drm/i915: Allow MI_LOAD_REGISTER_REG between whitelisted registers.

Allowing register copies where the source and destination are both
whitelisted should be safe, and is useful.  For example, Mesa uses
this to load the command streamer math registers with data from the
pipeline statistics counters.

v2: Reject writes to OACONTROL (and reads as well :(
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1462521014-13595-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_cmd_parser.c

@@ -215,7 +215,8 @@ static const struct drm_i915_cmd_descriptor hsw_render_cmds[] = {
 	CMD(  MI_RS_CONTEXT,                    SMI,    F,  1,      S  ),
 	CMD(  MI_LOAD_SCAN_LINES_INCL,          SMI,   !F,  0x3F,   M  ),
 	CMD(  MI_LOAD_SCAN_LINES_EXCL,          SMI,   !F,  0x3F,   R  ),
-	CMD(  MI_LOAD_REGISTER_REG,             SMI,   !F,  0xFF,   R  ),
+	CMD(  MI_LOAD_REGISTER_REG,             SMI,   !F,  0xFF,   W,
+	      .reg = { .offset = 1, .mask = 0x007FFFFC, .step = 1 }    ),
 	CMD(  MI_RS_STORE_DATA_IMM,             SMI,   !F,  0xFF,   S  ),
 	CMD(  MI_LOAD_URB_MEM,                  SMI,   !F,  0xFF,   S  ),
 	CMD(  MI_STORE_URB_MEM,                 SMI,   !F,  0xFF,   S  ),
@@ -1098,6 +1099,11 @@ static bool check_cmd(const struct intel_engine_cs *engine,
 					return false;
 				}
 
+				if (desc->cmd.value == MI_LOAD_REGISTER_REG) {
+					DRM_DEBUG_DRIVER("CMD: Rejected LRR to OACONTROL\n");
+					return false;
+				}
+
 				if (desc->cmd.value == MI_LOAD_REGISTER_IMM(1))
 					*oacontrol_set = (cmd[offset + 1] != 0);
 			}
@@ -1113,6 +1119,12 @@ static bool check_cmd(const struct intel_engine_cs *engine,
 					return false;
 				}
 
+				if (desc->cmd.value == MI_LOAD_REGISTER_REG) {
+					DRM_DEBUG_DRIVER("CMD: Rejected LRR to masked register 0x%08X\n",
+							 reg_addr);
+					return false;
+				}
+
 				if (desc->cmd.value == MI_LOAD_REGISTER_IMM(1) &&
 				    (offset + 2 > length ||
 				     (cmd[offset + 1] & reg->mask) != reg->value)) {
@@ -1301,6 +1313,7 @@ int i915_cmd_parser_get_version(struct drm_i915_private *dev_priv)
 	 * 4. L3 atomic chicken bits of HSW_SCRATCH1 and HSW_ROW_CHICKEN3.
 	 * 5. GPGPU dispatch compute indirect registers.
 	 * 6. TIMESTAMP register and Haswell CS GPR registers
+	 * 7. Allow MI_LOAD_REGISTER_REG between whitelisted registers.
 	 */
-	return 6;
+	return 7;
 }