diff --git a/src/gallium/drivers/iris/iris_screen.c b/src/gallium/drivers/iris/iris_screen.c
index 8d0628d248e..140f012747e 100644
--- a/src/gallium/drivers/iris/iris_screen.c
+++ b/src/gallium/drivers/iris/iris_screen.c
@@ -394,7 +394,7 @@ iris_init_screen_caps(struct iris_screen *screen)
    caps->constant_buffer_offset_alignment = 32;
    caps->min_map_buffer_alignment = IRIS_MAP_BUFFER_ALIGNMENT;
    caps->shader_buffer_offset_alignment = 4;
-   caps->max_shader_buffer_size = (unsigned)MIN2(screen->isl_dev.max_buffer_size, INT32_MAX); // INT32_MAX is correct.
+   caps->max_shader_buffer_size = ROUND_DOWN_TO((unsigned)MIN2(screen->isl_dev.max_buffer_size, INT32_MAX), 256);
    caps->texture_buffer_offset_alignment = 16; // XXX: u_screen says 256 is the minimum value...
    caps->linear_image_pitch_alignment = 1;
    caps->linear_image_base_address_alignment = 1;