linux-kernel - Re: [PATCH] drm/amd/display: Fix logical vs bitwise bug in get_embedded_panel_info_v2

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Message-ID: <5da812b5-7e16-4343-b060-9960180949fe@amd.com>
Date: Fri, 21 Nov 2025 12:36:16 -0700
From: Alex Hung <alex.hung@....com>
To: Dan Carpenter <dan.carpenter@...aro.org>,
 Harry Wentland <harry.wentland@....com>,
 Harshit Mogalapalli <harshit.m.mogalapalli@...il.com>
Cc: Leo Li <sunpeng.li@....com>, Rodrigo Siqueira <siqueira@...lia.com>,
 Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Hamish Claxton <hamishclaxton@...il.com>,
 Aurabindo Pillai <aurabindo.pillai@....com>,
 Zaeem Mohamed <zaeem.mohamed@....com>,
 Michael Strauss <michael.strauss@....com>, amd-gfx@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drm/amd/display: Fix logical vs bitwise bug in
 get_embedded_panel_info_v2_1()

This patch passes this week's promotion test.

Reviewed-by: Alex Hung <alex.hung@....com>

On 10/31/25 07:02, Dan Carpenter wrote:
> The .H_SYNC_POLARITY and .V_SYNC_POLARITY variables are 1 bit bitfields
> of a u32.  The ATOM_HSYNC_POLARITY define is 0x2 and the
> ATOM_VSYNC_POLARITY is 0x4.  When we do a bitwise negate of 0, 2, or 4
> then the last bit is always 1 so this code always sets .H_SYNC_POLARITY
> and .V_SYNC_POLARITY to true.
> 
> This code is instead intended to check if the ATOM_HSYNC_POLARITY or
> ATOM_VSYNC_POLARITY flags are set and reverse the result.  In other
> words, it's supposed to be a logical negate instead of a bitwise negate.
> 
> Fixes: ae79c310b1a6 ("drm/amd/display: Add DCE12 bios parser support")
> Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
> ---
> Please note that I have not tested this.  It's straight forward enough to
> see that logical negate was intended, but it's always good to test things
> as well.
> 
> Harshit and I only recently created this static checker warning.
> 
>   drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c b/drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c
> index 04eb647acc4e..550a9f1d03f8 100644
> --- a/drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c
> +++ b/drivers/gpu/drm/amd/display/dc/bios/bios_parser2.c
> @@ -1480,10 +1480,10 @@ static enum bp_result get_embedded_panel_info_v2_1(
>   	/* not provided by VBIOS */
>   	info->lcd_timing.misc_info.HORIZONTAL_CUT_OFF = 0;
>   
> -	info->lcd_timing.misc_info.H_SYNC_POLARITY = ~(uint32_t) (lvds->lcd_timing.miscinfo
> -			& ATOM_HSYNC_POLARITY);
> -	info->lcd_timing.misc_info.V_SYNC_POLARITY = ~(uint32_t) (lvds->lcd_timing.miscinfo
> -			& ATOM_VSYNC_POLARITY);
> +	info->lcd_timing.misc_info.H_SYNC_POLARITY = !(lvds->lcd_timing.miscinfo &
> +						       ATOM_HSYNC_POLARITY);
> +	info->lcd_timing.misc_info.V_SYNC_POLARITY = !(lvds->lcd_timing.miscinfo &
> +						       ATOM_VSYNC_POLARITY);
>   
>   	/* not provided by VBIOS */
>   	info->lcd_timing.misc_info.VERTICAL_CUT_OFF = 0;