[bug report] drm/bridge: synopsys: Add DW DPTX Controller support library

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Andy Yan,

Commit 86eecc3a9c2e ("drm/bridge: synopsys: Add DW DPTX Controller
support library") from Aug 22, 2025 (linux-next), leads to the
following Smatch static checker warning:

	drivers/gpu/drm/bridge/synopsys/dw-dp.c:730 dw_dp_voltage_max()
	warn: bitwise AND is zero '0x3 & 0x18'

drivers/gpu/drm/bridge/synopsys/dw-dp.c
    728 static u8 dw_dp_voltage_max(u8 preemph)
    729 {
--> 730         switch (preemph & DP_TRAIN_PRE_EMPHASIS_MASK) {
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This mask will always be zero (DP_TRAIN_PRE_EMPH_LEVEL_0).

    731         case DP_TRAIN_PRE_EMPH_LEVEL_0:
    732                 return DP_TRAIN_VOLTAGE_SWING_LEVEL_3;
    733         case DP_TRAIN_PRE_EMPH_LEVEL_1:
    734                 return DP_TRAIN_VOLTAGE_SWING_LEVEL_2;
    735         case DP_TRAIN_PRE_EMPH_LEVEL_2:
    736                 return DP_TRAIN_VOLTAGE_SWING_LEVEL_1;
    737         case DP_TRAIN_PRE_EMPH_LEVEL_3:
    738         default:
    739                 return DP_TRAIN_VOLTAGE_SWING_LEVEL_0;
    740         }
    741 }

The problem is the inconsistent >> 3 shifting.  Here is how the
caller looks like:

   755                  p = drm_dp_get_adjust_request_pre_emphasis(status, i);

p is a 0x3 << 3 mask

   756                  p >>= DP_TRAIN_PRE_EMPHASIS_SHIFT;

We shift it >> 3 for convenience

   757  
   758                  if (v != adj->voltage_swing[i] || p != adj->pre_emphasis[i])
   759                          changed = true;
   760  
   761                  if (p >=  (DP_TRAIN_PRE_EMPH_LEVEL_3 >> DP_TRAIN_PRE_EMPHASIS_SHIFT)) {
   762                          adj->pre_emphasis[i] = DP_TRAIN_PRE_EMPH_LEVEL_3 >>
   763                                                 DP_TRAIN_PRE_EMPHASIS_SHIFT;
   764                          adj->pre_max_reached[i] = true;
   765                  } else {
   766                          adj->pre_emphasis[i] = p;
   767                          adj->pre_max_reached[i] = false;
   768                  }
   769  
   770                  v = min(v, dw_dp_voltage_max(p));
                                                     ^
But the dw_dp_voltage_max() function expects the unshifted value.

There is another similar warning but it's probably deliberate.
drivers/gpu/drm/bridge/synopsys/dw-dp.c:1072 dw_dp_send_sdp() warn: bitwise AND is zero '0x1 & 0x2'

drivers/gpu/drm/bridge/synopsys/dw-dp.c
  1066  
  1067          if (sdp->flags & DW_DP_SDP_VERTICAL_INTERVAL)
  1068                  regmap_update_bits(dp->regmap, DW_DP_SDP_VERTICAL_CTRL,
  1069                                     EN_VERTICAL_SDP << nr,
  1070                                     EN_VERTICAL_SDP << nr);
  1071  
  1072          if (sdp->flags & DW_DP_SDP_HORIZONTAL_INTERVAL)
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
DW_DP_SDP_HORIZONTAL_INTERVAL is never used.

  1073                  regmap_update_bits(dp->regmap, DW_DP_SDP_HORIZONTAL_CTRL,
  1074                                     EN_HORIZONTAL_SDP << nr,
  1075                                     EN_HORIZONTAL_SDP << nr);
  1076  

regards,
dan carpenter