drm/i915: Workaround to bump rc6 voltage to 450

[Dan Carpenter]

Hi Ben,

The patch 31643d54a739: "drm/i915: Workaround to bump rc6 voltage to 
450" from Sep 26, 2012, leads to the following warning:
"drivers/gpu/drm/i915/intel_pm.c:2662 gen6_enable_rps()
	 warn: boolean comparison inside select"

drivers/gpu/drm/i915/i915_reg.h
  4260  #define   GEN6_ENCODE_RC6_VID(mv)               (((mv) / 5) - 245) < 0 ?: 0
                  ^^^^^^^^^^^^^^^^^^^^^^
It returns either 1 or 0 depending on if mv is >= 1230.

  4261  #define   GEN6_DECODE_RC6_VID(vids)             (((vids) * 5) > 0 ? ((vids) * 5) + 245 : 0)

I think this is trying to reverse the other macro but the order of
operations are wrong.  It should be "(vids + 245) * 5".

drivers/gpu/drm/i915/intel_pm.c
  2654          rc6vids = 0;
  2655          ret = sandybridge_pcode_read(dev_priv, GEN6_PCODE_READ_RC6VIDS, &rc6vids);
  2656          if (IS_GEN6(dev) && ret) {
  2657                  DRM_DEBUG_DRIVER("Couldn't check for BIOS workaround\n");
  2658          } else if (IS_GEN6(dev) && (GEN6_DECODE_RC6_VID(rc6vids & 0xff) < 450)) {
  2659                  DRM_DEBUG_DRIVER("You should update your BIOS. Correcting minimum rc6 voltage (%dmV->%dmV)\n",
  2660                            GEN6_DECODE_RC6_VID(rc6vids & 0xff), 450);
  2661                  rc6vids &= 0xffff00;
  2662                  rc6vids |= GEN6_ENCODE_RC6_VID(450);
                                   ^^^^^^^^^^^^^^^^^^^^^^^
This is the only caller and 450 is less than 1230 we store zero.

  2663                  ret = sandybridge_pcode_write(dev_priv, GEN6_PCODE_WRITE_RC6VIDS, rc6vids);
  2664                  if (ret)
  2665                          DRM_ERROR("Couldn't fix incorrect rc6 voltage\n");
  2666          }

regards,
dan carpenter