drm/i915/crt: Remove 0xa0 probe for VGA

This is a moral revert of 6ec3d0c0e9c0c605696e91048eebaca7b0c36695.

Following the fix to reset the GMBUS controller after a NAK, we finally
utilize the 0xa0 probe for a CRT connection. And discover that the code
is broken. Shock.

There are a number of issues, but following a key insight from Dave
Airlie, that 0xA0 is an invalid address on a 7-bit bus (though not if we
were to enable 10-bit addressing), and would look like the EDID port
0x50, it is possible to see where the confusion starts.

In short, a write to 0xA0 is accepted by the GMBUS controller which we
interpreted as meaning the existence of a connection (a slave on the
other end of the wire ACKing the write). That was false.

During testing with a broken GMBUS implementation, which never reset an
earlier NAK, this test always reported a NAK and so we proceeded on to
the next test.

Reported-and-tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35904
Reported-and-tested-by:  Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=32612
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/drivers/gpu/drm/i915/intel_crt.c b/drivers/gpu/drm/i915/intel_crt.c
index 8342259f3160d3eeaf118890c58825e845535982..d03fc05b39c0e008236406a9820f459103154f10 100644 (file)
--- a/drivers/gpu/drm/i915/intel_crt.c
+++ b/drivers/gpu/drm/i915/intel_crt.c
@@ -269,21 +269,6 @@ static bool intel_crt_detect_hotplug(struct drm_connector *connector)
        return ret;
 }
 
-static bool intel_crt_ddc_probe(struct drm_i915_private *dev_priv, int ddc_bus)
-{
-       u8 buf;
-       struct i2c_msg msgs[] = {
-               {
-                       .addr = 0xA0,
-                       .flags = 0,
-                       .len = 1,
-                       .buf = &buf,
-               },
-       };
-       /* DDC monitor detect: Does it ACK a write to 0xA0? */
-       return i2c_transfer(&dev_priv->gmbus[ddc_bus].adapter, msgs, 1) == 1;
-}
-
 static bool intel_crt_detect_ddc(struct drm_connector *connector)
 {
        struct intel_crt *crt = intel_attached_crt(connector);
@@ -293,11 +278,6 @@ static bool intel_crt_detect_ddc(struct drm_connector *connector)
        if (crt->base.type != INTEL_OUTPUT_ANALOG)
                return false;
 
-       if (intel_crt_ddc_probe(dev_priv, dev_priv->crt_ddc_pin)) {
-               DRM_DEBUG_KMS("CRT detected via DDC:0xa0\n");
-               return true;
-       }
-
        if (intel_ddc_probe(&crt->base, dev_priv->crt_ddc_pin)) {
                struct edid *edid;
                bool is_digital = false;