misc: rp1-pio: Don't just reuse the same DMA buf

A missing pointer increment meant that not only was the same buffer
being reused again and again, there was also no protection against
using it simultaneously for multiple transfers. Fix that basic bug, and
also move a similar increment to before the transfer is started, which
feels less racy.

See: #6919

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

Author: pelwell

drivers/misc/rp1-pio.c

@@ -850,9 +850,9 @@ static int rp1_pio_sm_rx_user(struct rp1_pio_device *pio, struct dma_info *dma,
 		if (ret < 0)
 			break;
 
+		dma->head_idx++;
 		dma_async_issue_pending(dma->chan);
 
-		dma->head_idx++;
 		bytes -= copy_bytes;
 	}
 
@@ -964,6 +964,7 @@ int rp1_pio_sm_xfer_data(struct rp1_pio_client *client, uint sm, uint dir,
 		return ret;
 	}
 
+	dma->head_idx++;
 	dma_async_issue_pending(dma->chan);
 
 	return 0;