drm/i915: program wm blocks to at least blocks required per line

In configurations with single DRAM channel, for usecases like
4K 60 Hz, FIFO underruns are observed quite frequently. Looks
like the wm0 watermark values need to bumped up because the wm0
memory latency calculations are probably not taking the DRAM
channel's impact into account.

As per the Bspec 49325, if the ddb allocation can hold at least
one plane_blocks_per_line we should have selected method2.
Assuming that modern HW versions have enough dbuf to hold
at least one line, set the wm blocks to equivalent to blocks
per line.

v2: styling and comments changes (Ville)
v3: Updated the reviewed-by tag
v4: max_t to max and patch styling (Ville)

References: https://gitlab.freedesktop.org/drm/intel/-/issues/4321
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220417093105.729014-1-vinod.govindapillai@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -5475,6 +5475,25 @@ static void skl_compute_plane_wm(const struct intel_crtc_state *crtc_state,
 	}
 
 	blocks = fixed16_to_u32_round_up(selected_result) + 1;
+	/*
+	 * Lets have blocks at minimum equivalent to plane_blocks_per_line
+	 * as there will be at minimum one line for lines configuration. This
+	 * is a work around for FIFO underruns observed with resolutions like
+	 * 4k 60 Hz in single channel DRAM configurations.
+	 *
+	 * As per the Bspec 49325, if the ddb allocation can hold at least
+	 * one plane_blocks_per_line, we should have selected method2 in
+	 * the above logic. Assuming that modern versions have enough dbuf
+	 * and method2 guarantees blocks equivalent to at least 1 line,
+	 * select the blocks as plane_blocks_per_line.
+	 *
+	 * TODO: Revisit the logic when we have better understanding on DRAM
+	 * channels' impact on the level 0 memory latency and the relevant
+	 * wm calculations.
+	 */
+	if (skl_wm_has_lines(dev_priv, level))
+		blocks = max(blocks,
+			     fixed16_to_u32_round_up(wp->plane_blocks_per_line));
 	lines = div_round_up_fixed16(selected_result,
 				     wp->plane_blocks_per_line);