linux-kernel - Re: [PATCH] drm/amd/display: Fix an uninitialized index variable

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Date:   Thu, 25 Feb 2021 13:33:56 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Arnd Bergmann <arnd@...db.de>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Nathan Chancellor <nathan@...nel.org>,
        Nicholas Kazlauskas <nicholas.kazlauskas@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Aurabindo Pillai <aurabindo.pillai@....com>,
        Stylon Wang <stylon.wang@....com>,
        Eryk Brol <eryk.brol@....com>,
        Bas Nieuwenhuizen <bas@...nieuwenhuizen.nl>,
        Bhawanpreet Lakha <Bhawanpreet.Lakha@....com>,
        Simon Ser <contact@...rsion.fr>,
        Qingqing Zhuo <qingqing.zhuo@....com>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] drm/amd/display: Fix an uninitialized index variable

On Thu, Feb 25, 2021 at 7:01 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> clang points out that the new logic uses an always-uninitialized
> array index:
>
> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9810:38: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
>                         timing  = &edid->detailed_timings[i];
>                                                           ^
> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9720:7: note: initialize the variable 'i' to silence this warning
>
> My best guess is that the index should have been returned by the
> parse_hdmi_amd_vsdb() function that walks an array here, so do that.
>
> Fixes: f9b4f20c4777 ("drm/amd/display: Add Freesync HDMI support to DM")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c    | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 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 b19b93c74bae..667c0d52dbfa 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -9736,7 +9736,7 @@ static bool parse_edid_cea(struct amdgpu_dm_connector *aconnector,
>         return false;
>  }
>
> -static bool parse_hdmi_amd_vsdb(struct amdgpu_dm_connector *aconnector,
> +static int parse_hdmi_amd_vsdb(struct amdgpu_dm_connector *aconnector,
>                 struct edid *edid, struct amdgpu_hdmi_vsdb_info *vsdb_info)
>  {
>         uint8_t *edid_ext = NULL;
> @@ -9746,7 +9746,7 @@ static bool parse_hdmi_amd_vsdb(struct amdgpu_dm_connector *aconnector,
>         /*----- drm_find_cea_extension() -----*/
>         /* No EDID or EDID extensions */
>         if (edid == NULL || edid->extensions == 0)
> -               return false;
> +               return -ENODEV;
>
>         /* Find CEA extension */
>         for (i = 0; i < edid->extensions; i++) {
> @@ -9756,14 +9756,15 @@ static bool parse_hdmi_amd_vsdb(struct amdgpu_dm_connector *aconnector,
>         }
>
>         if (i == edid->extensions)
> -               return false;
> +               return -ENODEV;
>
>         /*----- cea_db_offsets() -----*/
>         if (edid_ext[0] != CEA_EXT)
> -               return false;
> +               return -ENODEV;
>
>         valid_vsdb_found = parse_edid_cea(aconnector, edid_ext, EDID_LENGTH, vsdb_info);
> -       return valid_vsdb_found;
> +
> +       return valid_vsdb_found ? i : -ENODEV;

Thanks for the patch!

I don't think we need a local variable to store the return value from
one function call that's immediately returned, ie.

return parse_edid_cea(aconnector, edid_ext, EDID_LENGTH, vsdb_info) ?
i : -ENODEV;

would suffice, but the patch is still fine as is.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>  }
>
>  void amdgpu_dm_update_freesync_caps(struct drm_connector *connector,
> @@ -9781,7 +9782,6 @@ void amdgpu_dm_update_freesync_caps(struct drm_connector *connector,
>         struct amdgpu_device *adev = drm_to_adev(dev);
>         bool freesync_capable = false;
>         struct amdgpu_hdmi_vsdb_info vsdb_info = {0};
> -       bool hdmi_valid_vsdb_found = false;
>
>         if (!connector->state) {
>                 DRM_ERROR("%s - Connector has no state", __func__);
> @@ -9857,8 +9857,8 @@ void amdgpu_dm_update_freesync_caps(struct drm_connector *connector,
>                         }
>                 }
>         } else if (edid && amdgpu_dm_connector->dc_sink->sink_signal == SIGNAL_TYPE_HDMI_TYPE_A) {
> -               hdmi_valid_vsdb_found = parse_hdmi_amd_vsdb(amdgpu_dm_connector, edid, &vsdb_info);
> -               if (hdmi_valid_vsdb_found && vsdb_info.freesync_supported) {
> +               i = parse_hdmi_amd_vsdb(amdgpu_dm_connector, edid, &vsdb_info);
> +               if (i >= 0 && vsdb_info.freesync_supported) {

reusing `i` here is safe, for now, but reuse of variables like this in
separate branches like this might not get noticed if the function is
amended in the future.

>                         timing  = &edid->detailed_timings[i];
>                         data    = &timing->data.other_data;
>
> --
> 2.29.2
>


-- 
Thanks,
~Nick Desaulniers