Re: [PATCH] drm/modes: remove unused variables

[Benjamin GAIGNARD]

+ Zhao Yakui

On 12/5/19 10:55 AM, Jani Nikula wrote:
> On Wed, 04 Dec 2019, Benjamin GAIGNARD <benjamin.gaignard@xxxxxx> wrote:
>> On 12/4/19 10:35 AM, Thomas Zimmermann wrote:
>>> Hi
>>>
>>> Am 19.11.19 um 14:47 schrieb Benjamin Gaignard:
>>>> When compiling with W=1 few warnings about unused variables show up.
>>>> This patch removes all the involved variables.
>>>>
>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxx>
>>>> ---
>>>>    drivers/gpu/drm/drm_modes.c | 22 +++-------------------
>>>>    1 file changed, 3 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
>>>> index 88232698d7a0..aca901aff042 100644
>>>> --- a/drivers/gpu/drm/drm_modes.c
>>>> +++ b/drivers/gpu/drm/drm_modes.c
>>>> @@ -233,7 +233,7 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
>>>>    		/* 3) Nominal HSync width (% of line period) - default 8 */
>>>>    #define CVT_HSYNC_PERCENTAGE	8
>>>>    		unsigned int hblank_percentage;
>>>> -		int vsyncandback_porch, vback_porch, hblank;
>>>> +		int vsyncandback_porch, hblank;
>>>>    
>>>>    		/* estimated the horizontal period */
>>>>    		tmp1 = HV_FACTOR * 1000000  -
>>>> @@ -249,7 +249,6 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
>>>>    		else
>>>>    			vsyncandback_porch = tmp1;
>>>>    		/* 10. Find number of lines in back porch */
>>>> -		vback_porch = vsyncandback_porch - vsync;
>>>>    		drm_mode->vtotal = vdisplay_rnd + 2 * vmargin +
>>>>    				vsyncandback_porch + CVT_MIN_V_PORCH;
>>>>    		/* 5) Definition of Horizontal blanking time limitation */
>>>> @@ -386,9 +385,8 @@ drm_gtf_mode_complex(struct drm_device *dev, int hdisplay, int vdisplay,
>>>>    	int top_margin, bottom_margin;
>>>>    	int interlace;
>>>>    	unsigned int hfreq_est;
>>>> -	int vsync_plus_bp, vback_porch;
>>>> -	unsigned int vtotal_lines, vfieldrate_est, hperiod;
>>>> -	unsigned int vfield_rate, vframe_rate;
>>>> +	int vsync_plus_bp;
>>>> +	unsigned int vtotal_lines;
>>>>    	int left_margin, right_margin;
>>>>    	unsigned int total_active_pixels, ideal_duty_cycle;
>>>>    	unsigned int hblank, total_pixels, pixel_freq;
>>>> @@ -451,23 +449,9 @@ drm_gtf_mode_complex(struct drm_device *dev, int hdisplay, int vdisplay,
>>>>    	/* [V SYNC+BP] = RINT(([MIN VSYNC+BP] * hfreq_est / 1000000)) */
>>>>    	vsync_plus_bp = MIN_VSYNC_PLUS_BP * hfreq_est / 1000;
>>>>    	vsync_plus_bp = (vsync_plus_bp + 500) / 1000;
>>>> -	/*  9. Find the number of lines in V back porch alone: */
>>>> -	vback_porch = vsync_plus_bp - V_SYNC_RQD;
>>>>    	/*  10. Find the total number of lines in Vertical field period: */
>>>>    	vtotal_lines = vdisplay_rnd + top_margin + bottom_margin +
>>>>    			vsync_plus_bp + GTF_MIN_V_PORCH;
>>>> -	/*  11. Estimate the Vertical field frequency: */
>>>> -	vfieldrate_est = hfreq_est / vtotal_lines;
>>>> -	/*  12. Find the actual horizontal period: */
>>>> -	hperiod = 1000000 / (vfieldrate_rqd * vtotal_lines);
>>>> -
>>>> -	/*  13. Find the actual Vertical field frequency: */
>>>> -	vfield_rate = hfreq_est / vtotal_lines;
>>>> -	/*  14. Find the Vertical frame frequency: */
>>>> -	if (interlaced)
>>>> -		vframe_rate = vfield_rate / 2;
>>>> -	else
>>>> -		vframe_rate = vfield_rate;
>>> The amount of unused code is quite large, which makes me wonder if
>>> there's something missing below where these variables are supposed to be
>>> used.
>>>
>>> If these variables can be removed, comments should mention that steps 9
>>> and 11 to 14 are being left out. After all, the function is fairly
>>> explicit about implementing the GTF algorithm step by step.
>>>
>>> Best regards
>>> Thomas
>> If the goal is to keep all the steps then I could prefix all problematic
>> variables with __maybe_unused macro.
> The effect is the same; it hides a potential bug that should be analyzed
> and fixed. If you have the time, please look at the code and figure out
> what it's supposed to do, and why isn't it using the information. Look
> at git blame and log, was it always so, or did something change?
>
> The warnings are about potential bugs. The objective or end goal is to
> fix the bugs, not to silence the warnings.
This code haven't change since it has been added by commit:
26bbdadad356e ("drm/mode: add the GTF algorithm in kernel space")
The variables that I'm removing are not used anywhere else.
The algorithm is copy from xserver/hw/xfree86/modes/xf86gtf.c where
vframe_rate and v_back_porch are used with (void) calls:
(void) v_back_porch;
(void) v_frame_rate;
It is another way avoid the warnings.
Note that if you start removing v_frame_rate then vfield_rate becomes 
unused, etc...

Benjamin

>
> BR,
> Jani.
>
>
>> Benjamin
>>
>>>>    	/*  15. Find number of pixels in left margin: */
>>>>    	if (margins)
>>>>    		left_margin = (hdisplay_rnd * GTF_MARGIN_PERCENTAGE + 500) /
>>>>
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