drm/i915/intel_i2c: use WAIT cycle, not STOP

The i915 is only able to generate a STOP cycle (i.e. finalize an i2c
transaction) during a DATA or WAIT phase.  In other words, the
controller rejects a STOP requested as part of the first transaction in a
sequence.

Thus, for the first transaction we must always use a WAIT cycle, detect
when the device has finished (and is in a WAIT phase), and then either
start the next transaction, or, if there are no more transactions,
generate a STOP cycle.

Note: Theoretically, the last transaction of a multi-transaction sequence
could initiate a STOP cycle.  However, this slight optimization is left
for another patch.  We return -ETIMEDOUT if the hardware doesn't
deactivate after the STOP cycle.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
[danvet: added comment to the code that gmbus can't generate STOP on
the very first cycle.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 18 insertions, 15 deletions

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index 9707868881ec..291e51ec309b 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -204,8 +204,7 @@ intel_gpio_setup(struct intel_gmbus *bus, u32 pin)
 }
 
 static int
-gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
-                bool last)
+gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
 {
         int reg_offset = dev_priv->gpio_mmio_base;
         u16 len = msg->len;
@@ -213,7 +212,6 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 
         I915_WRITE(GMBUS1 + reg_offset,
                    GMBUS_CYCLE_WAIT |
-                   (last ? GMBUS_CYCLE_STOP : 0) |
                    (len << GMBUS_BYTE_COUNT_SHIFT) |
                    (msg->addr << GMBUS_SLAVE_ADDR_SHIFT) |
                    GMBUS_SLAVE_READ | GMBUS_SW_RDY);
@@ -239,8 +237,7 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 }
 
 static int
-gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
-                bool last)
+gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
 {
         int reg_offset = dev_priv->gpio_mmio_base;
         u16 len = msg->len;
@@ -256,7 +253,6 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
         I915_WRITE(GMBUS3 + reg_offset, val);
         I915_WRITE(GMBUS1 + reg_offset,
                    GMBUS_CYCLE_WAIT |
-                   (last ? GMBUS_CYCLE_STOP : 0) |
                    (msg->len << GMBUS_BYTE_COUNT_SHIFT) |
                    (msg->addr << GMBUS_SLAVE_ADDR_SHIFT) |
                    GMBUS_SLAVE_WRITE | GMBUS_SW_RDY);
@@ -289,7 +285,8 @@ gmbus_xfer(struct i2c_adapter *adapter,
                                                struct intel_gmbus,
                                                adapter);
         struct drm_i915_private *dev_priv = bus->dev_priv;
-        int i, reg_offset, ret;
+        int i, reg_offset;
+        int ret = 0;
 
         mutex_lock(&dev_priv->gmbus_mutex);
 
@@ -303,20 +300,17 @@ gmbus_xfer(struct i2c_adapter *adapter,
         I915_WRITE(GMBUS0 + reg_offset, bus->reg0);
 
         for (i = 0; i < num; i++) {
-                bool last = i + 1 == num;
-
                 if (msgs[i].flags & I2C_M_RD)
-                        ret = gmbus_xfer_read(dev_priv, &msgs[i], last);
+                        ret = gmbus_xfer_read(dev_priv, &msgs[i]);
                 else
-                        ret = gmbus_xfer_write(dev_priv, &msgs[i], last);
+                        ret = gmbus_xfer_write(dev_priv, &msgs[i]);
 
                 if (ret == -ETIMEDOUT)
                         goto timeout;
                 if (ret == -ENXIO)
                         goto clear_err;
 
-                if (!last &&
-                    wait_for(I915_READ(GMBUS2 + reg_offset) &
+                if (wait_for(I915_READ(GMBUS2 + reg_offset) &
                              (GMBUS_SATOER | GMBUS_HW_WAIT_PHASE),
                              50))
                         goto timeout;
@@ -324,15 +318,24 @@ gmbus_xfer(struct i2c_adapter *adapter,
                         goto clear_err;
         }
 
+        /* Generate a STOP condition on the bus. Note that gmbus can't generata
+         * a STOP on the very first cycle. To simplify the code we
+         * unconditionally generate the STOP condition with an additional gmbus
+         * cycle. */
+        I915_WRITE(GMBUS1 + reg_offset, GMBUS_CYCLE_STOP | GMBUS_SW_RDY);
+
         /* Mark the GMBUS interface as disabled after waiting for idle.
          * We will re-enable it at the start of the next xfer,
          * till then let it sleep.
          */
-        if (wait_for((I915_READ(GMBUS2 + reg_offset) & GMBUS_ACTIVE) == 0, 10))
+        if (wait_for((I915_READ(GMBUS2 + reg_offset) & GMBUS_ACTIVE) == 0,
+                     10)) {
                 DRM_INFO("GMBUS [%s] timed out waiting for idle\n",
                          adapter->name);
+                ret = -ETIMEDOUT;
+        }
         I915_WRITE(GMBUS0 + reg_offset, 0);
-        ret = i;
+        ret = ret ?: i;
         goto out;
 
 clear_err: