linux-kernel - Re: [PATCH] drm/i915/display: Ensure that ret is always initialized in icl_combo_phy_verify

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Message-ID: <20200716053124.GE2588642@mdroper-desk1.amr.corp.intel.com>
Date:   Wed, 15 Jul 2020 22:31:24 -0700
From:   Matt Roper <matthew.d.roper@...el.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        José Roberto de Souza 
        <jose.souza@...el.com>, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] drm/i915/display: Ensure that ret is always initialized
 in icl_combo_phy_verify_state

On Wed, Jul 15, 2020 at 09:27:42PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> drivers/gpu/drm/i915/display/intel_combo_phy.c:268:3: warning: variable
> 'ret' is uninitialized when used here [-Wuninitialized]
>                 ret &= check_phy_reg(dev_priv, phy, ICL_PORT_TX_DW8_LN0(phy),
>                 ^~~
> drivers/gpu/drm/i915/display/intel_combo_phy.c:261:10: note: initialize
> the variable 'ret' to silence this warning
>         bool ret;
>                 ^
>                  = 0
> 1 warning generated.
> 
> In practice, the bug this warning appears to be concerned with would not
> actually matter because ret gets initialized to the return value of
> cnl_verify_procmon_ref_values. However, that does appear to be a bug
> since it means the first hunk of the patch this fixes won't actually do
> anything (since the values of check_phy_reg won't factor into the final
> ret value). Initialize ret to true then make all of the assignments a
> bitwise AND with itself so that the function always does what it should
> do.
> 
> Fixes: 239bef676d8e ("drm/i915/display: Implement new combo phy initialization step")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1094
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Reviewed-by: Matt Roper <matthew.d.roper@...el.com>

> ---
>  drivers/gpu/drm/i915/display/intel_combo_phy.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_combo_phy.c b/drivers/gpu/drm/i915/display/intel_combo_phy.c
> index eccaa79cb4a9..a4b8aa6d0a9e 100644
> --- a/drivers/gpu/drm/i915/display/intel_combo_phy.c
> +++ b/drivers/gpu/drm/i915/display/intel_combo_phy.c
> @@ -258,7 +258,7 @@ static bool phy_is_master(struct drm_i915_private *dev_priv, enum phy phy)
>  static bool icl_combo_phy_verify_state(struct drm_i915_private *dev_priv,
>  				       enum phy phy)
>  {
> -	bool ret;
> +	bool ret = true;
>  	u32 expected_val = 0;
>  
>  	if (!icl_combo_phy_enabled(dev_priv, phy))
> @@ -276,7 +276,7 @@ static bool icl_combo_phy_verify_state(struct drm_i915_private *dev_priv,
>  				     DCC_MODE_SELECT_CONTINUOSLY);
>  	}
>  
> -	ret = cnl_verify_procmon_ref_values(dev_priv, phy);
> +	ret &= cnl_verify_procmon_ref_values(dev_priv, phy);
>  
>  	if (phy_is_master(dev_priv, phy)) {
>  		ret &= check_phy_reg(dev_priv, phy, ICL_PORT_COMP_DW8(phy),
> 
> base-commit: ca0e494af5edb59002665bf12871e94b4163a257
> -- 
> 2.28.0.rc0
> 

-- 
Matt Roper
Graphics Software Engineer
VTT-OSGC Platform Enablement
Intel Corporation
(916) 356-2795