UPSTREAM: drm/i915/intel_i2c: use double-buffered writes

The GMBUS controller GMBUS3 register is double-buffered.  Take advantage
of this  by writing two 4-byte words before the first wait for HW_RDY.
This helps keep the GMBUS controller from becoming idle during long writes.

In fact, during experiments using the GMBUS interrupts, the HW_RDY
interrupt would only trigger for transactions >4 bytes after 2 writes
to GMBUS3.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit 7a39a9d4767e8d22d60f2c4bf5eece4f4398c274)

Change-Id: I6fe6fff8b73e841b74324c237c63b8484ac8b6d9
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index 99a04f8..f02e52a 100644 (file)
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -262,13 +262,6 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
                   GMBUS_SLAVE_WRITE | GMBUS_SW_RDY);
        POSTING_READ(GMBUS2 + reg_offset);
        while (len) {
-               if (wait_for(I915_READ(GMBUS2 + reg_offset) &
-                            (GMBUS_SATOER | GMBUS_HW_RDY),
-                            50))
-                       return -ETIMEDOUT;
-               if (I915_READ(GMBUS2 + reg_offset) & GMBUS_SATOER)
-                       return -ENXIO;
-
                val = loop = 0;
                do {
                        val |= *buf++ << (8 * loop);
@@ -276,6 +269,13 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 
                I915_WRITE(GMBUS3 + reg_offset, val);
                POSTING_READ(GMBUS2 + reg_offset);
+
+               if (wait_for(I915_READ(GMBUS2 + reg_offset) &
+                            (GMBUS_SATOER | GMBUS_HW_RDY),
+                            50))
+                       return -ETIMEDOUT;
+               if (I915_READ(GMBUS2 + reg_offset) & GMBUS_SATOER)
+                       return -ENXIO;
        }
        return 0;
 }