I haven't found any method for getting the length of a response, so this
just uses the requested rx_len

Signed-off-by: John Keeping <redacted>
---
Unchanged in v2
---
  drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 54 ++++++++++++++++++++++++++++++++++
  1 file changed, 54 insertions(+)

diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
index f780401e8b5e..f0cb005c6d76 100644
--- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
+++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c

@@ -675,6 +675,54 @@ static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
  	return dw_mipi_dsi_gen_pkt_hdr_write(dsi, hdr_val);
  }
  
+static int dw_mipi_dsi_dcs_read(struct dw_mipi_dsi *dsi,
+				const struct mipi_dsi_msg *msg)
+{
+	const u8 *tx_buf = msg->tx_buf;
+	u8 *rx_buf = msg->rx_buf;
+	size_t i;
+	int ret, val;
+
+	dsi_write(dsi, DSI_PCKHDL_CFG, EN_CRC_RX | EN_ECC_RX | EN_BTA);
+	dsi_write(dsi, DSI_GEN_HDR, GEN_HDATA(tx_buf[0]) | GEN_HTYPE(msg->type));

+
+	ret = readl_poll_timeout(dsi->base + DSI_CMD_PKT_STATUS,
+				 val, !(val & GEN_RD_CMD_BUSY), 1000,
+				 CMD_PKT_STATUS_TIMEOUT_US);
+	if (ret < 0) {
+		dev_err(dsi->dev, "failed to read command response\n");
+		return ret;
+	}
+
+	for (i = 0; i < msg->rx_len;) {
+		u32 pld = dsi_read(dsi, DSI_GEN_PLD_DATA);
+		while (i < msg->rx_len) {
+			rx_buf[i] = pld & 0xff;
+			pld >>= 8;
+			i++;
+		}
+	}
+
+	return msg->rx_len;
+}
+
+static int dw_mipi_dsi_set_max_return_packet_size(struct dw_mipi_dsi *dsi,
+						  size_t len)
+{
+	u8 val[] = { len & 0xff, (len >> 8) & 0xff };
+	struct mipi_dsi_msg msg = {
+		.channel = dsi->channel,
+		.type = MIPI_DSI_SET_MAXIMUM_RETURN_PACKET_SIZE,
+		.tx_buf = val,
+		.tx_len = 2,
+	};
+
+	if (len > 0xffff)
+		return -EINVAL;
+
+	return dw_mipi_dsi_dcs_short_write(dsi, &msg);
+}
+
  static ssize_t dw_mipi_dsi_host_transfer(struct mipi_dsi_host *host,
  					 const struct mipi_dsi_msg *msg)
  {

@@ -692,6 +740,12 @@ static ssize_t dw_mipi_dsi_host_transfer(struct mipi_dsi_host *host,
  	case MIPI_DSI_DCS_LONG_WRITE:
  		ret = dw_mipi_dsi_dcs_long_write(dsi, msg);
  		break;
+	case MIPI_DSI_DCS_READ:
+		ret = dw_mipi_dsi_set_max_return_packet_size(dsi, msg->rx_len);
+		if (ret < 0)
+			return ret;
+		ret = dw_mipi_dsi_dcs_read(dsi, msg);
+		break;
  	default:
  		dev_err(dsi->dev, "unsupported message type 0x%02x\n",
  			msg->type);