * IRQ_DISABLE_UNLAZY ensures any pending falling edge is latched and
+	 * fires naturally on the next ad_sd_enable_irq() call.
 	 */
 	if (sigma_delta->rdy_gpiod) {
 		pending_event = gpiod_get_value(sigma_delta->rdy_gpiod);
-	} else {
+	} else if (sigma_delta->info->has_registers) {
 		unsigned int status_reg;
 
 		ret = ad_sd_read_reg(sigma_delta, AD_SD_REG_STATUS, 1, &status_reg);
@@ -274,6 +279,8 @@ static int ad_sigma_delta_clear_pending_event(struct ad_sigma_delta *sigma_delta
 			return ret;
 
 		pending_event = !(status_reg & AD_SD_REG_STATUS_RDY);
+	} else {
+		return 0;
 	}
 
 	if (!pending_event)
@@ -578,6 +585,7 @@ static int ad_sd_buffer_postenable(struct iio_dev *indio_dev)
 
 err_unlock:
 	sigma_delta->keep_cs_asserted = false;
+	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
 	sigma_delta->bus_locked = false;
 	spi_bus_unlock(sigma_delta->spi->controller);
 	spi_unoptimize_message(&sigma_delta->sample_msg);

-- 
2.43.0