drm/i915: Fix DDC probe for passive adapters

Passive DP->DVI/HDMI dongles on DP++ ports show up to the system as HDMI
devices, as they do not have a sink device in them to respond to any AUX
traffic. When probing these dongles over the DDC, sometimes they will
NAK the first attempt even though the transaction is valid and they
support the DDC protocol. The retry loop inside of
drm_do_probe_ddc_edid() would normally catch this case and try the
transaction again, resulting in success.

That, however, was thwarted by the fix for [1]:

commit 9292f37e
Author: Eugeni Dodonov <eugeni.dodonov@intel.com>
Date:   Thu Jan 5 09:34:28 2012 -0200

    drm: give up on edid retries when i2c bus is not responding

This added code to exit immediately if the return code from the
i2c_transfer function was -ENXIO in order to reduce the amount of time
spent in waiting for unresponsive or disconnected devices. That was
possible because the underlying i2c bit banging algorithm had retries of
its own (which, of course, were part of the reason for the bug the
commit fixes).

Since its introduction in

commit f899fc64
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jul 20 15:44:45 2010 -0700

    drm/i915: use GMBUS to manage i2c links

we've been flipping back and forth enabling the GMBUS transfers, but
we've settled since then. The GMBUS implementation does not do any
retries, however, bailing out of the drm_do_probe_ddc_edid() retry loop
on first encounter of -ENXIO. This, combined with Eugeni's commit, broke
the retry on -ENXIO.

Retry GMBUS once on -ENXIO on first message to mitigate the issues with
passive adapters.

This patch is based on the work, and commit message, by Todd Previte
<tprevite@gmail.com>.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=41059

v2: Don't retry if using bit banging.

v3: Move retry within gmbux_xfer, retry only on first message.

v4: Initialize GMBUS0 on retry (Ville).

v5: Take index reads into account (Ville).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85924
Cc: Todd Previte <tprevite@gmail.com>
Cc: stable@vger.kernel.org
Tested-by: Oliver Grafe <oliver.grafe@ge.com> (v2)
Tested-by: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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 @@ gmbus_xfer(struct i2c_adapter *adapter,
 			 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: