Re: [PATCH v4 4/4] drm/amd/display: Add DC_FP helper to check FPU state

["Christian König" <christian.koenig@amd.com>]

Am 27.07.21 um 02:52 schrieb Rodrigo Siqueira:
> 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>

> ---
>   .../gpu/drm/amd/display/amdgpu_dm/dc_fpu.c    | 19 +++++++++++++++++++
>   .../gpu/drm/amd/display/amdgpu_dm/dc_fpu.h    |  1 +
>   .../drm/amd/display/dc/dcn20/dcn20_resource.c |  2 ++
>   .../gpu/drm/amd/display/dc/dml/dcn2x/dcn2x.c  | 18 ++++++++++++++++++
>   4 files changed, 40 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> index 33807d746e76..c9f47d167472 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> @@ -46,6 +46,25 @@
>   
>   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.
> + */
> +inline void dc_assert_fp_enabled(void)
> +{
> +	int *pcpu, depth = 0;
> +
> +	pcpu = get_cpu_ptr(&fpu_recursion_depth);
> +	depth = *pcpu;
> +	put_cpu_ptr(&fpu_recursion_depth);
> +
> +	ASSERT(depth > 1);
> +}
> +
>   /**
>    * dc_fpu_begin - Enables FPU protection
>    * @function_name: A string containing the function name for debug purposes
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.h b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.h
> index fb54983c5c60..b8275b397920 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.h
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.h
> @@ -27,6 +27,7 @@
>   #ifndef __DC_FPU_H__
>   #define __DC_FPU_H__
>   
> +void 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);
>   
> diff --git a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
> index 988d7c02199c..e3e01b17c164 100644
> --- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
> +++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
> @@ -2357,7 +2357,9 @@ 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;
>   }
> 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];
>