diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index 651ade67ad2e..1b410291da53 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -262,11 +262,25 @@ 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.
+	 *
+	 * This is safe for all current registerless devices: ad7191 and ad7780
+	 * (with powerdown GPIO) are reset between conversions by CS deassertion,
+	 * so there is no stale result to drain; ad7780 (without powerdown GPIO)
+	 * and max11205 are continuously-converting and cycle ~DRDY at the output
+	 * data rate regardless of whether the previous result was read, so the
+	 * next falling edge fires naturally.
+	 *
+	 * A future registerless device that holds ~DRDY asserted until data is
+	 * read would be broken by this early return and would need either
+	 * num_resetclks set or a rdy-gpio.
 	 */
 	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);
@@ -274,11 +288,24 @@ 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)
 		return 0;
 
+	/*
+	 * With num_resetclks = 0, data_read_len is 0 and the drain sequence
+	 * below would compute memset(data + 2, 0xff, 0 - 1), underflowing to
+	 * SIZE_MAX and corrupting the heap. There is no safe way to drain the
+	 * stale result without knowing the data register size; it will be
+	 * consumed by the first ad_sd_read_reg() call in
+	 * ad_sigma_delta_single_conversion().
+	 */
+	if (!data_read_len)
+		return 0;
+
 	/*
 	 * In general the size of the data register is unknown. It varies from
 	 * device to device, might be one byte longer if CONTROL.DATA_STATUS is