diff --git a/src/gallium/drivers/iris/iris_context.h b/src/gallium/drivers/iris/iris_context.h
index e38ec77d1ec..a1cdb75a732 100644
--- a/src/gallium/drivers/iris/iris_context.h
+++ b/src/gallium/drivers/iris/iris_context.h
@@ -475,6 +475,7 @@ struct iris_vtable {
    void (*populate_gs_key)(const struct iris_context *ice,
                            struct brw_gs_prog_key *key);
    void (*populate_fs_key)(const struct iris_context *ice,
+                           const struct shader_info *info,
                            struct brw_wm_prog_key *key);
    void (*populate_cs_key)(const struct iris_context *ice,
                            struct brw_cs_prog_key *key);
diff --git a/src/gallium/drivers/iris/iris_program.c b/src/gallium/drivers/iris/iris_program.c
index 2fe842e9807..502cbebb024 100644
--- a/src/gallium/drivers/iris/iris_program.c
+++ b/src/gallium/drivers/iris/iris_program.c
@@ -1494,7 +1494,7 @@ iris_update_compiled_fs(struct iris_context *ice)
       struct iris_screen *screen = (struct iris_screen *)ice->ctx.screen;
       const struct gen_device_info *devinfo = &screen->devinfo;
    struct brw_wm_prog_key key = { KEY_INIT(devinfo->gen) };
-   ice->vtbl.populate_fs_key(ice, &key);
+   ice->vtbl.populate_fs_key(ice, &ish->nir->info, &key);
 
    if (ish->nos & (1ull << IRIS_NOS_LAST_VUE_MAP))
       key.input_slots_valid = ice->shaders.last_vue_map->slots_valid;
diff --git a/src/gallium/drivers/iris/iris_state.c b/src/gallium/drivers/iris/iris_state.c
index 99f006e8dfa..6fe3e04d92a 100644
--- a/src/gallium/drivers/iris/iris_state.c
+++ b/src/gallium/drivers/iris/iris_state.c
@@ -3442,6 +3442,7 @@ iris_populate_gs_key(const struct iris_context *ice,
  */
 static void
 iris_populate_fs_key(const struct iris_context *ice,
+                     const struct shader_info *info,
                      struct brw_wm_prog_key *key)
 {
    struct iris_screen *screen = (void *) ice->ctx.screen;
@@ -3458,8 +3459,8 @@ iris_populate_fs_key(const struct iris_context *ice,
 
    key->alpha_test_replicate_alpha = fb->nr_cbufs > 1 && zsa->alpha.enabled;
 
-   /* XXX: only bother if COL0/1 are read */
-   key->flat_shade = rast->flatshade;
+   key->flat_shade = rast->flatshade &&
+      (info->inputs_read & (VARYING_BIT_COL0 | VARYING_BIT_COL1));
 
    key->persample_interp = rast->force_persample_interp;
    key->multisample_fbo = rast->multisample && fb->samples > 1;