staging:iio:ad7476: Avoid alloc/free for each sample in buffered mode

The ad7476 driver has only support for 1 channel ADCs. So the upper limit for
the buffer size is the size of one sample plus the size of the timestamp.
Preallocate a buffer large enough to hold this to avoid having to allocate and
free a new buffer for each sample being captured.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/ad7476.h

@@ -33,8 +33,11 @@ struct ad7476_state {
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
+	 * Make the buffer large enough for one 16 bit sample and one 64 bit
+	 * aligned 64 bit timestamp.
 	 */
-	unsigned char			data[2] ____cacheline_aligned;
+	unsigned char data[ALIGN(2, sizeof(s64)) + sizeof(s64)]
+			____cacheline_aligned;
 };
 
 enum ad7476_supported_device_ids {

drivers/staging/iio/adc/ad7476_ring.c

@@ -26,28 +26,20 @@ static irqreturn_t ad7476_trigger_handler(int irq, void  *p)
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct ad7476_state *st = iio_priv(indio_dev);
 	s64 time_ns;
-	__u8 *rxbuf;
 	int b_sent;
 
-	rxbuf = kzalloc(indio_dev->scan_bytes, GFP_KERNEL);
-	if (rxbuf == NULL)
-		goto done;
-
-	b_sent = spi_read(st->spi, rxbuf,
-			  st->chip_info->channel[0].scan_type.storagebits / 8);
+	b_sent = spi_sync(st->spi, &st->msg);
 	if (b_sent < 0)
 		goto done;
 
 	time_ns = iio_get_time_ns();
 
 	if (indio_dev->scan_timestamp)
-		memcpy(rxbuf + indio_dev->scan_bytes - sizeof(s64),
-			&time_ns, sizeof(time_ns));
+		((s64 *)st->data)[1] = time_ns;
 
-	iio_push_to_buffer(indio_dev->buffer, rxbuf);
+	iio_push_to_buffer(indio_dev->buffer, st->data);
 done:
 	iio_trigger_notify_done(indio_dev->trig);
-	kfree(rxbuf);
 
 	return IRQ_HANDLED;
 }