drm/i915: Restore GMBUS operation after a failed bit-banging fallback

When the GMBUS based i2c transfer times out, we try to fall back to
bit-banging and retry the operation that way. However if the bit-banging
attempt also fails, we should probably go back to the GMBUS method for
the next attempt. Maybe there simply wasn't anyone one the bus at this
time.

There's also a bit of a mess going on with the force_bit handling.
It's supposed to be a ref count actually, and it is as far as
intel_gmbus_force_bit() is concerned. But it's treated as just a
flag by the timeout based bit-banging fallback. I suppose that's
fine since we should never end up in the timeout fallback case
if force_bit was already non-zero. However now that we want to restore
things back to where they were after the bit-banging attempt failed,
we're going to have to do things a bit differently to avoid clobbering
the force_bit count as set up by intel_gmbus_force_bit(). So let's
dedicate the high bit as a flag for the low level timeout based fallback
and treat the rest of the bits as a ref count just as before.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457366220-29409-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -996,6 +996,7 @@ struct intel_fbc_work;
 
 struct intel_gmbus {
 	struct i2c_adapter adapter;
+#define GMBUS_FORCE_BIT_RETRY (1U << 31)
 	u32 force_bit;
 	u32 reg0;
 	i915_reg_t gpio_reg;

drivers/gpu/drm/i915/intel_i2c.c

@@ -579,7 +579,6 @@ timeout:
 	 * Hardware may not support GMBUS over these pins? Try GPIO bitbanging
 	 * instead. Use EAGAIN to have i2c core retry.
 	 */
-	bus->force_bit = 1;
 	ret = -EAGAIN;
 
 out:
@@ -597,10 +596,15 @@ gmbus_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs, int num)
 	intel_display_power_get(dev_priv, POWER_DOMAIN_GMBUS);
 	mutex_lock(&dev_priv->gmbus_mutex);
 
-	if (bus->force_bit)
+	if (bus->force_bit) {
 		ret = i2c_bit_algo.master_xfer(adapter, msgs, num);
-	else
+		if (ret < 0)
+			bus->force_bit &= ~GMBUS_FORCE_BIT_RETRY;
+	} else {
 		ret = do_gmbus_xfer(adapter, msgs, num);
+		if (ret == -EAGAIN)
+			bus->force_bit |= GMBUS_FORCE_BIT_RETRY;
+	}
 
 	mutex_unlock(&dev_priv->gmbus_mutex);
 	intel_display_power_put(dev_priv, POWER_DOMAIN_GMBUS);