[PATCH] drm/i915/dsi: Fix command mode line time calculation

[Jinman Ma]

The command mode path in gen11_dsi_set_transcoder_timings() first
truncates the AFE clock period to an integer number of nanoseconds, and
then truncates the resulting line time to microseconds.

At high link rates, 1000000 / afe_clk() can evaluate to zero. Even at
lower rates, the successive integer divisions can make line_time_us
zero. DIV_ROUND_UP(400, line_time_us) then triggers a divide error
during the atomic commit and prevents i915 KMS from initializing.

Calculate the line time directly in nanoseconds using 64-bit arithmetic.
Use bits per pixel with the per-lane AFE bit clock and retain nanosecond
precision until calculating how many lines are needed for 400
microseconds.

Tested on a Huawei MateBook E with an Alder Lake-P GPU and a 2560x1600
dual-link DSI panel. The change prevents the divide error and allows KMS
to reach the display manager.

Signed-off-by: Jinman Ma <justdreemurr@xxxxxxxxxx>
---
This does not completely fix the panel. After entering Plasma, the image
still shows visible tearing or jitter, sometimes with an obvious offset
between the two halves of the dual-link panel. Switching to a text
console can also leave the panel black. I am continuing to investigate
this as a separate timing or synchronization issue.

Please let me know which additional i915 debug logs or register dumps
would be most useful for the remaining issue. I'm pleasured to collect them.
---
 drivers/gpu/drm/i915/display/icl_dsi.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/icl_dsi.c b/drivers/gpu/drm/i915/display/icl_dsi.c
index ea0cdb782..c89e24de7 100644
--- a/drivers/gpu/drm/i915/display/icl_dsi.c
+++ b/drivers/gpu/drm/i915/display/icl_dsi.c
@@ -917,16 +917,19 @@ gen11_dsi_set_transcoder_timings(struct intel_encoder *encoder,
 	if (is_vid_mode(intel_dsi)) {
 		vtotal = adjusted_mode->crtc_vtotal;
 	} else {
-		int bpp, line_time_us, byte_clk_period_ns;
+		u64 line_time_ns;
+		int bpp;
 
 		if (crtc_state->dsc.compression_enable)
 			bpp = fxp_q4_to_int(crtc_state->dsc.compressed_bpp_x16);
 		else
 			bpp = mipi_dsi_pixel_format_to_bpp(intel_dsi->pixel_format);
 
-		byte_clk_period_ns = 1000000 / afe_clk(encoder, crtc_state);
-		line_time_us = (htotal * (bpp / 8) * byte_clk_period_ns) / (1000 * intel_dsi->lane_count);
-		vtotal = vactive + DIV_ROUND_UP(400, line_time_us);
+		line_time_ns = DIV_ROUND_UP_ULL((u64)htotal * bpp * 1000000,
+						afe_clk(encoder, crtc_state) *
+						intel_dsi->lane_count);
+		vtotal = vactive +
+			DIV_ROUND_UP_ULL(400 * 1000, line_time_ns);
 	}
 	vsync_start = adjusted_mode->crtc_vsync_start;
 	vsync_end = adjusted_mode->crtc_vsync_end;
-- 
2.55.0