1 files changed, 17 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index 56e437e31580..ae628001fd97 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -435,7 +435,7 @@ gmbus_xfer(struct i2c_adapter *adapter,
                                                struct intel_gmbus,
                                                adapter);
         struct drm_i915_private *dev_priv = bus->dev_priv;
-        int i, reg_offset;
+        int i = 0, inc, try = 0, reg_offset;
         int ret = 0;
 
         intel_aux_display_runtime_get(dev_priv);
@@ -448,12 +448,14 @@ gmbus_xfer(struct i2c_adapter *adapter,
 
         reg_offset = dev_priv->gpio_mmio_base;
 
+retry:
         I915_WRITE(GMBUS0 + reg_offset, bus->reg0);
 
-        for (i = 0; i < num; i++) {
+        for (; i < num; i += inc) {
+                inc = 1;
                 if (gmbus_is_index_read(msgs, i, num)) {
                         ret = gmbus_xfer_index_read(dev_priv, &msgs[i]);
-                        i += 1;  /* set i to the index of the read xfer */
+                        inc = 2; /* an index read is two msgs */
                 } else if (msgs[i].flags & I2C_M_RD) {
                         ret = gmbus_xfer_read(dev_priv, &msgs[i], 0);
                 } else {
@@ -525,6 +527,18 @@ clear_err:
                          adapter->name, msgs[i].addr,
                          (msgs[i].flags & I2C_M_RD) ? 'r' : 'w', msgs[i].len);
 
+        /*
+         * Passive adapters sometimes NAK the first probe. Retry the first
+         * message once on -ENXIO for GMBUS transfers; the bit banging algorithm
+         * has retries internally. See also the retry loop in
+         * drm_do_probe_ddc_edid, which bails out on the first -ENXIO.
+         */
+        if (ret == -ENXIO && i == 0 && try++ == 0) {
+                DRM_DEBUG_KMS("GMBUS [%s] NAK on first message, retry\n",
+                              adapter->name);
+                goto retry;
+        }
+
         goto out;
 
 timeout: