linux-kernel - Re: [PATCH] drm/amd/display: Pass app_tf by value rather than by reference

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Date:   Fri, 22 Feb 2019 05:48:45 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] drm/amd/display: Pass app_tf by value rather than by
 reference

On Mon, Dec 10, 2018 at 04:42:01PM -0700, Nathan Chancellor wrote:
> Clang warns when an expression that equals zero is used as a null
> pointer constant (in lieu of NULL):
> 
> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:4435:3:
> warning: expression which evaluates to zero treated as a null pointer
> constant of type 'const enum color_transfer_func *'
> [-Wnon-literal-null-conversion]
>                 TRANSFER_FUNC_UNKNOWN,
>                 ^~~~~~~~~~~~~~~~~~~~~
> 1 warning generated.
> 
> This warning is caused by commit bb47de736661 ("drm/amdgpu: Set FreeSync
> state using drm VRR properties") and it could be solved by using NULL
> instead of TRANSFER_FUNC_UNKNOWN or casting TRANSFER_FUNC_UNKNOWN as a
> pointer. However, after looking into it, there doesn't appear to be a
> good reason to pass app_tf by reference as it is never mutated along the
> way. This is the only code path in which app_tf is used:
> 
> mod_freesync_build_vrr_infopacket ->
>     build_vrr_infopacket_v2 ->
>         build_vrr_infopacket_fs2_data
> 
> Neither mod_freesync_build_vrr_infopacket or build_vrr_infopacket_v2
> modify app_tf's value and build_vrr_infopacket_fs2_data expects just
> the value so we can avoid dereferencing anything by just passing in
> app_tf's value to mod_freesync_build_vrr_infopacket and
> build_vrr_infopacket_v2.
> 
> There is no functional change because build_vrr_infopacket_fs2_data
> doesn't do anything if TRANSFER_FUNC_UNKNOWN is passed to it, the same
> as not calling build_vrr_infopacket_fs2_data at all like before this
> change when NULL was used for app_tf.
> 
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  drivers/gpu/drm/amd/display/modules/freesync/freesync.c | 7 +++----
>  drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h  | 2 +-
>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
> index 620a171620ee..520665a9d81a 100644
> --- a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
> +++ b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
> @@ -656,7 +656,7 @@ static void build_vrr_infopacket_v1(enum signal_type signal,
>  
>  static void build_vrr_infopacket_v2(enum signal_type signal,
>  		const struct mod_vrr_params *vrr,
> -		const enum color_transfer_func *app_tf,
> +		enum color_transfer_func app_tf,
>  		struct dc_info_packet *infopacket)
>  {
>  	unsigned int payload_size = 0;
> @@ -664,8 +664,7 @@ static void build_vrr_infopacket_v2(enum signal_type signal,
>  	build_vrr_infopacket_header_v2(signal, infopacket, &payload_size);
>  	build_vrr_infopacket_data(vrr, infopacket);
>  
> -	if (app_tf != NULL)
> -		build_vrr_infopacket_fs2_data(*app_tf, infopacket);
> +	build_vrr_infopacket_fs2_data(app_tf, infopacket);
>  
>  	build_vrr_infopacket_checksum(&payload_size, infopacket);
>  
> @@ -676,7 +675,7 @@ void mod_freesync_build_vrr_infopacket(struct mod_freesync *mod_freesync,
>  		const struct dc_stream_state *stream,
>  		const struct mod_vrr_params *vrr,
>  		enum vrr_packet_type packet_type,
> -		const enum color_transfer_func *app_tf,
> +		enum color_transfer_func app_tf,
>  		struct dc_info_packet *infopacket)
>  {
>  	/* SPD info packet for FreeSync */
> diff --git a/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h b/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
> index 949a8b62aa98..063af6258fd9 100644
> --- a/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
> +++ b/drivers/gpu/drm/amd/display/modules/inc/mod_freesync.h
> @@ -145,7 +145,7 @@ void mod_freesync_build_vrr_infopacket(struct mod_freesync *mod_freesync,
>  		const struct dc_stream_state *stream,
>  		const struct mod_vrr_params *vrr,
>  		enum vrr_packet_type packet_type,
> -		const enum color_transfer_func *app_tf,
> +		enum color_transfer_func app_tf,
>  		struct dc_info_packet *infopacket);
>  
>  void mod_freesync_build_vrr_params(struct mod_freesync *mod_freesync,
> -- 
> 2.20.0
> 

Gentle ping on this patch, it would be nice to get this fixed and into
5.1!

Thanks,
Nathan