drm/amd/display: work around fp code being emitted outside of DC_FP_START/END

The dcn20_validate_bandwidth function would have code touching the
incorrect registers emitted outside of the boundaries of the
DC_FP_START/END macros, at least on ppc64le. Work around the
problem by wrapping the whole function instead.
Signed-off-by: NDaniel Kolesa <daniel@octaforge.org>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.6.x

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c

@@ -3068,25 +3068,32 @@ static bool dcn20_validate_bandwidth_internal(struct dc *dc, struct dc_state *co
 	return out;
 }
 
-
-bool dcn20_validate_bandwidth(struct dc *dc, struct dc_state *context,
-		bool fast_validate)
+/*
+ * This must be noinline to ensure anything that deals with FP registers
+ * is contained within this call; previously our compiling with hard-float
+ * would result in fp instructions being emitted outside of the boundaries
+ * of the DC_FP_START/END macros, which makes sense as the compiler has no
+ * idea about what is wrapped and what is not
+ *
+ * This is largely just a workaround to avoid breakage introduced with 5.6,
+ * ideally all fp-using code should be moved into its own file, only that
+ * should be compiled with hard-float, and all code exported from there
+ * should be strictly wrapped with DC_FP_START/END
+ */
+static noinline bool dcn20_validate_bandwidth_fp(struct dc *dc,
+		struct dc_state *context, bool fast_validate)
 {
 	bool voltage_supported = false;
 	bool full_pstate_supported = false;
 	bool dummy_pstate_supported = false;
 	double p_state_latency_us;
 
-	DC_FP_START();
 	p_state_latency_us = context->bw_ctx.dml.soc.dram_clock_change_latency_us;
 	context->bw_ctx.dml.soc.disable_dram_clock_change_vactive_support =
 		dc->debug.disable_dram_clock_change_vactive_support;
 
 	if (fast_validate) {
-		voltage_supported = dcn20_validate_bandwidth_internal(dc, context, true);
-
-		DC_FP_END();
-		return voltage_supported;
+		return dcn20_validate_bandwidth_internal(dc, context, true);
 	}
 
 	// Best case, we support full UCLK switch latency
@@ -3115,7 +3122,15 @@ bool dcn20_validate_bandwidth(struct dc *dc, struct dc_state *context,
 
 restore_dml_state:
 	context->bw_ctx.dml.soc.dram_clock_change_latency_us = p_state_latency_us;
+	return voltage_supported;
+}
 
+bool dcn20_validate_bandwidth(struct dc *dc, struct dc_state *context,
+		bool fast_validate)
+{
+	bool voltage_supported = false;
+	DC_FP_START();
+	voltage_supported = dcn20_validate_bandwidth_fp(dc, context, fast_validate);
 	DC_FP_END();
 	return voltage_supported;
 }