linux-kernel - Re: [PATCH linux-next] drm/i915/display: Remove unused variable and its assignment.

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Date:   Mon, 18 Oct 2021 11:57:06 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     luo penghao <cgel.zte@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, luo penghao <luo.penghao@....com.cn>,
        Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH linux-next] drm/i915/display: Remove unused variable and
 its assignment.

On Mon, Oct 18, 2021 at 08:43:31AM +0000, luo penghao wrote:
> Variable is not used in functions, and its assignment is redundant too.
> So it should be deleted.
> 
> The clang_analyzer complains as follows:
> 
> drivers/gpu/drm/i915/display/intel_dpll.c:1653:2 warning:
> Value stored to 'bestm1' is never read.
> 
> drivers/gpu/drm/i915/display/intel_dpll.c:1651:2 warning:
> Value stored to 'bestn' is never read.
> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: luo penghao <luo.penghao@....com.cn>
> ---
>  drivers/gpu/drm/i915/display/intel_dpll.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dpll.c b/drivers/gpu/drm/i915/display/intel_dpll.c
> index b84ed4a..28b1616 100644
> --- a/drivers/gpu/drm/i915/display/intel_dpll.c
> +++ b/drivers/gpu/drm/i915/display/intel_dpll.c
> @@ -1644,13 +1644,11 @@ static void chv_prepare_pll(const struct intel_crtc_state *crtc_state)
>  	enum pipe pipe = crtc->pipe;
>  	enum dpio_channel port = vlv_pipe_to_channel(pipe);
>  	u32 loopfilter, tribuf_calcntr;
> -	u32 bestn, bestm1, bestm2, bestp1, bestp2, bestm2_frac;
> +	u32 bestm2, bestp1, bestp2, bestm2_frac;
>  	u32 dpio_val;
>  	int vco;
>  
> -	bestn = crtc_state->dpll.n;

This one we could use. For some reason we hardcode it to
1 now, which is correct for our use cases but I don't really
see a reason to hardcode it here. We are supposed to calculate
it correctly after all, and chv_crtc_clock_get() also just blindly
reads it out.

>  	bestm2_frac = crtc_state->dpll.m2 & 0x3fffff;
> -	bestm1 = crtc_state->dpll.m1;

This one is a bit trickier since I don't think the spec even
gives us other values. But we could assert that it's correct.

Some something along these lines I think would be best:
+ drm_WARN_ON(&dev_priv->drm, bestm1 != 2);
  vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW1(port),
                 DPIO_CHV_M1_DIV_BY_2 |
- 		 1 << DPIO_CHV_N_DIV_SHIFT);
+ 		 bestn << DPIO_CHV_N_DIV_SHIFT);

-- 
Ville Syrjälä
Intel