staging:iio:ad7606: Avoid allocating buffer for each data capture

Currently the ad7606 driver dynamically allocates and frees a transfer
buffer each time a sample capture is performed in buffered mode, which
introduces unnecessary overhead. The driver state struct already contains a
buffer that is used for transfers in one-shot mode. This buffer is large
enough to hold all samples, but not the timestamp that might be present in
buffered mode. Extend the buffer size to be able to contain the timestamp
and update the buffered capture function to use this buffer.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7606.h

@@ -68,9 +68,9 @@ struct ad7606_state {
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
+	 * 8 * 16-bit samples + 64-bit timestamp
 	 */
-
-	unsigned short			data[8] ____cacheline_aligned;
+	unsigned short			data[12] ____cacheline_aligned;
 };
 
 struct ad7606_bus_ops {

drivers/staging/iio/adc/ad7606_ring.c

@@ -46,15 +46,10 @@ static void ad7606_poll_bh_to_ring(struct work_struct *work_s)
 	struct ad7606_state *st = container_of(work_s, struct ad7606_state,
 						poll_work);
 	struct iio_dev *indio_dev = iio_priv_to_dev(st);
-	__u8 *buf;
 	int ret;
 
-	buf = kzalloc(indio_dev->scan_bytes, GFP_KERNEL);
-	if (!buf)
-		return;
-
 	if (gpio_is_valid(st->pdata->gpio_frstdata)) {
-		ret = st->bops->read_block(st->dev, 1, buf);
+		ret = st->bops->read_block(st->dev, 1, st->data);
 		if (ret)
 			goto done;
 		if (!gpio_get_value(st->pdata->gpio_frstdata)) {
@@ -67,22 +62,21 @@ static void ad7606_poll_bh_to_ring(struct work_struct *work_s)
 			goto done;
 		}
 		ret = st->bops->read_block(st->dev,
-			st->chip_info->num_channels - 1, buf + 2);
+			st->chip_info->num_channels - 1, st->data + 1);
 		if (ret)
 			goto done;
 	} else {
 		ret = st->bops->read_block(st->dev,
-			st->chip_info->num_channels, buf);
+			st->chip_info->num_channels, st->data);
 		if (ret)
 			goto done;
 	}
 
-	iio_push_to_buffers_with_timestamp(indio_dev, buf,
+	iio_push_to_buffers_with_timestamp(indio_dev, st->data,
 					   iio_get_time_ns(indio_dev));
 done:
 	gpio_set_value(st->pdata->gpio_convst, 0);
 	iio_trigger_notify_done(indio_dev->trig);
-	kfree(buf);
 }
 
 int ad7606_register_ring_funcs_and_init(struct iio_dev *indio_dev)