iio: adc: ad_sigma_delta: fix CS assertion and registerless device handling

drivers/iio/adc/ad_sigma_delta.c

@@ -205,7 +205,7 @@
 }
 EXPORT_SYMBOL_NS_GPL(ad_sd_reset, IIO_AD_SIGMA_DELTA);
 
 static bool ad_sd_disable_irq(struct ad_sigma_delta *sigma_delta)
 {
 	guard(spinlock_irqsave)(&sigma_delta->irq_lock);
 
@@ -246,23 +246,50 @@
 
 	/*
 	 * 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 status_reg;
 
 		ret = ad_sd_read_reg(sigma_delta, AD_SD_REG_STATUS, 1, &status_reg);
 		if (ret)
 			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
@@ -428,11 +455,10 @@
 out:
 	ad_sd_disable_irq(sigma_delta);
 
-	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
-	ad_sigma_delta_disable_one(sigma_delta, chan->address);
-
 out_unlock:
 	sigma_delta->keep_cs_asserted = false;
+	ad_sigma_delta_set_mode(sigma_delta, AD_SD_MODE_IDLE);
+	ad_sigma_delta_disable_one(sigma_delta, chan->address);
 	sigma_delta->bus_locked = false;
 	spi_bus_unlock(sigma_delta->spi->controller);
 out_release:
@@ -572,6 +598,9 @@
 	return 0;
 
 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);
 
 	return ret;
@@ -662,7 +691,7 @@
 	if (sigma_delta->status_appended) {
 		u8 converted_channel;
 
 		converted_channel = data[status_pos] & sigma_delta->info->status_ch_mask;
 		if (converted_channel != sigma_delta->slots[sigma_delta->current_slot]) {
 			/*
 			 * Desync occurred during continuous sampling of multiple channels.