[PATCH 7/7] drm/panel: sitronix-st7789v: Check display ID

[Miquel Raynal]

A very basic debugging rule when a device is connected for the first
time is to access a read-only register which contains known data in
order to ensure the communication protocol is properly working. This
driver lacked any read helper which is often a critical peace for
fastening bring-ups.

Add a read helper and use it to verify the communication with the panel
is working as soon as possible in order to fail early if this is not the
case.

Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>
---
 .../gpu/drm/panel/panel-sitronix-st7789v.c    | 78 +++++++++++++++++++
 1 file changed, 78 insertions(+)

diff --git a/drivers/gpu/drm/panel/panel-sitronix-st7789v.c b/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
index 7de192a3a8aa..fb21d52a7a63 100644
--- a/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
+++ b/drivers/gpu/drm/panel/panel-sitronix-st7789v.c
@@ -113,6 +113,9 @@
 			return val;		\
 	} while (0)
 
+#define ST7789V_IDS { 0x85, 0x85, 0x52 }
+#define ST7789V_IDS_SIZE 3
+
 struct st7789v_panel_info {
 	const struct drm_display_mode *display_mode;
 	u16 width_mm;
@@ -165,6 +168,77 @@ static int st7789v_write_data(struct st7789v *ctx, u8 cmd)
 	return st7789v_spi_write(ctx, ST7789V_DATA, cmd);
 }
 
+static int st7789v_read_data(struct st7789v *ctx, u8 cmd, u8 *buf,
+			     unsigned int len)
+{
+	struct spi_transfer xfer[2] = { };
+	struct spi_message msg;
+	u16 txbuf = ((ST7789V_COMMAND & 1) << 8) | cmd;
+	u16 rxbuf[4] = {};
+	u8 bit9 = 0;
+	int ret, i;
+
+	switch (len) {
+	case 1:
+	case 3:
+	case 4:
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	spi_message_init(&msg);
+
+	xfer[0].tx_buf = &txbuf;
+	xfer[0].len = sizeof(txbuf);
+	spi_message_add_tail(&xfer[0], &msg);
+
+	xfer[1].rx_buf = rxbuf;
+	xfer[1].len = len * 2;
+	spi_message_add_tail(&xfer[1], &msg);
+
+	ret = spi_sync(ctx->spi, &msg);
+	if (ret)
+		return ret;
+
+	for (i = 0; i < len; i++) {
+		buf[i] = rxbuf[i] >> i | (bit9 << (9 - i));
+		if (i)
+			bit9 = rxbuf[i] & GENMASK(i - 1, 0);
+	}
+
+	return 0;
+}
+
+static int st7789v_check_id(struct drm_panel *panel)
+{
+	const u8 st7789v_ids[ST7789V_IDS_SIZE] = ST7789V_IDS;
+	struct st7789v *ctx = panel_to_st7789v(panel);
+	bool invalid_ids = false;
+	int ret, i;
+	u8 ids[3];
+
+	ret = st7789v_read_data(ctx, MIPI_DCS_GET_DISPLAY_ID, ids, ST7789V_IDS_SIZE);
+	if (ret) {
+		dev_err(panel->dev, "Failed to read IDs\n");
+		return ret;
+	}
+
+	for (i = 0; i < ST7789V_IDS_SIZE; i++) {
+		if (ids[i] != st7789v_ids[i]) {
+			invalid_ids = true;
+			break;
+		}
+	}
+
+	if (invalid_ids) {
+		dev_err(panel->dev, "Unrecognized panel IDs");
+		return -EIO;
+	}
+
+	return 0;
+}
+
 static const struct drm_display_mode default_mode = {
 	.clock = 7000,
 	.hdisplay = 240,
@@ -237,6 +311,10 @@ static int st7789v_prepare(struct drm_panel *panel)
 	gpiod_set_value(ctx->reset, 0);
 	msleep(120);
 
+	ret = st7789v_check_id(panel);
+	if (ret)
+		return ret;
+
 	ST7789V_TEST(ret, st7789v_write_command(ctx, MIPI_DCS_EXIT_SLEEP_MODE));
 
 	/* We need to wait 120ms after a sleep out command */
-- 
2.34.1