linux-kernel - Re: [PATCH v3] drm/amd/display: Fix exception handling in dm_validate_stream_and

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Message-ID: <ba89d859-f132-4e7d-ba29-47331e02152a@amd.com>
Date: Mon, 16 Jun 2025 15:22:06 -0600
From: Alex Hung <alex.hung@....com>
To: Markus Elfring <Markus.Elfring@....de>, amd-gfx@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org, Alex Deucher <alexander.deucher@....com>,
 Aurabindo Pillai <aurabindo.pillai@....com>,
 Christian König <christian.koenig@....com>,
 Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>,
 Dominik Kaszewski <dominik.kaszewski@....com>,
 Fangzhi Zuo <Jerry.Zuo@....com>, Harry Wentland <harry.wentland@....com>,
 Leo Li <sunpeng.li@....com>, Mario Limonciello <mario.limonciello@....com>,
 Roman Li <roman.li@....com>, Simona Vetter <simona@...ll.ch>,
 Tom Chung <chiahsuan.chung@....com>, Wayne Lin <Wayne.Lin@....com>
Cc: LKML <linux-kernel@...r.kernel.org>, kernel-janitors@...r.kernel.org,
 lkp@...el.com, oe-kbuild-all@...ts.linux.dev, llvm@...ts.linux.dev,
 cocci@...ia.fr, Melissa Wen <mwen@...lia.com>,
 Fangzhi Zuo <jerry.zuo@....com>
Subject: Re: [PATCH v3] drm/amd/display: Fix exception handling in
 dm_validate_stream_and_context()



On 6/10/25 00:10, Markus Elfring wrote:
> From: Markus Elfring <elfring@...rs.sourceforge.net>
> Date: Tue, 10 Jun 2025 07:42:40 +0200
> 
> The label “cleanup” was used to jump to another pointer check despite of
> the detail in the implementation of the function “dm_validate_stream_and_context”
> that it was determined already that corresponding variables contained
> still null pointers.
> 
> 1. Thus return directly if
>     * a null pointer was passed for the function parameter “stream”
>       or
>     * a call of the function “dc_create_plane_state” failed.
> 
> 2. Use a more appropriate label instead.
> 
> 3. Delete two questionable checks.
> 
> 4. Omit extra initialisations (for the variables “dc_state” and “dc_plane_state”)
>     which became unnecessary with this refactoring.
> 
> 
> This issue was detected by using the Coccinelle software.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202506100312.Ms4XgAzW-lkp@intel.com/
> Fixes: 5468c36d6285 ("drm/amd/display: Filter Invalid 420 Modes for HDMI TMDS")
> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> ---
> 
> V3:
> * Another function call was renamed.
> 
> * Recipient lists were adjusted once more.
> 
> V2:
> * The change suggestion was rebased on source files of
>    the software “Linux next-20250606”.
> 
> * Recipient lists were adjusted accordingly.
> 
> 
>   .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 20 ++++++++-----------
>   1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index 78816712afbb..7dc80b2fbd30 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -7473,19 +7473,19 @@ static enum dc_status dm_validate_stream_and_context(struct dc *dc,
>   						struct dc_stream_state *stream)
>   {
>   	enum dc_status dc_result = DC_ERROR_UNEXPECTED;
> -	struct dc_plane_state *dc_plane_state = NULL;
> -	struct dc_state *dc_state = NULL;
> +	struct dc_plane_state *dc_plane_state;
> +	struct dc_state *dc_state;
>   
>   	if (!stream)
> -		goto cleanup;
> +		return dc_result;
>   
>   	dc_plane_state = dc_create_plane_state(dc);
>   	if (!dc_plane_state)
> -		goto cleanup;
> +		return dc_result;

I think the two early returns look fine, but the rest of the changes 
reduces the readability and reusability.

>   
>   	dc_state = dc_state_create(dc, NULL);
>   	if (!dc_state)
> -		goto cleanup;
> +		goto release_plane_state;
>   
>   	/* populate stream to plane */
>   	dc_plane_state->src_rect.height  = stream->src.height;
> @@ -7522,13 +7522,9 @@ static enum dc_status dm_validate_stream_and_context(struct dc *dc,
>   	if (dc_result == DC_OK)
>   		dc_result = dc_validate_global_state(dc, dc_state, DC_VALIDATE_MODE_ONLY);
>   
> -cleanup:
> -	if (dc_state)
> -		dc_state_release(dc_state);
> -
> -	if (dc_plane_state)
> -		dc_plane_state_release(dc_plane_state);
> -
> +	dc_state_release(dc_state);
> +release_plane_state:
> +	dc_plane_state_release(dc_plane_state);

This clean was intended to be reused for now and for future changes, and 
the changes here remove the reusability. Also "cleanup" is commonly used 
already.

>   	return dc_result;
>   }
>   

I guess the intention was to reduce goto statements. If that's the case, 
it would be better to eliminate all goto and then to remove cleanup + 
two checks.

On the other hand, I don't see anything wrong with goto/cleanup approach 
either. Multiple exits in a function do not hurt if managed correctly.