drm/amd/display: disable SubVP + DRR to prevent underflow

[Why&How]
Temporarily disable SubVP+DRR since Xorg has an architectural limitation
where freesync will not work in a multi monitor configuration. SubVP+DRR
requires that freesync be working.

Whether OS has variable refresh setting enabled or not, the state on
the crtc remains same unless an application requests VRR. Due to this,
there is no way to know whether freesync will actually work or not
while we are on the desktop from the kernel's perspective.

If userspace does not have a limitation with multi-display freesync (for
example wayland), then this feature can be enabled by adding a
dcfeaturemask option to amdgpu on the kernel cmdline like:

amdgpu.dcfeaturemask=0x200

Signed-off-by: Aurabindo Pillai <[e-mail]>
Reviewed-by: Rodrigo Siqueira <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

···
 	if (amdgpu_dc_feature_mask & DC_DISABLE_LTTPR_DP2_0)
 		init_data.flags.allow_lttpr_non_transparent_mode.bits.DP2_0 = true;
 
+	/* Disable SubVP + DRR config by default */
+	init_data.flags.disable_subvp_drr = true;
+	if (amdgpu_dc_feature_mask & DC_ENABLE_SUBVP_DRR)
+		init_data.flags.disable_subvp_drr = false;
+
 	init_data.flags.seamless_boot_edp_requested = false;
 
 	if (check_seamless_boot_capability(adev)) {

drivers/gpu/drm/amd/display/dc/dc.h

···
 	bool force_bios_enable_lttpr;
 	uint8_t force_bios_fixed_vs;
 	int sdpif_request_limit_words_per_umc;
-
+	bool disable_subvp_drr;
 };
 
 enum visual_confirm {

drivers/gpu/drm/amd/display/dc/dml/dcn32/dcn32_fpu.c

···
 	int16_t stretched_drr_us = 0;
 	int16_t drr_stretched_vblank_us = 0;
 	int16_t max_vblank_mallregion = 0;
+	const struct dc_config *config = &dc->config;
+
+	if (config->disable_subvp_drr)
+		return false;
 
 	// Find SubVP pipe
 	for (i = 0; i < dc->res_pool->pipe_count; i++) {

drivers/gpu/drm/amd/include/amd_shared.h

···
 	DC_DISABLE_LTTPR_DP2_0 = (1 << 6), //0x40, disabled by default
 	DC_PSR_ALLOW_SMU_OPT = (1 << 7), //0x80, disabled by default
 	DC_PSR_ALLOW_MULTI_DISP_OPT = (1 << 8), //0x100, disabled by default
+	DC_ENABLE_SUBVP_DRR = (1 << 9), // 0x200, disabled by default
 };
 
 enum DC_DEBUG_MASK {