drm/i915: Add i915.enable_sagv modparam

Currently we have no sane way to forcibly disable SAGV, which
makes debugging things a PITA. Manually poking at the pcode
mailbox with it's various SAGV/QGV/PSF formats is no fun,
and likely to be clobbered by the driver anyway.

Let's add a modparam for this.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230322181219.5511-1-ville.syrjala@linux.intel.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

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

@@ -411,6 +411,9 @@ static bool intel_crtc_can_enable_sagv(const struct intel_crtc_state *crtc_state
 	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
 	struct drm_i915_private *i915 = to_i915(crtc->base.dev);
 
+	if (!i915->params.enable_sagv)
+		return false;
+
 	if (DISPLAY_VER(i915) >= 12)
 		return tgl_crtc_can_enable_sagv(crtc_state);
 	else
@@ -3696,6 +3699,7 @@ static int intel_sagv_status_show(struct seq_file *m, void *unused)
 	};
 
 	seq_printf(m, "SAGV available: %s\n", str_yes_no(intel_has_sagv(i915)));
+	seq_printf(m, "SAGV modparam: %s\n", str_enabled_disabled(i915->params.enable_sagv));
 	seq_printf(m, "SAGV status: %s\n", sagv_status[i915->display.sagv.status]);
 	seq_printf(m, "SAGV block time: %d usec\n", i915->display.sagv.block_time_us);
 

drivers/gpu/drm/i915/i915_params.c

@@ -121,6 +121,9 @@ i915_param_named_unsafe(enable_psr2_sel_fetch, bool, 0400,
 	"(0=disabled, 1=enabled) "
 	"Default: 0");
 
+i915_param_named_unsafe(enable_sagv, bool, 0600,
+	"Enable system agent voltage/frequency scaling (SAGV) (default: true)");
+
 i915_param_named_unsafe(force_probe, charp, 0400,
 	"Force probe options for specified supported devices. "
 	"See CONFIG_DRM_I915_FORCE_PROBE for details.");

drivers/gpu/drm/i915/i915_params.h

@@ -57,6 +57,7 @@ struct drm_printer;
 	param(bool, enable_dpt, true, 0400) \
 	param(bool, psr_safest_params, false, 0400) \
 	param(bool, enable_psr2_sel_fetch, true, 0400) \
+	param(bool, enable_sagv, true, 0600) \
 	param(int, disable_power_well, -1, 0400) \
 	param(int, enable_ips, 1, 0600) \
 	param(int, invert_brightness, 0, 0600) \