linux-kernel - Re: [PATCH] drm/amd/display: avoid passing enum as NULL pointer

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Date:   Thu, 7 Mar 2019 08:34:20 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Anthony Koo <Anthony.Koo@....com>, Aric Cyr <Aric.Cyr@....com>,
        Harmanprit Tatla <Harmanprit.Tatla@....com>,
        Ken Chalmers <ken.chalmers@....com>,
        SivapiriyanKumarasamy <sivapiriyan.kumarasamy@....com>,
        Kees Cook <keescook@...omium.org>,
        Bayan Zabihiyan <Bayan.Zabihiyan@....com>,
        Tony Cheng <Tony.Cheng@....com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amd/display: avoid passing enum as NULL pointer

On Thu, Mar 07, 2019 at 11:34:29AM +0100, Arnd Bergmann wrote:
> The mod_freesync_build_vrr_infopacket() function uses rather obscure
> calling conventions, where an enum is passed in through a pointer,
> and a NULL pointer is expected to behave the same way as the zero-value
> (TRANSFER_FUNC_UNKNOWN).
> 
> Trying to build this with clang results in a warning:
> 
> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:4601:3: error: expression which evaluates to zero treated
>       as a null pointer constant of type 'const enum color_transfer_func *' [-Werror,-Wnon-literal-null-conversion]
> 
> Passing it by value instead of by reference makes the code simpler
> and more conventional but should not change the behavior at all.
> 
> Fixes: c2791297013e ("drm/amd/display: Add color bit info to freesync infoframe")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  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 94a84bc57c7a..6f32fe129880 100644
> --- a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
> +++ b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
> @@ -724,7 +724,7 @@ 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,
> +		const enum color_transfer_func app_tf,
>  		struct dc_info_packet *infopacket)
>  {
>  	unsigned int payload_size = 0;
> @@ -732,8 +732,7 @@ 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);
>  
> @@ -757,7 +756,7 @@ 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,
> +		const 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 4222e403b151..645793b924cf 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,
> +		const 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
> 

Just as an FYI, I sent the same fix when the warning first hit which was
recently accepted:

https://cgit.freedesktop.org/~agd5f/linux/commit/?id=672e78cab819ebe31e3b9b8abac367be8a110472

Just waiting for it to hit mainline.

Cheers,
Nathan