drm/i915/dsb: Add i915.enable_dsb module parameter

As we extend the use of DSB for critical pipe/plane register
programming, it'll be nice to have an escape valve at hand,
in case things go very poorly. To that end, add a i915.enable_dsb
modparam by which we can force the driver to take the pure mmio
path instead.

v2: Use 0400 permissions for the actual modparam (Jani)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611133344.30673-9-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_display_params.c

@@ -54,6 +54,9 @@ intel_display_param_named_unsafe(enable_dc, int, 0400,
 intel_display_param_named_unsafe(enable_dpt, bool, 0400,
 	"Enable display page table (DPT) (default: true)");
 
+intel_display_param_named_unsafe(enable_dsb, bool, 0400,
+	"Enable display state buffer (DSB) (default: true)");
+
 intel_display_param_named_unsafe(enable_sagv, bool, 0400,
 	"Enable system agent voltage/frequency scaling (SAGV) (default: true)");
 

drivers/gpu/drm/i915/display/intel_display_params.h

@@ -31,6 +31,7 @@ struct drm_i915_private;
 	param(int, vbt_sdvo_panel_type, -1, 0400) \
 	param(int, enable_dc, -1, 0400) \
 	param(bool, enable_dpt, true, 0400) \
+	param(bool, enable_dsb, true, 0600) \
 	param(bool, enable_sagv, true, 0600) \
 	param(int, disable_power_well, -1, 0400) \
 	param(bool, enable_ips, true, 0600) \

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -460,6 +460,9 @@ struct intel_dsb *intel_dsb_prepare(struct intel_atomic_state *state,
 	if (!HAS_DSB(i915))
 		return NULL;
 
+	if (!i915->display.params.enable_dsb)
+		return NULL;
+
 	/* TODO: DSB is broken in Xe KMD, so disabling it until fixed */
 	if (!IS_ENABLED(I915))
 		return NULL;