drm/amd/display: Add DC_FP helper to check FPU state

To fully isolate FPU operations in a single place, we must avoid
situations where compilers spill FP values to registers due to FP enable
in a specific C file. Note that even if we isolate all FPU functions in
a single file and call its interface from other files, the compiler
might enable the use of FPU before we call DC_FP_START. Nevertheless, it
is the programmer's responsibility to invoke DC_FP_START/END in the
correct place. To highlight situations where developers forgot to use
the FP protection before calling the DC FPU interface functions, we
introduce a helper that checks if the function is invoked under FP
protection. If not, it will trigger a kernel warning.

Changes since V1:
- Remove fp_enable variables
- Rename dc_is_fp_enabled to dc_assert_fp_enabled
- Replace wrong variable type

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>

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

@@ -41,6 +41,28 @@
 
 static DEFINE_PER_CPU(int, fpu_recursion_depth);
 
+/**
+ * dc_assert_fp_enabled - Check if FPU protection is enabled
+ *
+ * This function tells if the code is already under FPU protection or not. A
+ * function that works as an API for a set of FPU operations can use this
+ * function for checking if the caller invoked it after DC_FP_START(). For
+ * example, take a look at dcn2x.c file.
+ *
+ * Return:
+ * Return true if we already enabled FPU protection, otherwise return false.
+ */
+inline bool dc_assert_fp_enabled(void)
+{
+	int *pcpu, depth = 0;
+
+	pcpu = get_cpu_ptr(&fpu_recursion_depth);
+	depth = this_cpu_read(fpu_recursion_depth);
+	put_cpu_ptr(&fpu_recursion_depth);
+
+	return depth > 1;
+}
+
 /**
  * dc_fpu_begin - Enables FPU protection
  * @function_name: A string containing the function name for debug purposes

drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.h

@@ -27,6 +27,7 @@
 #ifndef __DC_FPU_H__
 #define __DC_FPU_H__
 
+bool dc_assert_fp_enabled(void);
 void dc_fpu_begin(const char *function_name, const int line);
 void dc_fpu_end(const char *function_name, const int line);
 

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

@@ -2355,7 +2355,9 @@ int dcn20_populate_dml_pipes_from_context(
 	}
 
 	/* populate writeback information */
+	DC_FP_START();
 	dc->res_pool->funcs->populate_dml_writeback_from_context(dc, res_ctx, pipes);
+	DC_FP_END();
 
 	return pipe_cnt;
 }

drivers/gpu/drm/amd/display/dc/fpu_operations/dcn2x.c

@@ -41,6 +41,22 @@
  *    that deals with FP register is contained within this call.
  * 3. All function that needs to be accessed outside this file requires a
  *    public interface that not uses any FPU reference.
+ * 4. Developers should not use DC_FP_START/END in this file, but they need to
+ *    ensure that the caller invokes it before access any function available in
+ *    this file. For this reason, public API in this file must invoke
+ *    ASSERT(dc_assert_fp_enabled());
+ *
+ * Let's expand a little bit more the idea in the code pattern number for. To
+ * fully isolate FPU operations in a single place, we must avoid situations
+ * where compilers spill FP values to registers due to FP enable in a specific
+ * C file. Note that even if we isolate all FPU functions in a single file and
+ * call its interface from other files, the compiler might enable the use of
+ * FPU before we call DC_FP_START. Nevertheless, it is the programmer's
+ * responsibility to invoke DC_FP_START/END in the correct place. To highlight
+ * situations where developers forgot to use the FP protection before calling
+ * the DC FPU interface functions, we introduce a helper that checks if the
+ * function is invoked under FP protection. If not, it will trigger a kernel
+ * warning.
  */
 
 static noinline void _dcn20_populate_dml_writeback_from_context(struct dc *dc,
@@ -83,5 +99,6 @@ static noinline void _dcn20_populate_dml_writeback_from_context(struct dc *dc,
 void dcn20_populate_dml_writeback_from_context(struct dc *dc,
 	struct resource_context *res_ctx, display_e2e_pipe_params_st *pipes)
 {
+	ASSERT(dc_assert_fp_enabled());
 	_dcn20_populate_dml_writeback_from_context(dc, res_ctx, pipes);
 }