drm/amd/display: Prevent Gating DTBCLK before It Is Properly Latched

[ Upstream commit cfa0904 ]

[why]
1. With allow_0_dtb_clk enabled, the time required to latch DTBCLK to 600 MHz
depends on the SMU. If DTBCLK is not latched to 600 MHz before set_mode completes,
gating DTBCLK causes the DP2 sink to lose its clock source.

2. The existing DTBCLK gating sequence ungates DTBCLK based on both pix_clk and ref_dtbclk,
but gates DTBCLK when either pix_clk or ref_dtbclk is zero.
pix_clk can be zero outside the set_mode sequence before DTBCLK is properly latched,
which can lead to DTBCLK being gated by mistake.

[how]
Consider both pixel_clk and ref_dtbclk when determining when it is safe to gate DTBCLK;
this is more accurate.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4701
Fixes: 5949e7c ("drm/amd/display: Enable Dynamic DTBCLK Switch")
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit d04eb0c)
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Fangzhi Zuo

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn35/dcn35_clk_mgr.c

@@ -377,6 +377,8 @@ void dcn35_update_clocks(struct clk_mgr *clk_mgr_base,
 	display_count = dcn35_get_active_display_cnt_wa(dc, context, &all_active_disps);
 	if (new_clocks->dtbclk_en && !new_clocks->ref_dtbclk_khz)
 		new_clocks->ref_dtbclk_khz = 600000;
+	else if (!new_clocks->dtbclk_en && new_clocks->ref_dtbclk_khz > 590000)
+		new_clocks->ref_dtbclk_khz = 0;
 
 	/*
 	 * if it is safe to lower, but we are already in the lower state, we don't have to do anything
@@ -418,7 +420,7 @@ void dcn35_update_clocks(struct clk_mgr *clk_mgr_base,
 
 			actual_dtbclk = REG_READ(CLK1_CLK4_CURRENT_CNT);
 
-			if (actual_dtbclk) {
+			if (actual_dtbclk > 590000) {
 				clk_mgr_base->clks.ref_dtbclk_khz = new_clocks->ref_dtbclk_khz;
 				clk_mgr_base->clks.dtbclk_en = new_clocks->dtbclk_en;
 			}

drivers/gpu/drm/amd/display/dc/dccg/dcn35/dcn35_dccg.c

@@ -1405,7 +1405,7 @@ static void dccg35_set_dtbclk_dto(
 				__func__, params->otg_inst, params->pixclk_khz,
 				params->ref_dtbclk_khz, req_dtbclk_khz, phase, modulo);
 
-	} else {
+	} else if (!params->ref_dtbclk_khz && !req_dtbclk_khz) {
 		switch (params->otg_inst) {
 		case 0:
 			REG_UPDATE(DCCG_GATE_DISABLE_CNTL5, DTBCLK_P0_GATE_DISABLE, 0);