[PATCH v3] drm/bridge: ti-sn65dsi86: Check bridge connection failure

[Wolfram Sang]

Read out and check the ID registers, so we can bail out if I2C
communication does not work or if the device is unknown. Tested on a
Renesas GrayHawk board (R-Car V4M) by using a wrong I2C address and by
not enabling RuntimePM for the device.

Signed-off-by: Wolfram Sang <wsa+renesas at sang-engineering.com>
---

Changes since v2:
* switched to a new approach suggested by Doug (Thanks!). We add a
  dedicated read instead of using the first read. This prevents creating
  the aux devices. As a side-gain, we check now if the chip at the address
  is really the one we want to support.

 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 87fffaa52bb0..8caa7918933d 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
@@ -36,6 +36,7 @@
 #include <drm/drm_print.h>
 #include <drm/drm_probe_helper.h>
 
+#define SN_DEVICE_ID_REGS			0x00	/* up to 0x07 */
 #define SN_DEVICE_REV_REG			0x08
 #define SN_DPPLL_SRC_REG			0x0A
 #define  DPPLL_CLK_SRC_DSICLK			BIT(0)
@@ -1875,6 +1876,7 @@ static int ti_sn65dsi86_probe(struct i2c_client *client)
 {
 	struct device *dev = &client->dev;
 	struct ti_sn65dsi86 *pdata;
+	u8 id_buf[8];
 	int ret;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
@@ -1918,6 +1920,16 @@ static int ti_sn65dsi86_probe(struct i2c_client *client)
 	if (ret)
 		return ret;
 
+	pm_runtime_get_sync(dev);
+	ret = regmap_bulk_read(pdata->regmap, SN_DEVICE_ID_REGS, id_buf, ARRAY_SIZE(id_buf));
+	pm_runtime_put_autosuspend(dev);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to read device id\n");
+
+	/* The ID string is stored backwards */
+	if (strncmp(id_buf, "68ISD   ", ARRAY_SIZE(id_buf)))
+		return dev_err_probe(dev, -EOPNOTSUPP, "unsupported device id\n");
+
 	/*
 	 * Break ourselves up into a collection of aux devices. The only real
 	 * motiviation here is to solve the chicken-and-egg problem of probe
-- 
2.47.2