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 cince V3:
- Rebase

Changes cince V2 (Christian):
- Do not use this_cpu_* between get/put_cpu_ptr().
- In the kernel documentation, better describe restrictions.
- Make dc_assert_fp_enabled trigger the ASSERT message.

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

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Anson Jacob <Anson.Jacob@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Aric Cyr <aric.cyr@amd.com>
Cc: Jun Lei <jun.lei@amd.com>
Cc: Dmytro Laktyushkin <dmytro.laktyushkin@amd.com>
Cc: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/drivers/gpu/drm/amd/display/dc/dml/dcn2x/dcn2x.c b/drivers/gpu/drm/amd/display/dc/dml/dcn2x/dcn2x.c
index 8f0f6220327d..c58522436291 100644
--- a/drivers/gpu/drm/amd/display/dc/dml/dcn2x/dcn2x.c
+++ b/drivers/gpu/drm/amd/display/dc/dml/dcn2x/dcn2x.c
@@ -43,6 +43,22 @@
  *    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 **must 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 functions in this file must invoke
+ *    dc_assert_fp_enabled();
+ *
+ * Let's expand a little bit more the idea in the code pattern. 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.
  */
 
 void dcn20_populate_dml_writeback_from_context(struct dc *dc,
@@ -51,6 +67,8 @@ void dcn20_populate_dml_writeback_from_context(struct dc *dc,
 {
 	int pipe_cnt, i;
 
+	dc_assert_fp_enabled();
+
 	for (i = 0, pipe_cnt = 0; i < dc->res_pool->pipe_count; i++) {
 		struct dc_writeback_info *wb_info = &res_ctx->pipe_ctx[i].stream->writeback_info[0];