staging:iio:ad7887: Preallocate sample buffer

We know that the sample buffer will at most need to hold two 16 bit samples and
the 64 bit aligned 64 bit timestamp. Preallocate a buffer large enough to hold
this instead of allocating and freeing it each time a sample is read.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7887.h

@@ -72,9 +72,11 @@ struct ad7887_state {
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
+	 * Buffer needs to be large enough to hold two 16 bit samples and a
+	 * 64 bit aligned 64 bit timestamp.
 	 */
-
-	unsigned char			data[4] ____cacheline_aligned;
+	unsigned char data[ALIGN(4, sizeof(s64)) + sizeof(s64)]
+		____cacheline_aligned;
 };
 
 enum ad7887_supported_device_ids {

drivers/staging/iio/adc/ad7887_ring.c

@@ -73,31 +73,20 @@ static irqreturn_t ad7887_trigger_handler(int irq, void *p)
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad7887_state *st = iio_priv(indio_dev);
 	s64 time_ns;
-	__u8 *buf;
 	int b_sent;
 
-	unsigned int bytes = bitmap_weight(indio_dev->active_scan_mask,
-					   indio_dev->masklength) *
-		st->chip_info->channel[0].scan_type.storagebits / 8;
-
-	buf = kzalloc(indio_dev->scan_bytes, GFP_KERNEL);
-	if (buf == NULL)
-		goto done;
-
 	b_sent = spi_sync(st->spi, st->ring_msg);
 	if (b_sent)
 		goto done;
 
 	time_ns = iio_get_time_ns();
 
-	memcpy(buf, st->data, bytes);
 	if (indio_dev->scan_timestamp)
-		memcpy(buf + indio_dev->scan_bytes - sizeof(s64),
+		memcpy(st->data + indio_dev->scan_bytes - sizeof(s64),
 		       &time_ns, sizeof(time_ns));
 
-	iio_push_to_buffer(indio_dev->buffer, buf);
+	iio_push_to_buffer(indio_dev->buffer, st->data);
 done:
-	kfree(buf);
 	iio_trigger_notify_done(indio_dev->trig);
 
 	return IRQ_HANDLED;