linux-kernel - Re: [PATCH] drm/i915: rework the error handling in *_dpll

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Date:   Fri, 4 Mar 2022 23:11:54 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     trix@...hat.com
Cc:     jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, nathan@...nel.org,
        ndesaulniers@...gle.com, matthew.d.roper@...el.com,
        lucas.demarchi@...el.com, airlied@...hat.com, imre.deak@...el.com,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/i915: rework the error handling in *_dpll_params

On Fri, Mar 04, 2022 at 01:03:55PM -0800, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> Clang static analysis reports this issue
> intel_dpll.c:472:31: warning: The left operand of '-'
>   is a garbage value [core.UndefinedBinaryOperatorResult]
>   this_err = abs(clock.dot - target);
>                  ~~~~~~~~~ ^
> 
> In a loop clock.dot is set on successful call to
> i9xx_calc_dpll_params().  If the call fails, the later
> *is_valid() will use the previous loop's clock.dot.

I don't think this can happen. intel_pll_is_valid() validates
all the dividers first and bails out if they are junk.

> 
> The *_dpll_params functions return an arithmetic statement
> with the clock.dot as the variable.  Change the error handler
> to set clock.dot to 0 and jump to the return statement.
> 
> Fixes: dccbea3b0704 ("drm/i915: calculate the port clock rate along with other PLL params")
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dpll.c | 32 ++++++++++++++---------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dpll.c b/drivers/gpu/drm/i915/display/intel_dpll.c
> index 0ae37fdbf2a5b..ba7cada704288 100644
> --- a/drivers/gpu/drm/i915/display/intel_dpll.c
> +++ b/drivers/gpu/drm/i915/display/intel_dpll.c
> @@ -309,11 +309,13 @@ int pnv_calc_dpll_params(int refclk, struct dpll *clock)
>  {
>  	clock->m = clock->m2 + 2;
>  	clock->p = clock->p1 * clock->p2;
> -	if (WARN_ON(clock->n == 0 || clock->p == 0))
> -		return 0;
> +	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
> +		clock->dot = 0;
> +		goto end;
> +	}
>  	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n);
>  	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
> -
> +end:
>  	return clock->dot;
>  }
>  
> @@ -326,11 +328,13 @@ int i9xx_calc_dpll_params(int refclk, struct dpll *clock)
>  {
>  	clock->m = i9xx_dpll_compute_m(clock);
>  	clock->p = clock->p1 * clock->p2;
> -	if (WARN_ON(clock->n + 2 == 0 || clock->p == 0))
> -		return 0;
> +	if (WARN_ON(clock->n + 2 == 0 || clock->p == 0)) {
> +		clock->dot = 0;
> +		goto end;
> +	}
>  	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n + 2);
>  	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
> -
> +end:
>  	return clock->dot;
>  }
>  
> @@ -338,11 +342,13 @@ int vlv_calc_dpll_params(int refclk, struct dpll *clock)
>  {
>  	clock->m = clock->m1 * clock->m2;
>  	clock->p = clock->p1 * clock->p2;
> -	if (WARN_ON(clock->n == 0 || clock->p == 0))
> -		return 0;
> +	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
> +		clock->dot = 0;
> +		goto end;
> +	}
>  	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n);
>  	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
> -
> +end:
>  	return clock->dot / 5;
>  }
>  
> @@ -350,12 +356,14 @@ int chv_calc_dpll_params(int refclk, struct dpll *clock)
>  {
>  	clock->m = clock->m1 * clock->m2;
>  	clock->p = clock->p1 * clock->p2;
> -	if (WARN_ON(clock->n == 0 || clock->p == 0))
> -		return 0;
> +	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
> +		clock->dot = 0;
> +		goto end;
> +	}
>  	clock->vco = DIV_ROUND_CLOSEST_ULL(mul_u32_u32(refclk, clock->m),
>  					   clock->n << 22);
>  	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
> -
> +end:
>  	return clock->dot / 5;
>  }
>  
> -- 
> 2.26.3

-- 
Ville Syrjälä
Intel